Temperature inversion is one of the most important meteorological phenomena, in which atmospheric lapse rate is inverted, that is, temperature increases with height instead of decreasing. Inversion plays a significant role in disturbing air quality of an area where it exists. Karachi is governed by the coastal arid climate under the influence of land and sea breeze. Meteorological characteristics at a coastal location are different than those at inland due to sea breeze and land breeze circulation and its impact on local weather. Karachi lies beyond the reach of both the monsoon and the western disturbance. The coast enjoys the constant sea breeze that keeps the surface temperature moderate.
Unfortunately, air pollution is increasing in the city day by day. Air temperature can play an important role in the build-up or dispersion of surface air pollution. As air rises and expands in the atmosphere, the temperature decreases with altitude. This happens under the normal atmospheric conditions or normal lapse rate. In general, lapse rate is the rate of decrease in the value of any meteorological element with elevation. In some cases, temperature falls up to some height and then starts increasing or becomes constant at a height of a few tens of metres to a few kilometres above the ground. The layer in which this increase or stop occurs is known as thermal inversion layer.
The observation and analysis of temperature inversion layer have never been undertaken on Karachi before. Criteria for identifying the inversion layers relates to temperature increase with height and do not include cases where the temperature remains constant in a vertical interval (isothermal layer).
The analysis of thermal inversion layer over Karachi has been carried out by using four-time daily temperature of Japanese Reanalysis data from 2000 to 2009. The Radio-sonde daily data of temperature and wind from April 2009 to December 2009 have been obtained from the Climate Data Processing Centre (CDPC) of the Pakistan Meteorological Department. The daily data of haze, visibility relative humidity, dust in suspension and mist have been taken from the Regional Meteorological Centre in Karachi for the year 2009. Radio-sonde measurements provide a vertical profile of temperature, pressure, dew point and horizontal winds and are well suited to study low-level temperature inversions. Radio-sonde data of daily temperature over Karachi have been collected from the CDPC. Routine radio-sonde profiles are made once daily.
RESULTS: Troposphere is the lowest shell of the atmosphere, in which uniform decreases in temperature with height. The troposphere is the part of the atmosphere in which all weather occurs – clouds, rain, snow, hurricanes and tornadoes. There are generally four types of inversion at different levels, that is, subsidence inversion, frontal inversion, radiation inversion and nocturnal inversion.
The surface and subsidence inversion layer have been considered only at lower troposphere. These are two basic types of low-level inversions. These two low-level types of inversion layers have been identified over Karachi. Surface inversion layer occurs when the favourable conditions are produced by the radiation cooling at the surface. It typically occurs at night when there are no incoming solar radiations to balance the surface cooling due to outgoing long wave radiation. This type of inversion is formed at the ground or at surface and is also called radiation or nocturnal inversion.
Commonly, this type of inversion is formed when the surface of earth cools faster than air. It mostly forms at night time or in the winter season. During the day, the surface layer of atmosphere receives the heat of conduction and radiation from the earth surface and they are warmed. On clear nights, the ground surface radiates heat and cools quickly. Usually, the maximum strength of surface inversion is around sunrise. Surface inversion breaks up in day time when the sun heats the ground. It plays the main role in air quality; resisting vertical motion of pollutants trapped below and cannot mix with air above inversion. Subsidence inversion occurs high above the ground, lifted away from the surface of the earth. This type of inversion is found due to sinking of air in high pressure area because high pressure promotes sinking air and slow sinking is responsible for development for large number of inversion forms in free atmosphere. Temperature inversion formed above the ground in lower part of the troposphere resists to convection of surface air due to resistance in air below the inversion and causes air pollution.
From the Pakistan
Journal of Meteorology, Vol 7, Issue 14
