HR: 1340h
AN: A53D-1436    [Abstracts]
TI: Characteristics of Non-precipitating Clouds as Observed by the Tropical Rainfall Measuring Mission Satellite
AU: * Miller, M A
EM: mamille4@ncsu.edu
AF: North Carolina State University, Dept. Marine, Earth, Atmos. Sci. Campus Box 8208, Raleigh, NC 27695, United States
AU: Yuter, S E
EM: seyuter@ncsu.edu
AF: North Carolina State University, Dept. Marine, Earth, Atmos. Sci. Campus Box 8208, Raleigh, NC 27695, United States
AB: Satellites are the only platform currently available to practically measure precipitation on a global scale. It is important to identify errors and uncertainties in satellite estimates of global precipitation so that these can be taken into account in merged products and other applications of these data. The Tropical Rainfall Measuring Mission (TRMM) satellite carries two instruments to measure precipitation: the TRMM Microwave Imager (TMI) and the Precipitation Radar (PR). Berg et al. (2006, J. Climate) examined differences in rainfall detection between the TMI and PR over the East China Sea where the TMI detected rain and the PR did not. Berg et al. theorized that aerosol loading suppresses precipitation in these clouds. They suggest that for a relatively high liquid water path (LWP) cloud (1 kg m^-2) with a plausible cloud droplet concentration for a polluted cloud (based on field observations from the Asian Particle Environmental Change Studies), the reflectivity could be reduced to approximately 18 dBZ – just below the PR minimum detection threshold. Although not observable by PR, clouds with a reflectivity near 18 dBZ would be observable by an S-band WSR-88D radar which has an operational minimum detectable threshold of 5 dBZ in precipitation mode. We tested their hypothesis by examining coastal radar data, visible and IR satellite data, and upper air sounding data. The Taiwan coastal S-band WSR-88D radar did not detect any echoes > 10 dBZ in the East China Sea features identified as raining in TMI but not PR. Infrared satellite and sounding observations indicate that these areas of spurious TMI precipitation occur where there are liquid-phase clouds with tops just below the freezing level and a low cloud base very near the surface, i.e. the cloud layer is unusually thick. These thick warm-phase clouds have a high enough LWP to be detected by the TMI. Since the reflectivity is below the detection threshold of the coastal radar, we infer that these are non-precipitating clouds. Including these non-precipitating clouds as precipitating may contribute to a positive bias in TRMM TMI rainfall estimates, particularly in regions where average rain rates are low. Within the global TRMM domain, clouds with radiative characteristics similar to the spurious TMI rainfall events in the East China Sea occur ~1% of the time (regionally as high as 10%) in a wide variety of settings including coastal regions and open ocean regions far removed from land including the southern oceans. These types of clouds do not occur in marine stratocumulus regions off the west coasts of South America, North America, and Africa. The variety of locations and lack of correlation with aerosol optical depth globally indicates that aerosols may be a contributing factor but are not the sole cause of the occurrence of low-cloud-top, high LWP clouds.
DE: 3311 Clouds and aerosols
DE: 3354 Precipitation (1854)
DE: 3360 Remote sensing
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting