Climate Change and Health Seminar: Urban heat islands: Theory, measurement and mitigation
October 19, 2020Dr. Xuhui Lee, Sara Shallenberger Brown Professor of Meterorology
Yale School of the Environment
September 21, 2020
Information
- ID
- 5779
- To Cite
- DCA Citation Guide
Transcript
- 00:03- Alright, I think we should start,
- 00:05so welcome everyone
- 00:07and welcome to our fourth,
- 00:10the first seminar of the Yale Center on Content
- 00:15in House in for 2020,
- 00:18and so today we are very please that you have dr. Xuhui Lee
- 00:25from the Yale School of Environment.
- 00:27So he's the Sara Shallenberger Brown
- 00:32Professor of Meteorology,
- 00:34he's also a director of the Yale Center
- 00:38for The Earth Observation,
- 00:41he also received the 2015 award
- 00:45for outstanding achievement in Balm meteorology
- 00:49from the American Meteorological Society.
- 00:52So without further ado,
- 00:57we will have doctors. Xuhui Lee
- 01:01- Thank you, Kai
- 01:02and also thank you Rob for having me in this event.
- 01:09Let me see, how do I, can you see my screen Okay?
- 01:15- Yes.
- 01:16Okay good.
- 01:18So I'm gonna go talk about some of all the work done
- 01:23on Urban Heat Island.
- 01:26Let me see if we can turn out the,
- 01:30so the title of my talk is Urban Heat Island Theory
- 01:36Measurement and Mitigation.
- 01:41So somewhere in that order,
- 01:42let me see if I can turn off my screen here.
- 01:45Okaynow, that's much better
- 01:48and so the work I'm presenting today
- 01:51is really a collection of things done by folks
- 01:55in my lab, current members and also past members so far
- 02:02of my lab, some of them are actually attending this event
- 02:07and I noticed that this event is being recorded,
- 02:10that's fine with me.
- 02:11There are a few slides where we don't have where we can....
- 02:15Where I showed you a sort of unpublished results
- 02:18so if you'd like to, if you want to share this recording
- 02:21with folks, please refrain from perhaps
- 02:24not sharing that part to people.
- 02:32So many of you are familiar
- 02:35with this kind of projections right?
- 02:36Projecting for temperature into the future
- 02:39to the end of the century
- 02:40depending on whether we take the aggressive mitigation
- 02:46or scenario or more of a business as new your scenario
- 02:49we will end up with very different temperature projection
- 02:52in the low emissions scenario,
- 02:55we expect maybe 1.5 degrees of increase, decrease dialysis
- 02:59increase near the end of the century
- 03:01but in a more sort of aggressive emission scenario RCP 8.5,
- 03:08the projection is that four degrees of decreases of warming
- 03:14towards the end of the century.
- 03:16So that's the kinda big picture.
- 03:19So what I would argue is that Heat stress
- 03:22is actually perhaps the most,
- 03:23the biggest climate threat to humans
- 03:27in stress associated with climate change.
- 03:29The reason is simple
- 03:31that we humans are warm blooded animals,
- 03:34We have a biological limit we cannot overcome,
- 03:38so we are warm blooded,
- 03:40we keep our body temperature at a constant value
- 03:43of the property 37 degrees Celsius
- 03:46and in a warm climate we need to maintain
- 03:49a temperature differential of at least two degrees
- 03:53between the thick body and the skin
- 03:56in order to for the metabolic heat
- 03:57to get discredited in the environment right?
- 04:01So that's a physiological limit barrier
- 04:03we cannot overcome if conditions
- 04:06in such that we cannot maintain
- 04:08a skin temperature lower than 35 degrees
- 04:11then we will suffer serious health consequences
- 04:16even death without of course the help of air conditioning.
- 04:22So that's the kind of the motivation
- 04:24for this kind of work off
- 04:26and of course we know that residents
- 04:29in the Urban Environment,
- 04:31urban residents suffer an additional Heat stress
- 04:35due to the Urban Heat Island.
- 04:36This is sort of classic depiction
- 04:38by Jumoke of what an urban heat Island looks like.
- 04:42If you have a bicycle for example
- 04:43your attach or sensor, something I would talk about it,
- 04:47you'd end up with this lecture
- 04:49and you move across a transect from rural to urban core.
- 04:56You would record temperature variations such way
- 05:00lower temperature in outside city,
- 05:03as you move to the center of city
- 05:06yo'll register very high temperature
- 05:08while relative to high temperature
- 05:10and this difference between urban
- 05:13versus rural temperature temperature
- 05:16is really what we call Urban Heat Island
- 05:17or intensity of therapy to time.
- 05:20So that's a well accepted sort of depiction
- 05:24of this phenomenon
- 05:27and so this is added heat
- 05:28that urban residents would experience,
- 05:31and this is a sort of spatial view
- 05:32for urban heat island here
- 05:34actually in the city of New Haven,
- 05:36the urban unite is very patchy.
- 05:38I have high spots here and there and some low spots there.
- 05:42So the high spots in the archaea shotguns area, right?
- 05:47And then that's this downtown area
- 05:49and then near the fringe of the city
- 05:52where you have a lot of trees, temperature is much lower.
- 05:56So that's the kind of urban heat island parent
- 05:59that you see in New Haven.
- 06:02So why Urban heat island is a concern?
- 06:05Well, you can just simply consider
- 06:07a probability distribution of temperature,
- 06:10this is a probability distribution temperature
- 06:12of maybe a rural background
- 06:14and Urban heat Island would shift
- 06:16this probability distribution just by a little bit,
- 06:19maybe by one degrees on average, right?
- 06:22But that one degree of shift in the mean
- 06:25would actually create a serious consequence
- 06:28in terms of heawave frequency
- 06:31and let's suppose the Heatwave threshold is here,
- 06:34now this is per heatwave threshold
- 06:36beyond which we will see problems
- 06:39with mobility and mortality
- 06:42and for Rural background, rural location,
- 06:46this is the area under this curve
- 06:49is your Heatwave frequency.
- 06:52Now for urban land, the simple shift in mean due to our heat
- 06:56on it, would change that frequency a lot,
- 07:00we increase that frequency a lot, right?
