Dry and moist atmospheric thermodynamics of clouds from GNSS radio occultation profiles

Abstract:

Radio occultations (RO) of the Global Navigation Satellite System (GNSS) signals have been used to characterize vertical profiles of radio refractivity that are related to atmospheric pressure, temperature, and water vapor profiles. Because GNSS signals are able to traverse clouds, they would seem the only satellite remote sensing technique capable of providing the thermodynamics of the troposphere within clouds.

However, to convert refractivity into temperatures refractivity is either assimilated into weather analyses such as ECMWF and NCEP, or converted into pressure and temperature at the top of the atmosphere by using an initial pressure and temperature from a model and, within the lower moist troposphere, the temperature profile is used from a model analysis to infer water vapor. As a consequence, current analyses of atmospheric thermodynamics using RO depend on the accuracy of the models used to analyze the refractivity profiles, most strongly so inside moist clouds where the temperature is that of the model.

Given the relevance of clouds in the thermodynamics parameterizations that can improve climate models, one might learn something by maximizing the independence of RO from the models we need to validate. This work compares the features of refractivity profiles within clear, overcast non-precipitating, and overcast precipitating clouds with precipitation information from TRMM, GPM, as a function of height and with cloud top properties from MODIS. When RO refractivity profiles are compared to those of dry adiabats, the static stability of the upper troposphere is visible. Comparison of precipitating and non-precipitating clouds to saturated and non-saturated adiabats show more features associated to precipitation thus providing clues about the thermodynamics changes within precipitating clouds.