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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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A. R. Webb, A. F. Bais, M. Blumthaler, G-P. Gobbi, A. Kylling, R. Schmitt, S. Thiel, F. Barnaba, T. Danielsen, W. Junkermann, A. Kazantzidis, P. Kelly, R. Kift, G. L. Liberti, M. Misslbeck, B. Schallhart, J. Schreder, and C. Topaloglou

Abstract

Results are presented from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) campaign to measure spectral global UV irradiance and actinic flux at the ground, beneath an atmosphere well defined by supporting measurements. Actinic flux is required to calculate photolysis rates for atmospheric chemistry, yet most spectral UV measurements are of irradiance. This work represents the first part of a project to provide algorithms for converting irradiances to actinic fluxes with specified uncertainties. The campaign took place in northern Greece in August 2000 and provided an intercomparison of UV spectroradiometers measuring different radiation parameters, as well as a comprehensive radiation and atmospheric dataset. The independently calibrated spectroradiometers measuring irradiance and actinic flux agreed to within 5%, while measurements of spectral direct irradiance differed by 9%. Relative agreement for all parameters proved to be very stable during the campaign. A polarization problem in the Brewer spectrophotometer was identified as a problem in making radiance distribution measurements with this instrument. At UV wavelengths actinic fluxes F were always greater than the corresponding irradiance E by a factor between 1.4 and 2.6. The value of the ratio F : E depended on wavelength, solar zenith angle, and the optical properties of the atmosphere. Both the wavelength and solar zenith angle dependency of the ratio decreased when the scattering in the atmosphere increased and the direct beam proportion of global irradiance decreased, as expected. Two contrasting days, one clear and one with higher aerosol and some cloud, are compared to illustrate behavior of the F : E ratio.

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