- 07:03And the other thing that you should notice
- 07:05of course as the urban heat Island,
- 07:08urban residents will actually experience
- 07:11a record temperatures not being seen by rural residents
- 07:15so again rural temperature stops here on,
- 07:18so this is a spread.
- 07:20So, but in the city,
- 07:22you will see temperature beyond the record, right?
- 07:26The record registering in the background sites.
- 07:29So that's also another issue
- 07:31that we should be concerned about Bob.
- 07:36So that is really the motivation
- 07:39for why we study the theory of urban heat island
- 07:44and why we want to come up with strategy
- 07:46to mitigate urban heat island, alright?
- 07:50So let me switch to give you
- 07:52a sort of review of theory
- 07:54of the urban heat island phenomenon.
- 07:56So this traits, they can be trace back to me many years ago
- 08:00to team Oaks textbook,
- 08:02in his textbook he listed the seven causes
- 08:05of urban heat Island of the seven,
- 08:08I highlight the four causes people consider it
- 08:11to be the major ones.
- 08:13The first one is increased absorption
- 08:15of short-wave radiation due to urban mophology
- 08:20and maybe due to the color of the landscape
- 08:23so they're committed...
- 08:25The conventional wisdom
- 08:26is that urban land tend to trap more solar radiation
- 08:29so that's a source of urban heat island.
- 08:32A second source of urban heat island of course
- 08:36is very easy to understand
- 08:38because there's an additional heat,
- 08:41anthropogenic heat from anthropogenic sources
- 08:44from automobile driving, driving automobiles bills,
- 08:47converts chemical energy in fossil fuel to mechanical energy
- 08:51that mechanical energy eventually dissipates
- 08:53as heat to the environment, right?
- 08:56And so another important source
- 08:57of anthropogenic heat is a space heating.
- 09:00We heat our houses or use of air conditioning
- 09:05and they will generate heat
- 09:08and dissipate heat to the environment.
- 09:11The third course is increased sensible heat storage
- 09:14on buildings and other facial structures can store energy
- 09:20solar energy, solar radiation energy in a daytime
- 09:24and that then they were released
- 09:26that energy at night causing nighttime urban warming,
- 09:30and finally not a major course is decreased evaporation
- 09:34You know that you'll replace natural vegetation,
- 09:37replacing, replace trees with artificial impervious surface
- 09:41you reduce evaporative cooling power right?
- 09:43So those are the four
- 09:44sort of major causes of Urban heat Island
- 09:49and so the, we understand
- 09:51those concepts in a conceptual way,
- 09:53in a qualitative way for a long time
- 09:56and so what we did was with a few years back
- 10:00was try to quantify those causes in a quantitative way.
- 10:05We believe, we know only by quantifying those causes
- 10:08that will then lay the foundation
- 10:11for sensible sort of measure of how to mitigate Binky Don.
- 10:18So I need to sort of take a step back
- 10:20and introduce this theory called
- 10:23The theory of intrinsic biophysical mechanism,
- 10:27this is theory was first developer to actually,
- 10:29to understand how perturbation changes surface temperature,
- 10:35changes near surface temperature amid arm,
- 10:38this theory is extended to talk,
- 10:40to the study of urban heat Island
- 10:43so some key points here.
- 10:45So this theory,
- 10:46This mechanism really is concerned with the process
- 10:49which how surface temperature responds
- 10:52to external perturbation by external perturbation,
- 10:56I mean a number of things.
- 10:57It could be addition additional aerosols to the atmosphere
- 11:00that will block sunlight penetration
- 11:04and an intercept sunlight penetration.
- 11:07And it could also be a change of urban, change of landscape
- 11:12a land use change replacing say, forest, we some open-end
- 11:16or natural land by urban man
- 11:20so those are considered to be external perturbation
- 11:24and so he helped Bob understand this process.
- 11:28There are two key components to that.
- 11:32Why is one called a local Longwave radiation feedback?
- 11:36And the other one is a change in energy redistribution
- 11:39but in the service in the overlaying atmosphere,
- 11:42I'm gonna explain those two processes in a little bit,
- 11:47so the way it quantified the surface temperature response
- 11:51is really just to do this sort of experiment
- 11:54or numerical experiment
- 11:57and then it goes quantified through measurement as well
- 12:01to the surface response really is the difference
- 12:04between temperature of old state before the perturbation
- 12:08and a new state after perturbation.
- 12:10So that's what the perturbation temperature signal
- 12:13is really the key here and we're trying to quantify.
- 12:18So let's take a look at,
- 12:20so let's go back to the case of deforestation study, right?
- 12:26The interest here is motivate your part
- 12:29by the new trying to send whether removing trees
- 12:33or adding trees or warm or cool the local temperature.
- 12:38So I, this is my favorite numerical example.
- 12:43This is a actual data collected over forest in Israel,
- 12:48semi arid climate conditions.
- 12:50This is how much solar energy reaches the forest
- 12:53and this is how much get reflected
- 12:54through its albedo reflected away from the surface,
- 13:00some escape of course to outer space,
- 13:04this is just a top of atmosphere.
- 13:06Now if you remove the forest
- 13:08and replace for us with some Shrub land,
- 13:12shrub land is much brighter, has higher albedo
- 13:16and so it's a short wave radiation
- 13:18well reflection will increase
- 13:22and so naturally you would think
- 13:24that the temperature would go down, right?
- 13:26Because now you have more
- 13:27or less short wave trapping solar radiation
- 13:32and so when the surface
- 13:34when we undergo what we call radiative feedback
- 13:38because when you have low absorption solar radiation,
- 13:42the surface cool and therefore they will have
- 13:46less Longwave radiation escaping to the from surface
- 13:49and eventually you will establish
- 13:51a new radiation liberate, right?
- 13:54Cause that process, the longwave adjustment,
- 13:57it's called Longwave feedback, that's a negative feedback
- 14:02and so if you allow just Longwave a radiation exchange,
- 14:07only allow radiation exchange to occur
- 14:10between the surface and atmosphere,
- 14:13this is you can come up with a simple prediction
- 14:16So the change of straight away radiation is dead ass
- 14:20that's your perturbation signal
- 14:22and the change of surface temperature Delta Ts right?
