LIRAD Observations of Tropical Cirrus Clouds in MCTEX. Part II: Optical Properties and Base Cooling in Dissipating Storm Anvil Clouds

Abstract

During the Maritime Continent Thunderstorm Experiment (MCTEX), several decaying storm anvils were observed. The anvil clouds exhibited typical patterns of fallout and decay over a number of hours of observation. The anvil bases were initially very attenuating to lidar pulses, and continued that way until anvil breakup commenced. During that time, the anvil base reached some characteristic altitude (?7 km) below which the cloud particles had evaporated fully. Some typical ?tongues? of fallout below such levels also occurred. Millimeter radar showed the storm anvil cloud tops to be much higher than detected by lidar until the anvil was well dissipated. The infrared properties of the anvils were calculated. In three of the four anvils studied, the calculated emittance never exceeded 0.8?0.85. In the remaining case, the cloud emittance approached unity only in the period before the anvil had descended appreciably. Radiative transfer calculations showed that the infrared emission originated mostly from the layer between cloud base and the height at which complete attenuation of the lidar pulse occurred. However, the correct blackbody emission at cloud base could only be obtained by assuming the existence of an additional layer, situated above the first, 1.8 km deep and with a specific backscatter coefficient. The depressed values of emittance were interpreted as a cooling (below those temperatures measured by radiosonde) for some distance above anvil cloud base due to evaporation of the cloud. Typically, this cooling amounted to about 10°C, depending on the layer thickness above cloud base at which cooling was occurring. A reexamination of data taken in 1981 at Darwin, Northern Territory, Australia, indicated a similar depression in emittance in all cases of attenuating storm anvils. A simple model of ice-mass evaporation saturating the ambient air was used to approximate the observed cooling in one anvil. Millimeter radar reflectivity measurements, which also yielded ice water content at cloud base, were also used to find equivalent cooling rates. By varying the mean volume diameter in the calculation, cooling rates similar to those found from the radiometric method could be obtained. The values of mean volume diameter agreed, within uncertainties, with those obtained by the lidar?radar method. Estimated cooling to over 1 km above cloud base confirms earlier work on anvil mammata. Values of backscatter-to-extinction ratio at the base of the anvils showed some consistent variations, indicating a change of ice-crystal habit, or size, with time.