On The Microphysical Properties Of Tropical Anvil Cirrus And Its Response To Changes In Environmental Conditions

Based on the dataset obtained during the recent international field campaigns ACTIVE and TWPICE(from November 2005 to February 2006),and combined with the satellite and radar data,the microphysical characteristics of anvil cirrus clouds in wet season in the Darwin area,Australia is analyzed.The evolution process and the sensitivity of cloud microphysical properties of anvil cirrus to changes in aerosol concentration is explored using the 3D dynamic cloud model of severe storm developed by the Institute of Atmospheric Physics,Chinese Academy of Science.The following conclusions are reached:1.The modeling shows that the evolution process of anvil cirrus can be divided into a fast phase and a slow phase.A case study shows that at the earlier stage of the slow phase,the horizontal differences in the cloud microphysical properties are more significant than that at other times,that is,the number concentration and ice water content(IWC)of ice particles are higher in the middle region than that close to the lateral boundary regions in anvil cirri.The size spectra are also broader,and the number concentration of aggregates account for a higher portion in the total amount in this region.At the decay stage,all the above parameters approach uniform in horizontal direction.Therefore,the heterogeneity of microphysical properties in anvil cirri should be considered in simulation of the climate of low latitudes.2.A statistical analysis of 15 cases(only the slow phase are considered with the height ranging from 9.7 to 14.9 km and temperature ranging from -70.4℃to -30.0℃) shows that the typical value of IWC is 10^-2g m^-3,and the number concentrations of ice particles measured by CIP(＞100μm),CPI(＞5μm),and CAS(0.54-50μm), are 10¹,10²,and 10³ L^-1,respectively.The particle size spectra in most cases are double mode with the peak diameters close to 10μm and 300μm,respectively.3.The changes of probability distributions of IWC in anvil cirri with temperature(or altitude)are compared among different experiments(ACTIVE/TWPICE and CEPEX),and among different periods in the Darwin area,Australia(e.g.,the establish and prevail period of Australian monsoon).The results show that the vertical distribution of IWC is to larger extent affected by the intensity of convection,that is,the stronger is the convection,and the higher the anvil cirrus can reach,the larger of the IWC in the upper part of anvil.4.For the case simulated here,heterogeneous nucleation is the main process responsible for formation of ice particles in tropical deep convection,and followed by homogeneous nucleation.It also shows that when the concentration of cloud condensation nuclei(CCN)increase,the maximum vertical velocity of the convection decrease,leading to a lower transport of hydrometeors to the upper part of the cloud and to a lower number concentration of ice particles in the anvil cirri.