- 14:24This is a parameter called Local climate sensitivity,
- 14:28that's more or less a constant the number
- 14:30and so in this particular numerical example
- 14:33you would predict by replacing for us Shrub land
- 14:37and you expect a coin of dot four degrees
- 14:40about five degrees, right?
- 14:42So that's an argument some people used
- 14:46to promote deforestation,
- 14:48they're saying deforestation actually maybe a good thing
- 14:51cause helps cool the local climate
- 14:56because a lot because of albedo effect,
- 14:58but of course that picture is not complete
- 15:01because in the real world,
- 15:03you not only how a radiative process irradiated feedback,
- 15:06you also have too what I called energy redistribution
- 15:11occurring between the surface and the atmosphere.
- 15:15So there are two processes;
- 15:16One is evaporation.
- 15:18Evaporation is a process
- 15:19where liquid water is converted to water vapor right?
- 15:23So that happens near, at the surface.
- 15:25so evaporation that will take away energy,
- 15:28take away late night Tiki damage that will consume energy
- 15:31and then when vapor gets to the top
- 15:34above the atmospheric boundary layer
- 15:36and condenses to form cloud,
- 15:37that energy latent heat get released.
- 15:40So the process is a process of energy redistribution.
- 15:44It reduced screwed energy, taking away energy away
- 15:46from the surface, and then put the energy back
- 15:49into the atmosphere above the boundary layer.
- 15:51So that's one energy redistribution process.
- 15:53A second energy redistribution process is connection,
- 15:57is really is due, is the result of an emotion result
- 16:02of triplet motion in the boundary layer.
- 16:04That process is dissipating energy from the ground
- 16:11to the lower atmosphere.
- 16:15So you can set up this kind of thought experiment
- 16:18to look at how the two, the processes play out, right?
- 16:24In this thought experiment
- 16:26Or you can also do this in numerical, in the motto as well.
- 16:31You put a forest next to an open land
- 16:36and the two patches of landscape are influenced
- 16:40by same atmospheric conditions in terms of temperature,
- 16:44background temperature,
- 16:45in terms of incoming solar radiation, long wave radiation
- 16:49and so basically the value
- 16:51that those quantities are the same
- 16:53across the two patches of land
- 16:55at this order called a Blending height
- 16:58which is typically taking its first mode
- 17:00of great height about 50 meters
- 17:02to a 100 meters above the surface right?
- 17:04And then, so in this kind of site pair analysis
- 17:09all a space for a time analysis that the contrast open land
- 17:14the contrast in temperature which an open land
- 17:17and the forest land is really your,
- 17:19is really the deforestation signal
- 17:21cause that's how we approach this particular problem, right?
- 17:27And so I don't want to get into too much
- 17:29of a mathematical details except to say,
- 17:32this is how we frame the problem,
- 17:36we combined what we call
- 17:37the one source of a model for heat transfer,
- 17:43surface energy balance conservation of energy at the surface
- 17:47to formulate our solution for surface temperature
- 17:50so in this One source Model heat is dissipated
- 17:54from the ground to Reference height
- 17:58and using some kind of resistance analog right?
- 18:01So the heat of efficiency of heat flux
- 18:04is really proportional to temperature difference
- 18:06between difference in temperature
- 18:07between the surface and temperature at a lower atmosphere
- 18:11at a per landing height.
- 18:12So you combine those two sort of considerations.
- 18:16You'll come up with a solution for surface temperature
- 18:21And then you do a sort of the perturbation to decide
- 18:25mathematically it's just,
- 18:26that's equivalent to differentiating this equation
- 18:31and so you then get perturbation signal.
- 18:34That's your temp deforestation signal
- 18:37by replacing it four of this open land,
- 18:38you get a temperature change,
- 18:40that's the temperature change mathematically
- 18:42and then the temperature changes
- 18:43then it's partitioned into three components.
- 18:45The first component has to do with changing albedo.
- 18:50I mentioned earlier using that Israel example,
- 18:53the second component has to do is back.
- 18:54The energy redistribution efficiency has changed
- 18:58due to a change of reference.
- 19:00So forest landscape is very rough and very efficient
- 19:05in generating triplets,
- 19:06It's very efficient in dissipating energy by triplets
- 19:09but open land, it's very smooth so it's not as efficient.
- 19:13So that itself will cause change in temperature
- 19:17and then the third component contribution
- 19:20is change of energy redistribution
- 19:23due to evaporation change or change of evaporation
- 19:26and that can go either way
- 19:28when you compare forest to open land
- 19:30depending a forest cover to open land
- 19:33depending on which one has higher evaporation potential.
- 19:37So that is the approach we use to study a deforestation
- 19:42and it later turns out that we have two prompters here,
- 19:46one is this local climate sensitivity prompter
- 19:50which is more or less constant
- 19:52but this prompt F is energy redistribution factor.
- 19:55Some people have done quite a bit of work
- 19:57on this prompter and turns out this prompers
- 19:59more like a property of the landscape.
- 20:03So for example, this is a study by Bright et al
- 20:07looking at Energy redistribution factor
- 20:09for different ecosystem.
- 20:13This is evergreen needle-leaf forest,
- 20:16deciduous broad-leaf forest
- 20:18evergreen broad-leaf forest
- 20:20and this is a two types of crop lands, rain fat irrigated
- 20:25and this is grassland.
- 20:26Typically when you compare a forest
- 20:29versus the grass open land,
- 20:31you find the energy redistribution factor
- 20:33much high for forest
- 20:35especially for tropical evergreen broad-leaf forest
- 20:39meaning that they are a disturbance,
- 20:44just external sort of perturbation
- 20:47will not change his temperature
- 20:48as much same perturbation occurring over grassland
- 20:53because over or at this kind of landscape,
- 20:56the energy is can be dissipated very quickly
- 20:59to the atmosphere
- 21:00and therefore is more resistant to change in temperature,
- 21:05and then later on TC from my lab did this calculation
- 21:11mapping the energy redistribution factor across the globe
- 21:16given the current distribution of vegetation types
- 21:19of course and you find a high value in tropical places
- 21:24and low Value elsewhere
- 21:25and then Nighttime value is much lower
- 21:28so there's, when you look at tables
- 21:30night contrast Daytime energy redistribution factors
- 21:33is much higher than at Nighttime
- 21:36meaning that same amount of changes
- 21:39of a disturbance would cause much higher response
- 21:43in temperature at nighttime than in the daytime.
- 21:46So that kind of day and night symmetry
- 21:48is also very important in the consideration
- 21:50of how land use change affects the surface temperature.
- 21:55So basically then we'd say okay well,
- 21:57let's just extend this to urban landscape right?
- 22:00You've sent the urban landscape now
- 22:02instead of contrasting for us was open ended.
- 22:05We are contrasting a natural land versus urban land.
- 22:08That's the urban heat Island signal right?
- 22:11And so you go through that little model you find
- 22:14then now you have five contributions
- 22:18five factors contributing.
- 22:19One is changing the albedo or radiation convection effect,
- 22:22evaporation effect changing storage
- 22:25and change your anthropogenic heat.
- 22:27So a few years ago, my former student lays out,
- 22:31did this attribution analysis based on
- 22:36this model and then did a partitioning
- 22:39of urban heat island intensity
- 22:41to and partition the urban heat Island
- 22:42intensinty to different factors
- 22:44and this is a very complex plot that maybe I should show you
- 22:48I tend to just read this particular diagram.
- 22:50This diagram is daytime
- 22:52urban heat island on in situation for four cities in East,
- 22:57Southeast United States including where we are
- 23:00and so this is sort of wet climate.
- 23:03So and this is the modis settling observed over here.
- 23:08He did in intensity,
- 23:09this a climate model calculate intensity.
- 23:12This is the summation of the in individual terms,
- 23:16individual contributions right?
- 23:18So in the case of cities, this part of the world actually
- 23:25Albedo effect is cooling
- 23:27so contrary to what many people believe
- 23:31turns out cities in this part of the country
- 23:35our axe is brighter than the background,
- 23:39but then the rural background
- 23:41is mostly forests are dark
- 23:43so the Albedo effect is cooling
- 23:46but so what's surprised us actually,
- 23:47is this connection effect right?
- 23:49It turns out
- 23:53in this this kind of climate,
- 23:56this region urban land is not efficient in dissipating heat
- 24:00than the background forest land
- 24:03and so as a result of loss of convection efficiency
- 24:07you have an obviously a lot of warming.
- 24:10So it's actually this loss efficiency
- 24:12dominates urban heat Island intensity,
- 24:16is much stronger than the effect
- 24:19of loss of evaporative cooling, right?
- 24:22So that's the that kind of interpretation
- 24:27of the based on that model
- 24:30and so this kind of attribution.
- 24:32this kind of practitioner is obviously very important
- 24:34when you've tried to formulate a mitigation strategy
- 24:38whether you want to say for example,
- 24:40you want to change our Albedo or change
- 24:44in evaporating
- 24:48client trees by improving evaporation.
- 24:51So you can use this to help determine
- 24:53which one is more efficient
- 24:54whether Albedo of change or change of gray infrastructure
- 24:59or tangible green infrastructure
- 25:01which one gives you more cooling power.
- 25:05And then so that study was done prior
- 25:10to Google earth engine
- 25:10not always before Google earth engine error.
- 25:17So we've hand picked a 60 some cities
- 25:21and we manually select a satellite data
- 25:24and that was a lot of work right?
- 25:26But now we Google Earth Engine
- 25:28the marking of Urban heat island much easier.
- 25:32I just want to draw your attention
- 25:34to the work done by TC again,
- 25:36he used the Google App Engine
- 25:39to map out basically the urban heat island
- 25:42for all the cities in the world.
- 25:45You can go to this link and you can pick any city.
- 25:49I can then, there's this interface allows you,
- 25:53this Explorer allows you to map out local urban heat Island
- 25:59and also variation of time change of urban heat island
- 26:03or the satellite air.
- 26:07Now let me switch gear here
- 26:09and speak about mitigation right?
- 26:12Mitigation and we know urban heat Island
- 26:15is not a a good thing, especially in hot weather conditions,
- 26:18it exacerbate the heat stress on our urban residents
- 26:23so we like to perhaps modified urban landscape
- 26:27to comeback, to control,
- 26:29to reduce the intensity of Urban heat island.
- 26:37So this is a sort of a summary
- 26:40of the kind of strategies that people are considering right?
- 26:43One strategy is white roof,
- 26:45you basically convert a dark roof to replace dark roof
- 26:49with some kind of a white shiny bright material
- 26:53to increase Albedo so you then cool the urban climate.
- 26:59The other strategy
- 27:00is strategy promoted by the city of Chicago you know,
- 27:05putting green vegetation on rooftop
- 27:08like indicate this case is a City Hall
- 27:13and a third strategy is the one
- 27:14that our school used is to convert a rooftop
- 27:18to Solar Panel to cover the rooftop with Solar Panel.
- 27:22The benefit there is that instead
- 27:24of allowing radiation
- 27:27to turn into heat,
- 27:29you actually capture solar radiation
- 27:31and convert some of it into electricity
- 27:35and therefore avoiding heating the local environment right?
- 27:39So that would also bring cooling benefits.
- 27:41It's a fourth approach is to use
- 27:44Street trees
- 27:47to help cool
- 27:49whenever you can
- 27:50wherever you can plant trees to cooll the local climate.
- 27:55So the question is which one is more effective, right?
- 28:00And if so how do you quantify that
- 28:04before I do give you a solid quantification,
- 28:07I just want to draw your attention to this case in Chicago.
- 28:12It turns out changing roof top albedo
- 28:14is not a theoretical concept,
- 28:17it's actually been actively promoted in many cities,
- 28:20city of Chicago was one of the pioneer cities
- 28:23promoting this idea, promoting this approach
- 28:26using a brighter reflective materials
- 28:29to help cool the local climate
- 28:31to help control
- 28:34the local urban heat Island,
- 28:36this is a work done by a former student
- 28:41of professor Ron Smith and myself.
- 28:44So he quantified change in urban out Albedo
- 28:48in Chicago after 1995, after that notorious heat wave
- 28:52that kills a hundreds of people
- 28:54and turns out we can actually,
- 28:56we were able to quantify change of the citywide Albedo
- 29:01the city over this time period,
- 29:03the city Albedo has increased by a little bit by 0.02,
- 29:09but, so you can actually quantify,
- 29:12this is a homework exercise.
- 29:14I'll ask my students to do when they do my class
- 29:17and this isn't in my book, sort of homework exercise
- 29:21you know the question ask,
- 29:23the question we're asking students to do is that,
- 29:26when the albedo,
- 29:27if Albedo is increased by this much estimate
- 29:30how much temperature reduction you get, right?
- 29:33So you can basically go back to that model
- 29:36that I presented you earlier
- 29:39but now the situation is much simpler,
- 29:41you don't need to worry
- 29:42about changing energy REdistribution
- 29:44because we have not changed urban form.
- 29:46We all only did,
- 29:47only what we did was just to change the roof of Albedo.
- 29:51So you have that single prompter problem
- 29:54and if you put numbers together,
- 29:56you'll find that the 0.02 Change increase in Albedo
- 30:00would cause a cooling on average of 1.5 degrees Celsius.
- 30:05That could be quite important in the event of a heat wave.
- 30:11Now let me share with you the pertinent results, right?
- 30:15So we, that in the case of Chicago,
- 30:18that's, what's really a local example
- 30:20and then we with lays work, we use climate models
- 30:24and in with fall, all kinds of scenarios
- 30:27considerations, climate consideration,
- 30:29climate scenarios also mitigation scenarios
- 30:32using our partition efforts.
- 30:35So this is a...
- 30:36Let me help you interpret this diagram a little bit.
- 30:37This is the condition for Mid summer day
- 30:43for cities in the United States average condition
- 30:46of all the cities in the United States
- 30:48not also the 60 some cities in the United States.
- 30:51So this is, would be the current background temperature.
- 30:54You get
- 30:56on a hot summer,
- 30:57at summer noontime in rural background,
- 31:02okay?
- 31:02And this is then the urban temperatures here
- 31:05on the current climate condition
- 31:07in a future climate near the end of century,
- 31:10the rural background will be up here
- 31:12and urban temperature would be up here.
- 31:14So we will forever residents,
- 31:16we were gonna expect this much of a temperature, right?
- 31:20We referenced to current rural background
- 31:24and so by implementing core roofs
- 31:27we are, we stay in the model,
- 31:29we change all the roofs to core to highly reflective roofs.
- 31:34We get this much of cooling,
- 31:36that's substacalling substantial right?
- 31:38Basically you raise all the urban heat Island effect
- 31:43and all some greenhouse effect
- 31:46and then we say, okay, let's plant street trees,
- 31:48well, there's only a limited space
- 31:52for planting street trees,
- 31:53but we planted street trees in the model anywhere we can
- 31:58and also we change reflect your pavements
- 32:01change your pavements to reflect your material.
- 32:04So this is what we call additive effects,
- 32:07it's like the IBL from mitigation wedge, right?
- 32:12People talk about when we talk about dealing
- 32:14with greenhouse mitigation here,
- 32:15you can use the same idea of a wedge idea
- 32:18to see the attitude of strategies
- 32:23for mitigating urban heat Island.
- 32:26So in this is very aggressive scenario of course
- 32:30we can raise all the Urban heat island
- 32:35and greenhouse effect.
- 32:36We actually have some additional cooling
- 32:39of course, it's highly idealized and real world,
- 32:42we cannot achieve this maximum cooling
- 32:46but it's instructive to show that indeed
- 32:48a core roof Australia is much more effective
- 32:53than street tree or reflect your payment.
- 32:59So spatially, this is what this looks lik, right?
- 33:02If you don't do
- 33:06any change to the urban landscape at the end of the century
- 33:09you will still have a lot of urban heat Island.
- 33:11This is circle,
- 33:14warm color circles
- 33:15indicate Urban heat island.
- 33:17We have a few cities that actually have cool like Island
- 33:22indicated by the cold color,
- 33:26but they never that's on average,
- 33:28you've got quite strong urban heat Island
- 33:31but if you use EPA white roof everywhere in this cities,
- 33:35you actually now have a cold Island almost
- 33:38across the whole country.
- 33:42This is of course in a Daytime situation
- 33:44but the white roof does not work as well
- 33:46for nighttime obviously, right?
- 33:47White roof works because it reflects sunlight in the daytime
- 33:52but at nighttime there's no sunlight took to stick off
- 33:54so you don't get much of a benefit at nighttime.
- 33:59So that still would be still is an important
- 34:02hurdle to overcome how do you call a nighttime temperature?
- 34:08The white roof would not be an effective approach for that.
- 34:18So that the calculation is done really theoretical right,
- 34:22in the theoretical calculation
- 34:24and we don't really get a sense
- 34:27of the kind of change we are calling for,
- 34:29the change Urban land form is really substantial.
- 34:33If you really want to follow this strategy
- 34:36I'll be implementing white roof everywhere.
- 34:39So for that
- 34:42we decided to well the triplets,
- 34:43do some visualization.
- 34:45This visualization is based on
- 34:48sense fly a data source
- 34:51sort of drawn data collected by this company
- 34:55over a neighborhood in a city in,
- 34:59I think in Switzerland
- 35:03and so we then use this to it to some animation.
- 35:06Let me see if can turn the animation over here.
- 35:12It does not, let me see
- 35:17way by control here.
- 35:29(indistinct) Okay there it's go
- 35:33So this is the current landscape, right?
- 35:35We're doing a fly by as if we were a bird
- 35:38looking at the landscape from different angles.
- 35:41It's a very pleasant landscape,
- 35:44you know, have a dark roof
- 35:46green lawn and street trees
- 36:00and then we say, okay well,
- 36:01we'd like to change this landscape
- 36:03because we are we are very concerned
- 36:05about urban heat Island.
