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Thomas L. Bell and R. Suhasini

Abstract

Radar or satellite observations of an area generate sequences of rain-rate maps. From a gridded map a histogram of rain rates can be obtained representing the relative areas occupied by rain rates of various strengths. The histograms vary with time as precipitating systems in the area evolve and decay and amounts of convective and stratiform rain in the area change. A method of decomposing the histograms into linear combinations of a few empirical distributions with time-dependent coefficients is developed, using principal component analysis as a starting point. When applied to a tropical Atlantic dataset (GATE), two distributions emerge naturally from the analysis, resembling stratiform and convective rain-rate distributions in that they peak at low and high rain rates, respectively. The two “modes” have different timescales and only the high-rain-rate mode has a statistically significant diurnal cycle. The ability of just two modes to describe rain variability over an area can explain why methods of estimating area-averaged rain rate from the area covered by rain rates above a certain threshold are so successful.

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C. Prabhakara, G. Dalu, G. L. Liberti, J. J. Nucciarone, and R. Suhasini

Abstract

Passive microwave measurements made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) reveal information about rain and precipitation-sized ice in the field of view (FOV) of the instruments. The brightness temperature T b measured at 37 GHZ, having an FOV of about 30 km, shows relatively strong emission from rain and only marginal effects caused by scattering by ice above the rain clouds. At frequencies below 37 GHz, where the FOV is larger and the volume extinction coefficient is weaker, it is found that the observations made by these radiometers do not yield appreciable additional information about rain. At 85 GHz (FOV ≈ 15 km), where the volume extinction coefficient is considerably larger, direct information about rain below the clouds is generally masked.

Based on the above considerations, 37-GHz observations with a 30-kin FOV from SMMR and SSM/I are selected for the purpose of rain-rate retrieval over oceans. An empirical method is developed to estimate the rain rate in which it is assumed that over an oceanic area the statistics of the observed T b's at 37 GHz in a rain storm are related to the rain-rate statistics in that storm. Also, in this method, the underestimation of rain rate, arising from the inability of the radiometer to respond sensitively to rain rate above a given threshold, is rectified with the aid of two parameters that depend on the total water vapor content in the atmosphere. The rain rates retrieved by this method compare favorably with radar observation. Monthly mean global maps of rain derived from this technique over the oceans are consistent with climatology.

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