The goal of this proposed work is to provide a physically-based interpretation of observed interannual variations in hydrometeor retrievals from TRMM (and other supporting platforms) that will enable a more accurate determination of the tropical water balance and contribute to improved precipitation algorithms in advance of the Global Precipitation Measurement mission, GPM.
The work proposed here has three climate-related objectives. They are to:
- Quantify regional to near-global variations of liquid and frozen hydrometeors at interannual to intradecadal time scales in terms of composition by various precipitation "regimes". (By regimes we mean bulk attributes of convection that are manifest as vertical structure, liquid / frozen composition, characteristic particle size etc.)
- Relate these variations in hydrometeor attributes to environmental factors that support precipitating clouds and define precipitation regimes (e.g. thermodynamic stability properties, vertical shear, vertical motion).
- Determine how interannual to intradecadal variability in precipitation / hydrometeor regime distributions are coupled to cloud and water vapor changes, particularly in the tropical upper troposphere.
These objectives will be accomplished through a number of interrelating tasks. The August 2001 TRMM orbit was boosted ~50 km which, from a climate perspective, has introduced a non-negligible reduction in sensitivity to rainfall. Our current work suggests that a drop of ~3% in PR precipitation has occurred at the surface and that this increases with height. To account for this effect in our subsequent analyses we will further quantify this artifact using both statistical and independent passive microwave measurements.
Using TRMM Level-2 reflectivities we will develop retrievals of stratiform and convective ice water content (IWC) and its vertical integral, ice water path (IWP) using a bulk reflectivity-mass retrieval (e.g. Rutledge and Petersen, 2001). IWP estimates from TRMM, NOAA AMSU-B and earlier SSM/T-2 platforms we will be evaluated and intercalibrated as record of IWP from 1993 to the present. Validation for these products will be taken from coincident multi-frequency and dual-polarimetric radar observations of ice water content in multiple precipitation regimes (cold and warm season). Building on efforts by Boccippio et al., (2003) we will employ a clustering approach to objectively classify precipitation systems according to their vertical reflectivity structure, hydrometeor phase, and convective / stratiform composition as seen by the TRMM PR. We will then search for preferred relationships between these canonical precipitation structures and vertical velocity, static stability, and vertical shear, etc. using reanalysis data. With the resulting data and analyses, relationships between ENSO-related variations in precipitating ice, tropical cirrus, and upper tropospheric humidity (UTH) will be studied. Joint frequency distributions of IWP as seen by both TRMM / passive microwave and the CERES sensors will be developed and related to UTH available from the SSM/T-2 and AMSU-B 183.3 GHz channel complex.
The planned research continues efforts to clarify reflectivity vs. attenuation based estimates of precipitation / hydrometeor signals. Isolating TRMM orbit boost effects will improve the S/N and reliability of interannual signals needed for climate research. Development of a precipitating ice climatology will be of direct use in advancing climate diagnostics studies and in validating prognostic condensate and convective parameterizations in global models.
