August 13, 2013
Sofia at her desk.
During my last couple of weeks in Hong Kong I was busy perfecting my Matlab program. I gradually added various layers of complexity to my code, accounting for more types of heat transfer in my calculations, and modeling the walls and streets I was looking at as three-dimensional slabs of concrete and asphalt instead of just surfaces. It always took a little while to integrate each addition into my code perfectly, as well as to study up on the science and calculations behind them, but once I got them to work it was cool to see how accounting for each new aspects of the urban environment changed the output of the program, and made it more realistic. For a while the calculated temperatures for the walls, street, and air between the buildings in my urban canyon model seemed a bit too high to be realistic, although the patterns they followed seemed logical, but after accounting for the effect of wind on heat transfer from the walls and street to the air, and the potential for heat storage in the walls and street based on the heat capacity of their materials, the temperatures started to look more like real-life urban temperatures, which was exciting. Working on this program reminded me a lot of my computer programming class at Cal, where you would struggle over our code for hours trying to figure out why it wasn’t spitting out the right answers to the test cases we were given, and when you finally figured out where you’d gone wrong and fixed it and it finally gave you the right answer, it felt like the greatest achievement in the world. Writing this program was a lot like that, except even cooler, because when I got it to do what I was trying to do it actually was simulating things in the real world that I was studying, instead of solving an arbitrary problem assigned in class. It was cool to really apply the things I’ve learned in school to studying urban heat islands.
After tweaking my program some more, I had to come up with some ways to validate my model, since it was a theoretical study and I needed to prove it had some application to the real world. To do this I used data from a study our professor had conducted, where he measured the temperatures of different types of concrete walls over the course of several days. One wall type was just a smooth concrete wall, and one had pattern of rectangular grooves, which closely resembled a bunch of side-by-side urban canyons. I used my program to simulate temperature for these two wall types, likening the second type of wall to a miniature urban canyon, and my modeled temperatures turned out to be really close to the measured temperatures during daytime hours. They deviated a bit during the nighttime, but this was most likely because we didn’t have any data on the wind speeds at the location where the study was conducted, and that has a big effect. This was determined to not be too huge of an issue though, since I was looking primarily at solar radiation as an energy source, which is only around during the daytime hours, so the nighttime hours were not as important.
Sofia in Hong Kong.
In addition I examined how valid my use of an infinite urban canyon model was, since in real life streets are not infinitely long. To do this I calculated the effective albedos (the fraction of the radiation that enters the canyon that is reflected back out) of finite urban canyons of varying lengths, and looked at how they changed as the lengths were increased. I found that beyond a length of about 300 meters, the length of a couple typical city blocks, the effective albedos showed almost no change, which meant that even though I was using a model of an infinite canyon to make calculations simpler, it was similar enough to real world situations to actually be meaningful.
After finishing up all these model validations I had to write a research paper to present everything I had discovered over my nine weeks in Hong Kong. This was a bit daunting since I had never written a formal scientific research paper before, but many hours and sixteen pages later, I finally summarized all my findings into a legitimate research paper. One of the graduate students, Alan, helped me by reading my paper and giving me advice, since he has written several papers himself, so I also learned a lot about writing research papers while I was here. I can’t believe my time in Hong Kong is already over, it has been such an incredible experience and I have learned so much this summer. Now when I walk around the streets of Hong Kong I can’t help but think about solar radiation incident angles and sky view factors, so my research has really made me look at cities in a new way, and I’m looking forward to bringing that knowledge back to Berkeley with me. Good-bye Hong Kong and thanks for a great summer!
July 10, 2013
Graphs from the program Sofia is working on.
Since my last blog post I have been continuing to develop my Matlab program to simulate the urban environment and the effect of solar radiation. Since cities are incredibly complex, and all unique, typically studies look at a simplification of a repeated unit of a city, called the “urban canyon.” The urban canyon consists of two buildings and the street between them, and this is the simplification model that my program uses to look at the interactions between buildings, streets, and solar radiation. It is a manageable unit to apply calculations because it is not based in any specific real city findings about it can be extended to any city in the world. At this point my program calculates how much solar radiation comes into the space between the two buildings, determines how much would be reflected out back into the atmosphere, and how much is absorbed by each building wall and the street, and finally calculates the surface temperature of the walls and street. This overview may sound simple, but it involves a lot of complicated calculations so I will give more details on how the program calculates all this information. Also, since my research is all theoretical, I have to check that it has some basis in reality by writing some secondary programs for some simple cases like how solar radiation would affect the temperature of just a horizontal slab of concrete on the ground over the course of a day. This allows me to see if my program has output that seems reasonable and could be expected in the real world, this is important in order in order to be confident my methodology is on the right track. So here’s the long explanation of what goes into writing this program:
First I had to determine how much solar radiation is coming in from the sky, which depends on time and geographical location. Solar radiation starts as direct beams from the sun but is broken up by particulate matter in the atmosphere so part of it becomes diffuse radiation, which is basically directionless radiation that hits surfaces on Earth from every direction. Then I had to determine how much direct and diffuse radiation hits each surface, which for direct radiation depends on the shading between buildings and for diffuse radiation depends on the sky view factor, which is basically how much of its “view” of the sky is not obstructed by neighboring buildings. Then I determined how much radiation was reflected between each wall and the street and how much was reflected back into the atmosphere, which depends on the view factors between the walls and the street and the sky view factor. The taller the buildings are the smaller the sky view factor becomes, so more radiation is trapped in the urban canyon. The radiation that isn’t reflected by each surface gets absorbed by it, and can be used to calculate the temperature of each surface by looking at the balance between the incoming solar radiation and the radiation that the surfaces emit based on their temperature, which is called long-wave radiate cooling.
Sofia meeting with her professor to give updates on her progress.
Last week we met with out professor and some of his graduate students to update him on our progress. I showed him my program and the graphs it produces of the radiation reflected out of the canyon and the surface temperatures of the walls and street in the canyon and how they change over the course of several days for different canyon aspect ratios. At this point I have observed from my program’s output that although deeper urban canyons (aka denser cities) trap a larger fraction of the incoming radiation, the tall buildings provide so much shading between each other that very little solar radiation makes it into the canyon at all, which produces a lower temperature within the canyon than a shallower canyon would, which might indicate that denser cities would in fact increase thermal comfort for their inhabitants. He seemed pleased with what I had accomplished so far and asked me to continue by looking into how the temperatures might change over longer periods of time and see if heat becomes stored in the deeper canyons since it is harder for heat to escape from then, and to write my findings in a research paper by the end of my time here. So for now I will continue adding new layers of complexity to my code and eventually begin writing my research paper.
June 23, 2013
A lot has happened since my last blog post. Our professor came back from Germany and we got to meet with him to get some more details about the specifics of what we are researching. Also some of his PhD students and another intern from mainland China arrived. In addition to the urban heat island effect, some researchers have also observed an urban cool island effect in cities for short periods during the morning. Some researchers have a theory that if cities are more dense they will have more of an urban cool island effect because there will be a lot of shading between adjacent buildings which will prevent sunlight from getting in and warning up the air. However, our professor doesn’t think there is ever a cool island effect in the city of Hong Kong and that is what I am investigating. Most of the studies done on the urban heat island effect look at situations with buildings that have relatively small ratios of building height to street width (basically a measure of how dense the city is), with ratios like 2:1 or 1:1, but in Hong Kong the building are incredibly tall and might have a ratio more like 10:1. This could mean that even if only a little solar radiation makes it in to the space between buildings, it will just get reflected back and forth essentially forever between them, trapping a lot more energy than shorter buildings and actually making it warmer. In addition, these studies are done in dry places where the temperature gets hot during the day but pretty cool at night, but in Hong Kong it is hot all the time, even at night, so our professor thinks these differences make it so there is no cool island effect.
Sofia on a ferry headed to Macau.
To investigate this I am working on writing a program with Matlab that will simulate the amount of incoming solar radiation in the space between two buildings and evaluate how much of the radiation is reflected back out into the atmosphere and how much is absorbed by the buildings, and then calculate the surface temperature of the building walls and the air temperature between them. A lot of the calculations are really complicated, for instance, I have to figure out the geometry of the shadows the buildings cast on each other and how the radiation is reflected between the two buildings once it hits one of them, so I have been reading lots of technical papers and textbooks about solar radiation.
Outside of work, Carter (the other intern here from Berkeley) and I have befriended a group of Yale students also living in the dorms here who we have been exploring Hong Kong with. A while back we had a day off from work because it was a holiday (The Dragon Boat Festival) so we traveled to a nearby town to watch the dragon boat races and eat rice and pork wrapped in banana leaves, a traditional food for the holiday. We also took a ferry to China’s other Special Administrative Region, Macau, which used to be a Portuguese colony, and ate delicious Portuguese food and saw pandas. We also went up to Victoria Peak, the highest point in Hong Kong, at night to see the beautiful city lights. Hong Kong is a very international city and last weekend I got to get in touch with my Swedish heritage by celebrating Swedish midsummer on a boat out in the harbor organized by the Swedish Chamber of Commerce in Hong Kong. The views of the city and the jungle-covered hills from the water were really beautiful. I also visited the nearby Ten Thousand Buddhas Monastery, which you have to climb a path of 431 steps lined with buddhas to get to. After the monastery I went to a nearby market where you could buy super cheap food on the street, like dumplings being made fresh while you waited in line to buy them, and tropical fruits which cost 20 HK dollars (about $2.50 US dollars) for three pounds of them! I am having a great time here learning a lot about both urban heat islands and the cuisine and culture of Hong Kong.
June 11, 2013
The streets of Hong Kong.
It’s a really interesting experience to be able to study the urban heat island effect here in the most urban place I have even been, Hong Kong. My internship is at the Chinese University of Hong Kong, which is a little bit outside the main city of Hong Kong. We are looking at the urban heat island effect, a well-documented but little-understood phenomenon that causes urban areas to be significantly warmer than the surrounding suburban and rural areas. This effect has been attributed to several different factors, but my research project involves studying the solar radiation budget and the geometry of the urban landscape to see if it can be used to determine the surface temperature of buildings and the effective albedo of the urban landscape, which is how much of the incoming solar radiation is reflected back out into the atmosphere. My first week involved a lot of technical reading to learn as much as I could about this topic and see what analysis has been done already.
The professor I am working with is currently in Germany but will be back soon, so for now his grad students have been showing me the ropes. They are very nice and friendly and in addition to providing me with lots of resources about thermal radiation and building geometry they even offered to take me hiking one day, which I am looking forward to. Over the weekend I got to see a little bit of Hong Kong, which is definitely the biggest and most crowded city I have ever been to, but surprisingly clean and orderly for a place with so many people, and also very beautiful, especially at night when all the buildings are illuminated. Stay tuned to here more about my research adventures!