Urbanization: The problem of local climate modification

Post date: Apr 3, 2017 3:02:08 AM

People migrate to cities due to different reasons such as easy access to schools, health facilities, jobs and transportation are few of the reasons. More people, therefore, live in urban areas than in rural. According to the United Nations estimate, in 2014 more than 54% of the world population dwell in cities. It was 30% in 1950. Projections indicate that 66% of the world's population will live in urban by 2050 . Most of urban people reside in relatively small areas with high number of inhabitants per square meter of land (UN, 2015):

"Close to half of the world’s urban dwellers reside in relatively small settlements of less than 500,000 inhabitants, while only around one in eight live in the 28 mega-cities with more than 10 million inhabitants."

It is ages since people noticed that urban air was different from rural air. However, it was air pollution which is the hallmark of the urban atmosphere. In 1818 Luke Howard (1772-1864) published the first edition of a book dealing with the climate of the city (Landsberg E., 1981). Howard clearly recognized a major alteration of meteorological element and called it "city fog". He compared mean temperature taken from measurements in the city and out of the city and found out the urban center was warmer than the surrounding countryside. Another discovery of this temperature anomaly was discovered in Paris metropolis by E. Renou (1815-1902) a few decades later. Micro-meteorological investigations of the urban landscape was stated by Wilhelm Schmidt (Landsberg E., 1981).

It is not only air pollution and high temperature which are attributed to urban weather and climate, but also atmospheric influence of of the city, causing an increase in rain towards the lee side of the city. This was asserted by August Schmauss (1877-1954) while making assessment for the city of Munich (Landsberg E., 1981) . Other studies also suggested there is an increase in urban rainfall as the cities grow.

On of the steps needed is to measure meteorological variables (temperature, humidity, wind precipitation) of importance in rural and urban environments and compare the results. Simple mean value difference can give an overview of the difference between rural and urban weather. However, topographical features, proximity to industrial sites, and leeward or windward measurement site may produce difference already even before the cities were built.

Comparison of such weather and climate elements can be made with certainty if there are observations from a site prior to urbanization for a considerable period of time. But such observation data available rarely.

Three major elements that influence the measured values of a metropolitan variable, M are basic climate of the region C, a difference introduced by location L (e.g. topography, water bodies, etc), and an alteration term U produced by urbanization ( Landsberg E., 1981). M is a statistical ensemble consisting of the three components.

M = C + L + U (1)

From measurements M is the only known quantity.

How can we determine U which is the urban effect?

Using sample at time 0 and later time t, and assuming invariance of C and L, one can determine U:

C + L + U (2)

C + L + 0 (3)

Urban influence is assume to be negligible at t = 0, but one can use the system only in the absence of trends in C, certainly not a good assumption.

The other scheme to establish an urban influence (Landsberg E., 1981) is to establish an urban-rural difference of variables measured at one or more sites in the two environments

- (4)

the subscripts u and re stand for urban and rural respectively. This is a quick approach that can give at least an estimate of the urban effect.

Another frequently used approach is the comparison of time trends in the two environments. Assuming that C and L are not time dependent one can state that

(5)

where i represents individual values of a time series of n members, such as successive yearly values, t is time, and

is the probability that a given weather type will occur in a given time interval t, perhaps suitably split by seasons.

Assumptions can also be made that industrial workweek and traffic create difference between workdays and weekends in human influences on some atmospheric parameters. Such comparison of Monday-Friday values with Saturday-Sunday values may reveal urban effects:

(6)

where

is a mean value of the element tested on workdays and is the value for non-workdays. This test also needs comparison of no differences in the values for comparable splits of days simultaneously in the same area not affected by urbanization. It helps us to find how far the city influence reaches.

Why should we study urban climate?

The desire to study urban climate emanates from: a) scientific curiosity as to how climate differs between urban and rural, b) application for urban design purposes, including building construction, highway and bridge design, drainage systems, stack designs for minimizing effluent nuisances, space heating, and air conditioning and similar pursuits, c) to identify the causes of extreme weather events like heat and flooding, and d) to monitor air pollution.

Urban energy losses

It is generally agreed that solar energy is lower over the cities. The loss could be due to availability of aerosols or pollutants in the urban atmosphere. The other reason why solar rays are lower is because of turbidity. Turbulence is higher over cities than the surrounding rural areas. Radiative reduction occur because of molecular scattering (Rayleigh scattering).

Turbidity is a useful measure of atmospheric suspensions, and is expressed as

(7)

where

solar radiation extra terrestrially (solar constant)

Solar radiation at the Earth's surface

correction factor of the seasonally changing distance of sun-earth

optical air mass

extinction factor for pure dry air (Rayleigh)

water vapor

This is a measure of ratio of the prevailing attenuation of solar radiation to that produced by a clean atmosphere where only molecular extinction (Rayleigh scatter and absorption) occurs.

Net energy flux