- 36:07So we then,
- 36:08we can artificially digitally alter the roof material
- 36:12to a white shiny high albedo material
- 36:16and then we'd do a fly by, right?
- 36:41So that, this is kind of landscape
- 36:43we are, we'll be looking at
- 36:45if we do implement that white roof strategy
- 36:49and of course, it's this very alien landscape,
- 36:52we are not very used to,
- 36:53a lot of people criticize us for saying that
- 36:55because they said, this is not a pleasant landscape
- 36:57to a city to be in
- 37:00and pass maybe you wouldn't be detrimental
- 37:05to pilots because they can't see the ground well
- 37:09and maybe they will get blinded
- 37:10by the Brighton yourself
- 37:15the roof.
- 37:16But anyway, so that's obviously a big change we need,
- 37:21we will be expecting
- 37:23but now let me switch gear a little bit
- 37:27to what we are doing now.
- 37:28So I won't pick a criticism
- 37:30of the work we have been doing is that
- 37:33we are using surface temperature
- 37:34as a measure of heat stress,
- 37:37temperature at the surface of landscape
- 37:39but people obviously, this is obviously is not accurate
- 37:44because to measure heat stress,
- 37:47you need to use air temperature
- 37:49and furthermore heat stress is not only caused
- 37:52by temperature, it's also caused by high humidity.
- 37:56So strictly you should,
- 37:58we should be using some kind of combined index,
- 38:01index that can combine both air temperature,
- 38:03not surface temperature but air temperature
- 38:06and also air humidity
- 38:09so that a perspective from the thermodynamic person,
- 38:12turns out the best way of measuring the combined effect
- 38:15is to use one called Wet-bulb temperature,
- 38:18in meteorology,
- 38:19this is how we measure Wet-bulb temperature, right?
- 38:22So we cover the thermometer with some kind of Wet cloth
- 38:25allowing the surface of the thermometer
- 38:27to be wet all the time
- 38:29and so, and allow the evaporation to occur at the surface
- 38:34and so the temperature you imagine
- 38:36that this situation is Wet-bulb temperature
- 38:40and so that's a thermodynamic parameter
- 38:42that meteorologists use a lot
- 38:45to characterize the thermal environment.
- 38:48It turns out though in a hot environment
- 38:52sweating is obviously is a way, it's the only way actually
- 38:56for us to maintain low skin temperature,
- 39:00a person who is sweating a lot
- 39:03can be considered essentially a big wet bulb
- 39:08cause we assume the body is exposed,
- 39:11no clothing and the whole body is covered with sweat
- 39:16so analogous to a wet bulb.
- 39:20So then you can use wet bulb temperature to see the effect
- 39:26of heat stress on human body
- 39:29and as I said earlier
- 39:32to stay alive
- 39:33just to survive hard environment
- 39:36we need to maintain a two degree difference
- 39:38between skin and a deep body temperature
- 39:42so that our body can dissipate heat
- 39:45to the environment right?
- 39:46But then it turns out if the We-bulb temperature
- 39:49of the environment goes beyond 35 degrees,
- 39:51this is no longer possible,
- 39:53we cannot, we wouldn't be able to be able
- 39:56to maintain a two degree difference.
- 39:58Our skin temperature would be higher than 35 degrees
- 40:02and if we don't have air conditioning.
- 40:06So without air conditioning we cannot survive
- 40:09when external environmental temperature
- 40:12or Wet-bulb temperature goes beyond 35 degrees.
- 40:15That's really the physiological barrier
- 40:19the limit that you know, determines the survivability
- 40:23or habitability of the law of the environment.
- 40:27So we are knowledge high trying to come up with a strategy
- 40:33of studying using a wet bulb
- 40:36instead of the surface temperature to quantity
- 40:38that's undergoing a new project,
- 40:40it's a collaborative project happening here at Yale,
- 40:45it's called Biking for Science and Health
- 40:48and so the idea is that we can use bicycles
- 40:50to help out map out temperature and humidity
- 40:54across urban and rural landscape
- 40:56and use that as a way of collecting data
- 40:59to validate a model calculation
- 41:02of course
- 41:04the project oe the objective of this project
- 41:07is much broader than only measuring temperature.
- 41:12So the broad objective
- 41:13is to integrate smart sensor technology
- 41:16with public bicycles
- 41:17or maybe private bicycles as well
- 41:19for urban environmental monitoring
- 41:22so T-Mobile for scientists
- 41:24including professor Dubrow as part of the team
- 41:29and so this is that the idea right?
- 41:32So we, what we want to do is to convert bicycles
- 41:34into measurement platform either volunteer cyclist bicycles,
- 41:39planning to volunteer cyclist or public bicycles.
- 41:43So and then, the smart sensor
- 41:46would sense the environmental conditions
- 41:48temperature humidity and in the future,
- 41:50we also want to measure air pollutants
- 41:53and so the sense of what, then you turn a cyclist smartphone
- 41:56into some kind of geolocation
- 41:58and data collection device and that data can then try
- 42:01and get transmitted to some kind of a server to allow
- 42:05and then in the case of public bicycles
- 42:08the data will be automatically transmitted to a data server,
- 42:12and then the data server
- 42:13would then dispatch data to different users
- 42:18and so that's the idea.
- 42:20So we are having some success
- 42:22in terms of designing a sensor,
- 42:25a smart sensor for temperature humidity.
- 42:27This is a patch of smart temperature humidity sensors,
- 42:32very small and this is a picture
- 42:34of all this smart sensors
- 42:37calibrate it against commercial sensors right?
- 42:41(indistinct)
- 42:42This is, oh sorry.
- 42:43Before I share with you some data,
- 42:45this is the kind of sensor right?
- 42:47It's very small
- 42:48or this is the interface, smartphone interface
- 42:52and this is to give you a scale of the sensor,
- 42:54a cache to the bicycle handlebar
- 42:58and so I'll show you that the idea we have
- 43:00is to recruit volunteer cyclists
- 43:02and eventually we can also implement sensors
- 43:06on public bicycles
- 43:07but in case of volunteer cyclists
- 43:09we are hoping,
- 43:10we are defining sort of kind of data interface.
