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A hot topic

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Professor Krishan Kumar, Visiting Professor at UTS, from Jawaharlal Nehru University in India, explores urban heat island formations and the effects they have. The focus of his research has been upon the vast city of Delhi and its rural environs.

Urban areas are places of high population density with large scale residential, industrial and transportation activities confined within relatively small areas. Urban structures differ from their rural counterparts in terms of building height, materials, shapes and sizes etc. Further, impervious surfaces such as roads with greyish black, bituminous surfaces constitute a large part of the urban land cover.

Image 1 (above) and image 3: Professor Kumar and students taking measurements of solar spectral irradiance with a Spectroradiometer,  and particulate matter with a Mini Volume Air Sampler.
All these factors bring about a change in the energy budget of urban areas in many different ways. The high rise buildings and their complex geometry leads to multiple reflections and reduce the sky view in urban areas. This makes the process of cooling during the night less efficient over the urban areas. In contrast, the night time cooling over the rural areas takes place quite rapidly. The urban materials also have high heat capacities which makes them cool off more slowly during the night. Most of the rainfall over the urban surfaces goes as surface run off due to their impervious nature. In contrast, rainfall over rural areas recharges the soil in terms of its moisture content. Thus, there is reduced evaporation from the urban surfaces and most of the incident solar energy is utilised towards warming the surface. Further, the lower vegetation cover over the urban areas also leads to reduced transpiration and as a result these places do not witness the cooling effect caused by vegetation.

The large scale residential, commercial and transportation activities over the urban areas are also associated with the release of a large amount of waste heat in the urban atmosphere. All these factors make the central parts of a city warmer than the surrounding rural areas. At places like Delhi, urban heat island formation leads to warmer nights in summer causing human discomfort. Thus people resort to use of air conditioners and water coolers inside their homes. This in turn contributes significantly to the energy and water demands during summer.

The satellite data show that central parts of Delhi may be up to 6-8ºC warmer than the surrounding rural areas in most times of the year except the monsoon season when the sky is generally cloudy and this difference reduces to about 2ºC. Energy demand then increases in the summer months as people are very uncomfortable during the night, something that did not happen in Delhi in the 1960s and 1970s when nights used to be pleasant and people used to sleep out in the open on their rooftops. The extra night heat also leads to a local circulation of pollutants within the city and while earlier steps were taken to improve air quality in Delhi (for example switching to compressed natural gas instead of diesel) the sheer number of vehicles has seen a rise in pollutants like nitrogen dioxides.

Delhi also experiences a dimming of the intensity of light during its winters (November, December) when its atmosphere is quite hazy. There is a difference of 40-100 watts pm² of less solar radiation in Delhi in those months compared to rural areas, attributed to a high aerosol load over Delhi. Aerosols also have a significant amount of what is known as black carbon content which absorbs the radiation that is lost by the earth’s surface each day and night. Temperature measurements of the lower and mid troposphere over Delhi have shown a general rising trend both during the day and night since 1973.

A significant part of the aerosols are caused by the biomass burns which happen after crops have been harvested, a widespread phenomenon in India as farmers prepare their fields for the next crop. Professor Kumar also discovered day time cool islands in Delhi in some seasons using satellite data, a finding which was also reflected in the ground meteorological recordings, much to the astonishment of others in the scientific community. It seems air pollution climatology really is a hot topic.

Professor Krishan Kumar’s visit was sponsored by the Key Technology Partnership (KTP) Visiting Fellows program at UTS, a scheme which provides support for international researchers to work collaboratively with research staff and students at UTS. He was nominated by Prof. Alfredo Huete, group leader of the Remote Sensing program within the Plant Functional Biology and Climate Change Cluster (C3), where there exists mutual collaborative interests in investigating the inter-relations among bushfires, air quality and vegetation dynamics and the use of satellite remote sensing technologies to monitor landscape- climate- human health interactions. Professors Kumar and Huete have initiated a long term collaborative research program involving exchange visits and co-supervision of students.    
For more information: KTP program, <http://www.uts.edu.au/partners-and-community/initiatives/internationalisation/key-technology-partnership-program>Alfredo Huete, <http://www.uts.edu.au/staff/alfredo.huete>C3 Remote Sensing Program, <http://www.uts.edu.au/research-and-teaching/our-research/climate-change-cluster/research-groups/remote-sensing>

Professor Krishan Kumar was interviewed by Ruby Vincent for A Question of Balance. Images provided by Professor Kumar. Summary text by Victor Barry, June 2015.

Click here to access a Conference paper by Krishan Kumar and colleagues that presents their research findings concerning the role of aerosols in the dynamics of urban heat island formation over Delhi.
here for access to the journal publication: aerosol climatology at Delhi in the western Indo-Gangetic Plain: Microphysics, long-term trends, and source strengths.

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