Monday, November 19, 2012

Hot Time, Summer in the City

From our German friend, Jörg Zimmermann a reminder that good planning requires good scientists and good politicians: 


Global warming is harder and harder to deny.  Plots of global temperature are clearly moving upward.  The Arctic sea ice melt reached a new record low this year, which definitively confirmed what we suspected in 2007, that the Arctic has entered a new climatic regime. Thus, we are now more and more concerned about the impact that global warming will have. One of the most discussed topics this year in Germany was how city dwellers will deal with increasingly common heat waves.  But first I want to tell you a story of a professor, who pasted grains of sand on paper ...

It is over 70 years since Professor Ludwig Prandtl glued sand onto paper.  His experiment consisted of blowing air over the paper and measuring the decrease in flow velocity as a function of the distance above the paper.  He wanted to discover the mechanism by which momentum was transferred in neutrally stratified air (ie where vertical motion was neither hindered nor promoted) due to turbulence and how to calculate it.  Today, the lower part of the atmosphere, the frictional layer, where obstacles on the ground directly influence the wind field, is named after him, the Prandtl layer (Eli:  aka the boundary layer in English, Prandtl’s work was mostly concerned with aerodynamics and the flow of air over an airfoil, but it has important connections with meteorology and how winds blow near the surface.  The key insight is that the speed of the wind at the surface, which does not move, must be zero.  The increase of speed above the surface depends on the roughness of the surface).  For winds moving over the surface, in most cases the boundary layer is a few tens of meters, though at time it can be more than 100 meters high.  The effect of the ground on the flow can be recognized because under conditions of neutral stratification, the wind speed increases logarithmically with increasing altitude - first quickly, but the rate of increase rapidly falls off with height. If you want to express it in a formula, the wind speed is proportional to the logarithm of the ratio of height to the roughness. The roughness is again an estimate of the average effective height of the barriers that slow down the air. The logarithm is multiplied by the ratio of the shear velocity and the Karman constant. The shear velocity is a measure of how under prevailing conditions an air packet would move, as well as how it would be decelerated via turbulence generated by friction with the surface. And von Karman's constant, that was, what Professor Prandtl wanted to determine with his sand. In his measurements, he found the number 0.42.

Since then there have always been laboratory and field experiments and campaigns where, among other things, people wanted to determine von Karaman’s constant more accurately than Prandtl with his sand. Over the decades, the number varied from 0.35 to 0.45. But when I looked last, the current value was back at 0.42. Only the error bars had become smaller.

But what have Prandtl grains of sand to do with climate change in cities?

Cities are areas where air is decelerated. The houses act like huge kernels of sand and calm the wind in urban areas. When air heats up in the cities, it can only slowly mix with cooler air outside. This will make it warmer in cities than in surrounding areas. At night, the heat island effect can account for more than 10 degrees of warming. Why do the cities heat up so much that the mixing of the air is a critical problem? The main reason is that sealed paved streets and buildings hinder evaporation of water. The evaporation of water converts sensible heat into latent heat. The latent heat is released only when water vapor condenses into droplets which, for example, can be the source of violent thunderstorms. This is in addition to industrial, commercial and household usage which contribute to heat production in cities. Further buildings and pavement on top of the ground hinder dissipation of heat into the soil. However it is the obstruction of air flow by the buildings that makes the heat island effect really important.

The heat island effect is not just a reason that temperatures at urban weather stations might increase faster than at rural stations, requiring care in constructing temperature time series, it also has implications for urban planning.  The question for planners is how to increase water evaporation in cities. This requires plants, water features and unpaved ground. Easy to say, but dense development is a consequence of efficient commercial usage. The higher the price of land, the harder it is to convince people to leave open spaces in a city.

So you need good arguments. The German Weather Service is increasingly providing these. In cities such as Munich and Frankfurt, the German authorities had already produced thick planning reports showing how climate change might affect the urban climate and where development should be less dense. Hills near the city which are not developed can work like air conditioners. When cool at night, dense cold air forms on the hilly heights and that air flows under gravity like a river into the valleys. If the cold air lanes are not blocked by buildings the city can breathe this cold fresh air.

The German Weather Service in Cologne is also busy. Measuring vehicles are driving through the city to map out where in hot weather it is especially warm and show how the Rhine can help cool the city somewhat.  As climate change accelerates the mayor and city councilors need to understand more and more its impacts on the urban climate, and what rules to follow for urban and green planning. Besides air conditioning the frequency of extreme weather events plays a major role. Ten years ago there were summer floods on the Elbe after extreme rainfall that caused havoc in the old town area of Dresden. The weather conditions in the southeast brought up warm, humid air masses that led to the heavy rains.  With increasing warming such events could be more frequent. Cities must therefore address the question of how to better protect against high water instead of, as before, by canalizing rivers which strengthens flooding. Here very different groups and research organizations in meteorology, hydrology and water management, urban planning, landscape architecture and ecology have to work together.

Interim results of such a project were presented to the city of Cologne in 2010 which welcomed them.  The report identified temperature differences of up to eight degrees between densely populated areas and those crisscrossed by green spaces. The project's results are available from the city on its website.(in German) Within this framework municipal officials need to understand that the challenge of climate change is long since not "if" it must be dealt with, but only "how", and what need to be done now.

Cologne has set up a system of 14 climate stations in the city to measure changes and guide the city's response


David B. Benson said...

And sometimes it becomes unbearably hot:

Russell said...

The problems of Prandtl layer friction and hindrance of transpirational cooling are compounded by low urban albedo.

Black asphalt roofs and paving redouble the solar heat load in areas where impermeable ground limits evaporative heat dissipation, and run off often gathers in water features with albedos as low as the asphalt surrounding them.

The downside of low urban albedo has not gone un-noticed.

27 centuries before the current vogue of white roofs and 'cool cities ' initiatives, Solon The Lawgiver issued an edict ordering Athenians to whitewash their houses once a year.

Anonymous said...

"The houses act like huge kernels of sand and calm the wind in urban areas."

Is "surface roughness" (which impacts the drag coefficient) really the reason that cities impede air flow?

Or is it mainly the increase in area normal to the direction of air flow? (ie, the fact that buildings block wind)


Russell said...

Now we know why the Anonymous Number does not figure much in fluid dynamics lectures.

Anonymous said...

Russell, please do enlighten me ... with physics (eg, drag equation) , as opposed to silly attempts at insults.


J Bowers said...

"Or is it mainly the increase in area normal to the direction of air flow? (ie, the fact that buildings block wind)"

Isn't that more surface roughness anyway, but big?

David B. Benson said...

That is indeed more surface roughness.