- 43:12This is work by TC and Yichen interface
- 43:16to so that when the data is sent
- 43:19to some kind of data center,
- 43:22the cyclist would receive a link.
- 43:26The link then allows the cyclist to view the bicycle route
- 43:30as well as the conditions, temperature condition
- 43:34and humidity and maybe in the future
- 43:36also air quality parameters and along the route by spiked
- 43:41we are still having trouble with the color scale yet
- 43:43but if this is the kind of general idea, right?
- 43:45And so you can actually look at data, put the data
- 43:48this kind of spaghetti plot under different map background.
- 43:52This is just pure simple map background.
- 43:55You can put it in a,
- 43:57you know, satellite background map background
- 43:59or you can put down in street map background.
- 44:03So this is not place still very much a work in progress.
- 44:07So I was up here and see if we have questions.
- 44:11I like leave some time to engage.
- 44:14I was discussion and questions.
- 44:17Thank you very much.
- 44:20- Thank you, (indistinct) for the wonderful presentation.
- 44:25We do have a lot of questions from the students.
- 44:29But if people,
- 44:31if you have your own questions
- 44:34please type your question in the chat box while
- 44:39Dr. Lee was answering to the students' questions.
- 44:42So the first question actually
- 44:46don't be you showed a very very interesting
- 44:48with us about them, why the core roofs
- 44:53and I had receive a lot
- 44:54of question from the students asking about the comparison
- 44:59between a white roof versus a green roof.
- 45:04They were particular interesting in whether,
- 45:08what do you think about like the disadvantage
- 45:11of the white roof compared to the green roof?
- 45:16- So my White roof is not very pleasant, right?
- 45:19You don't like that in your neighborhood
- 45:21and if I showed you with that, a drone sort of animation
- 45:25the landscape's not that pleasant to look at
- 45:28but in terms of cooling this surface climate,
- 45:31white roof is much much more effective than green roof.
- 45:34I'll tell you why, in green roof, you have to,
- 45:38first of all, it's very difficult to plant trees
- 45:41on a roof right?
- 45:44So trees tend to sustain evaporation much more
- 45:49than grass than shrubs
- 45:50and so, but if you just planted shrubs
- 45:53and grass on rooftop, you have to constantly irrigate them
- 45:58in order to get cooling benefit
- 46:00and then your irrigation is not easy
- 46:02especially if you have a tall buildings
- 46:04and think about pumping water
- 46:06up to the rooftop and irrigate right?
- 46:08So that's itself is a very energy intensive endeavor.
- 46:14So absence of the radiation green roof really won't do much
- 46:21to the local temperature
- 46:23but I should have knowledge of obviously green roof
- 46:26is much more pleasant right?
- 46:28It's maybe has other benefits
- 46:31beyond just cooling the local landscape.
- 46:33So that's a debate obviously that's people should,
- 46:38that aspect should be considered
- 46:41when you look at a white roof versus a green roof.
- 46:44So if you look at the cooling power street vegetation
- 46:49is more effective
- 46:51than green roof.
- 46:52So you've put green roof here,
- 46:54the effect is really tiny compared to a quarrel for white.
- 47:01- Thanks, I think we will get more questions
- 47:03on these from the audience,
- 47:04but I will move on to the other question from the students.
- 47:09The other questions students are wondering is like
- 47:13you introduce us about the concept of urban heat Island
- 47:17and students are wondering like a lot of the mitigations
- 47:22we take for the urban area that's that has also impact
- 47:26for the adjacent rural areas.
- 47:29Like if we do all these,
- 47:32why move
- 47:34in urban area,
- 47:36does it also like
- 47:38simultaneously reduce
- 47:41the heat exposure in the rural area?
- 47:45- Yeah, that's a very good question.
- 47:46I think, so that really the question maybe can be brought
- 47:50in a little bit to say that's changing urban forms
- 47:55whatever way does the have effect
- 47:57on regional climate or even global climate?
- 48:00Right?
- 48:01The answer is probably no,
- 48:02because we are we are talking about change,
- 48:07intensive changes that's
- 48:08but the intensive change,
- 48:11is only occurs in a very tiny fraction of the landscape.
- 48:17Urban land is what 2% of the whole terrestrial land surface
- 48:21and so, and in that we have intensive modification
- 48:25that intensive modification will manifest itself
- 48:29in localized response but outside of urban area
- 48:33that the benefit is really really not that bad.
- 48:38So the answer is probably, no,
- 48:42unless we are dealing with like a huge metropolitan region
- 48:46maybe in India, where you have clusters of cities,
- 48:50a lot of cities cluster together
- 48:52maybe then there, you might have some effect
- 48:55on background temperature.
- 48:59- Thanks, I think, yeah.
- 49:01I think if we got a follow up customer
- 49:03regarding the green roofs
- 49:05so they were asking one of your paper,
- 49:09The Jaw and The Shoes article,
- 49:12in that paper, there's mixed implementation
- 49:16of the white and green roofs
- 49:18and the given the green roofs lead
- 49:20to increase the evaporation
- 49:22and likely increase humidity with wide roofs
- 49:26and green roofs have under
- 49:30donor's state effects
- 49:31due to green roofs contributing to the Web-bulb temperature
- 49:36- Yeah, yeah, that's an excellent point
- 49:39and so if you take that humidity into consideration
- 49:43you probably don't actually,
- 49:46you want to avoid a green roof
- 49:50because green roof
- 49:53on one hand you will reduce the air temperature.
- 49:55but on the other hand, it will increase humidity, right?
- 50:00So the reduction air temperature could be totally erased
- 50:03or the effect of temperature reduction could totally raise
- 50:06by enhanced humidity factors.
- 50:09And so, and of course in this analysis,
- 50:12the solid dollar analysis
- 50:15we have not brought in the concept of wet bulb,
- 50:18but if we bring wet bulb into consideration
- 50:20that may be an argument we should consider seriously.
- 50:25- Yeah, I'll also from the audience
- 50:27a question regarding the implementing of the
- 50:32core roof policy,
- 50:34have you considered whether you paint all the roofs white
- 50:38or use how they are scattered
- 50:41painting within the city?
