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Robert F. Adler and Ida M. Hakkarinen

Abstract

Aircraft passive microwave observations at 18, 37, 92, and 183 GHz of light oceanic precipitation are studied in conjunction with visible and infrared observations and ground-based radar data. Microwave signatures for clear, cloudy, and precipitating conditions are defined, with results in general agreement with previous theoretical results. Emission signatures are evident at 18, 37, and 92 GHz with clouds and precipitation producing an increase in brightness temperature (T b) over that observed over the low-emissivity ocean background. Polarization differences at 18 and 37 GHz also decrease in precipitation areas to minima of 30 K at 18 GHZ and 15 K at 37 GHz. The 92-GHz T b shows a double-valued relationship, with an increase in cloudy and very lightly raining areas and a subsequent decrease for higher rain rates and deeper clouds where the ice scattering process becomes important. The 183-GHz observations display a distinct sensitivity to small amounts of ice.

Simple channel differences are shown to compare favorably to the rain field, including polarization differences at 18 and 37 GHz and frequency differences between 92 and 37 GHz and between 183 and 92 GHz.

The cases examined include both stratiform and convective structures, and the results indicate that this difference may be important in microwave T b-rain rate relationships. Some of the observed differences may be due to the presence or absence of a melting layer, observed as a radar “bright band.”

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Christian Kummerow, Robert A. Mack, and Ida M. Hakkarinen

Abstract

A multichannel statistical approach is used to retrieve rainfall rates from brightness temperatures (TB's) observed by passive microwave radiometers flown on a high-altitude NASA aircraft. Brightness temperature statistics are based upon data generated by a cloud radiative model. This model simulates variabilities in the underlying geophysical parameters of interest, and computes their associated TB's in each of the available channels. By further imposing the requirement that the observed TB's agree with the TB values corresponding to the retrieved parameters through the cloud radiative transfer model, the results can be made to agree quite well with coincident radar-derived rainfall rates. Some information regarding the cloud vertical structure is also obtained by such an added requirement.

The applicability of this technique to satellite retrievals is also investigated. Data which might be observed by satellite-borne radiometers, including the effects of nonuniformly filled footprints, are simulated by the cloud radiative model for this purpose. Results from statistics generated using different hydrometeor vertical profiles in the cloud radiative model are examined. It is found that errors in the retrieved rainfall rates, and retrieval biases, decrease with increasing agreement between simulated TB's and those corresponding to the retrieved geophysical parameters.

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Robert F. Adler, Robert A. Mack, N. Prasad, Ida M. Hakkarinen, and H-Y. M. Yeh

Abstract

Aircraft passive microwave observations of deep atmospheric convection at frequencies between 18 and 183 GHz are presented in conjunction with visible and infrared satellite and aircraft observations and ground-based radar observations. Deep convective cores are indicated in the microwave data by negative brightness temperature (TB) deviations from the land background (270 K) to extreme TB values below 100 K at 37, 92, and 183 GHz and below 200 K at 18 GHz. These TB minima, due to scattering by ice held aloft by the intense updrafts, are well correlated with areas of high radar reflectivity. For this land background case, TB is inversely correlated with rain rate at all frequencies due to TB-ice-rain correlations. Mean ΔT between vertically polarized and horizontally polarized radiance in precipitation areas is approximately 6 K at both 18 GHz and 37 GHz, indicating nonspherical precipitation size ice particles with a preferred horizontal orientation. Convective cores not observed in the visible and infrared data are clearly defined in the microwave observations and borders of convective rain areas are well defined using the high-frequency (90 GHz and greater) microwave observations.

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Christian Kummerow, Ida M. Hakkarinen, Harold F. Pierce, and James A. Weinman

Abstract

This study presents the first quantitative retrievals of vertical profiles of precipitation derived from multispectral passive microwave radiometry. Measurements of microwave brightness temperature (Tb) obtained by a NASA high-altitude research aircraft are related to profiles of rainfall rate through a multichannel piecewise-linear statistical regression procedure. Statistics for Tb are obtained from a set of cloud radiative models representing a wide variety of convective, stratiform, and anvil structures. The retrieval scheme itself determines which cloud model best fits the observed meteorological conditions. Retrieved rainfall rate profiles are converted to equivalent radar reflectivity for comparison with observed reflectivities from a ground-based research radar. Results for two cases studies a stratiform rain situation and an intense convective thunderstorm, show that the radiometrically derived profiles capture the major features of the observed vertical structure of hydrometeor density.

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Robert F. Adler, Andrew J. Negri, Peter R. Keehn, and Ida M. Hakkarinen

Abstract

This paper describes a method to combine geosynchronous IR and low-orbit microwave data to estimate mean monthly rainfall useful for climate studies. The IR data have the advantage of high time resolution (important for rapidly changing precipitation patterns and for the detection of diurnal signals) but lack a strong physical connection between the remotely sensed signal and the surface rainfall. The microwave data provide a stronger relation between the radiance and the rainfall but provide poor time sampling of the rainfall signal.

The microwave technique uses the brightness temperature at 37 and 86 GHz from the Special Sensor Microwave/Imager instrument on board the Defense Meteorological Satellite Program (DMSP) satellite to define raining areas over water and land and uses the 86-GHz scattering signal to assign rain rate based on cloud model-microwave calculations. The microwave results are generally good for both individual swaths and monthly totals, except for a glaring underestimation of shallow, orographic rain systems over the southern coast of Japan. The IR techniques used are the GOES precipitation index of Arkin and Meisner and the convective-stratiform technique of Adler and Negri.

Initially the IR estimates are computed separately using hourly data from the Japanese Geostationary Meteorological Satellite. Calibration or adjustment factors are derived by dividing the microwave monthly estimate by a second IR estimate (made with the microwave sampling that simulates the observations from an IR radiometer on board the DMSP satellite). The spatial array of coefficients are then multiplied by the original IR monthly estimates (produced from all the hourly data) to produce the merged IR-Microwave monthly estimates. The results show that in areas where the base (microwave) technique performs well, that is, has a relatively small bias, the combined microwave-IR monthly total estimates have better error statistics than either the microwave or IR techniques individually.

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