Environmental Thermodynamics Affect Radiative Impact of Deep Convective Cloud Systems

Jensen, M., Brookhaven National Laboratory

Atmospheric Thermodynamics and Vertical Structures

Cloud Modeling

Jensen, M.P., A. Del Genio, Radiative and Microphysical Characteristics of Deep Convective System in the Tropical Western Pacific, Journal of Applied Meteorology, Vol. 42, No. 9, pp. 1234-1254.

Deep convective systems (often referred to as thunder clouds over land) are one of the most complex elements of the climate system. The extreme environmental conditions contained in a convective system play a significant role in the transfer of heat and energy between the Earth surface and the atmosphere. This is particularly true in the tropics, where consistently warm ocean temperatures and high surface air humidity result in many deep convective systems. These systems heat the atmosphere by the release of latent heat when water vapor condenses and through absorption of solar and infrared radiation.

There are differing views among scientists on the relative importance of these two heating terms in determining the evolution and lifetime of tropical convection. To evaluate the relative contribution of radiative versus latent heating, climate researchers constructed representations of the structure of 17 tropical storm systems. They used combined measurements from groundbased instruments at the Department of Energy's Atmospheric Radiation Measurement (ARM) Program sites at Manus, Papua New Guinea, and Naura Island, and from satellite observations obtained from the Tropical Rainfall Measuring Mission (TRMM). TRMM is jointly conducted by the National Aeronautics and Space Administration and the National Space Development Agency of Japan. Both deep convective (cloud top heights over 10 km) and midlevel convective cases (cloud top heights of about 4 to 7 km) were studied.

Radar reflectivity data from the TRMM precipitation radar were used to identify daytime storm systems (cloud conditions near local noon have the greatest impact on the solar energy budget) at the ARM Manus and Nauru sites, and to infer their ice/liquid water content. These values of cloud water were used to compute the radiation budget of the atmosphere and surface. The modeled radiation fields were then compared with observations at the surface using ARM data and at the top of the atmosphere using satellite data. For cases of mature deep convection near local solar noon, the maximum value of radiative heating was approximately 10 to 30 percent of the maximum value of the latent heating. However, the maximum value in this radiative heating profile was generally located higher in the atmosphere, near cloud top; a secondary heating maximum was located near cloud base. Deep convective cloud systems contain geometrically extensive layers with small ice crystals and large optical thickness near cloud top (12-14 km). These layers result in shortwave heating and longwave cooling at cloud tops. Midlevel convective clouds do not develop this deep layer of small ice crystals and, therefore maximum radiative heating and cooling in these systems tend to occur around the freezing level (typically 5 to 6 km), which is about the same location as maximum latent heating.

These distinct differences between the radiative heating of deep convection and midlevel convection led the researchers to hypothesize a link between the thermodynamics of the environment (i.e., the local vertical structure of temperature and moisture) and the vertical extent and structure of convective systems. If these preliminary relationships are confirmed by further research, they may provide a useful constraint on the upper limit of tropical cloud forcing (the effect of clouds on the planetary radiation budget) used in climate models.