- 50:44So do you consider the difference of the painting
- 50:48depend all the buildings, all you does a scattered because.
- 50:52- So in this calculation, we except hypothetical calculation
- 50:57we just combine all the routes to a high Albedo material,
- 51:03in actual implementation I think you cannot do that
- 51:06because there's no point actually doing
- 51:09a one size fits all situation
- 51:10because if you have North facing roofs right,
- 51:13then the deflections doesn't doesn't matter as much
- 51:17I saw spacing roof.
- 51:18So maybe you need to differentiate North facing
- 51:20versus South facing roofs.
- 51:22In the city of Chicago,
- 51:23they actually have grades,
- 51:26if you have very steep roof, they ask you,
- 51:28they recommend certain kind of Albedo values
- 51:31when you have less steep roofs,
- 51:33they recommend other kind Albedo
- 51:35so he said,
- 51:38it's mixed of strategy.
- 51:42By now all lot of cities actually
- 51:44aggressively promoting spokes,
- 51:46those kinds of reflect humid roof materials.
- 51:50- Thanks, I guess the audience
- 51:53and the students are very interested in this topic though.
- 51:55They have accurately both the students
- 51:58and audience ask a question regarding
- 52:01have you ever considered
- 52:02all these heat Island mitigation matters?
- 52:05They may have some side effects on the air quality
- 52:09so how you
- 52:12kissing that in your own modeling?
- 52:15- Yeah, there's a...
- 52:17So people say maybe for white roof material implementation
- 52:21it's best to it in clean cities
- 52:23where there's no, air quality is not a big concern
- 52:27in progic cities When you put in a white roof,
- 52:31you can change
- 52:34the way that the structure of the boundary layer
- 52:37essentially what happens is if you have a white roof
- 52:40you are not heating the low atmosphere as much.
- 52:44You're reflecting a lot of sunlight away
- 52:46without us to the upper atmosphere
- 52:47and to the outer space, right?
- 52:49So what happens then is you end up
- 52:51with a shallow a boundary layer
- 52:54but there's less mixing power, less mixing volumes,
- 52:57so you end up with higher air pollution concentration.
- 53:01So that's the, it could be a serious societal effect
- 53:04especially imploded seedlings.
- 53:05So that's another,
- 53:11this the harm you could say perhaps caused
- 53:14by air quality.
- 53:16That's a very good point.
- 53:19- Thanks, another aspect of the students are wondering
- 53:25is like you showed a little bit about
- 53:28the different
- 53:31like riddles from the satellite,
- 53:32from the modeling
- 53:34and the students are particularly interesting
- 53:36in wanting these kind of modeling.
- 53:39So how can you actually
- 53:43simulate the interactions
- 53:45with the global warming and also all the biophysical drivers
- 53:51of the urban heat Island in the continent models?
- 53:55- Okay, so in the climate models right,
- 53:58they, a lot of models don't actually have
- 54:00what we call a city landscape that so,
- 54:04but the the model we use
- 54:06have what we call subgrid parameterization,
- 54:09so within each Greek cell
- 54:12you have different parches for that type of land
- 54:15so some great cells were contained
- 54:20urban land tile, urban tile
- 54:22and some would have no, if there's no urban.
- 54:26So this model actually can calculate
- 54:29within which is great cell, temperature, humidity,
- 54:33and so on within for each tile.
- 54:38So typically when you download a data though,
- 54:41the data is aggregate to the Greek cell
- 54:43that was so you don't see subgrade kind of a pattern.
- 54:48You don't see a subgrade pattern
- 54:50but we are able to re redo the calculation
- 54:54and retrieve data within each Greek Model grade data
- 54:59for vegetations tile and offer urban tile.
- 55:02So that essentially set up the problem
- 55:03for us to have to do then compare
- 55:06those subgrade tile data to get the urban heat Island calc
- 55:09apart from the climate models.
- 55:11That's how a client model handles landscape heterogeneity
- 55:15within a model grid cell.
- 55:19- Thanks, I think due to the time limitation,
- 55:22final question
- 55:23is the students and audience are very interested that
- 55:27in like, what's your recommendations for our daily life
- 55:31in as an individual,
- 55:32is it more eco-friendly to have solar panels
- 55:36or have a quarter of a solar.
- 55:39- Solar panels are very interesting, right?
- 55:42You need to do a very
- 55:44sort of a careful calculation,
- 55:47to look at the benefits.
- 55:48So solar panel dependent if it's true false
- 55:50for why is that you, like I said
- 55:53you convert a local solar radiation to electricity
- 55:57and in doing so, you don't heat the environment,
- 56:01you don't allow radiation to heat the environment
- 56:04but the commercial efficiency is not very high.
- 56:06It's not as high as reflection by core roof.
- 56:11So on its own, you would say the cooling benefit
- 56:15of solar panel is not as high as core roof,
- 56:18but then you have an added benefit
- 56:21of electricity generated by solar energy right?
- 56:24So you
- 56:27offset the demand for fossil fuel energy.
- 56:30So that benefits more broad
- 56:32modular views is you're offsetting energy demand
- 56:36for fossil fuel
- 56:37and therefore you cool the whole club global climate.
- 56:41So there's that, there's a benefit to that
- 56:44so that you need to consider both sides
- 56:47local Coolig versus global cooling
- 56:50versus and offsetting energy
- 56:53and so that'd be a hard subject
- 56:56that need to be debated, right?
- 56:59But I think if you are,
- 57:00if you want to conserve your electricity bill,
- 57:03if you want to reduce your electricity bill in your house
- 57:06that you're,
- 57:07the best approach is actually having a core roof.
- 57:11If you have at a core roof on your rooftop,
- 57:13then the demand for AC will be substantially reduced.
- 57:18You will have a lot of electricity saving in that way.
- 57:23That's has to be demonstrated by a lot of people actually
- 57:26- One fourth session.
- 57:28Thank you for all the insightful discussion
- 57:31and also the presentation
- 57:33and with that, I think we thanked Dr. Lee
- 57:37for this wonderful presentation
- 57:39and I thank you all for coming for our seminar.
- 57:44- Bye - See you guys.