Tuesday, 4 June 2002: 11:29 AM
A study of ice-phase microphysics in TRMM-LBA deep convective clouds
Although deep convection can be seen in a state of quasi-equilibrium with a large-scale, destabilizing forcing, multiple scales are important to shape the characteristics of convection. This includes forcings on the mesoscale, cloud-scale circulations, turbulence and, on the extreme, microphysical processes. The effect of cumulus convection on the redistribution of the water vapor in the atmosphere strongly depends on the detrained water substance and, therefore, is very sensitive to the cloud microstructure. In fact, it is at the scale of microphysical processes that the latent heat associated with convection is actually exchanged. Microphysical variables such as hydrometeor size, fall velocity and shape (in the case of ice particles) can influence evaporation and sublimation and hence the water vapor transport and the heating profile. Those uncertainties can place severe limitations to our capabilities of modeling tropical convection. Surprisingly, however, there are only few and insufficient microphysical observations in the tropics. To what extent the microphysics of Amazon convection is influenced by its dynamics and can in turn influence the larger scales is still unknown. In this context, the Tropical Rainfall Measuring Mission Large Scale Biosphere-Atmosphere Experiment in Amazonia (TRMM-LBA) provided the first opportunity to extensively investigate the microphysical structure of Amazon convection. In this work, data obtained by instrumentation aboard the Citation II aircraft from University of North Dakota, during TRMM-LBA were analyzed. The aircraft instrumentation included a set of optical/spectrometer probes to measure concentration and size of hydrometeors (FSSP, 2DC, 1DP and HVPS), with distinction to the Two-Dimensional Cloud Probe (2DC), which samples clouds and describes their microphysics characteristics, such as particle concentration, size and also ice crystal habit. Two flights from the TRMM/LBA campaign were chosen to be analyzed in this work: February 10 and 17. In both flights, the instrumented aircraft penetrated deep convective systems in multiple heights above the freezing level. Different ice crystal habits were observed, including columns, rosettes and dendrites, as well as crystals with superimposed habits (for instance, CP1a- and CP2a-type). In addition, aggregates and graupel particles were found, indicating that those convective systems have a highly complex microstructure. However, despite the multiplicity of ice crystal shapes, it was observed that their size distribution as a function of diameter follows approximately a power law with different slopes for stratiform and convective regions.