The need to understand the complementarity of the radar and radiometer is important not only to the Tropical Rain Measuring Mission (TRMM) program but to a growing number of multi-instrumented airborne experiment that combine single or dual-frequency radars with multichannel radiometers. The method of analysis used in this study begins with the derivation of dual-wavelength radar equations for the estimation of a two-parameter drop size distribution (DSD). Defining a "storm model" as the set of parameters that characterize snow density, cloud water, water vapor, and features of the melting layer, then to each storm model there will usually correspond a set of range-profiled drop size distributions that are approximate solutions of the radar equations. To test these solutions, a radiative transfer model is used to compute the brightness temperatures for the radiometric frequencies of interest. A storm model or class of storm models is considered optimum if it provides the best reproduction of the radar and radiometer measurements. Tests of the method are made for stratiform rain using simulated storm models as well as measured airborne data. Preliminary results show that the best correspondence between the measured and estimated radar profiles usually can be obtained by using a moderate snow density (0.1-0.2 g/cm/sup -3/), the Maxwell-Garnett mixing formula for partially melted hydrometeors (water matrix with snow inclusions), and low to moderate values of the integrated cloud liquid water (less than 1 kg/m/sup -/2). The storm-model parameters that yield the best reproductions of the measured radar reflectivity factors also provide brightness temperatures at 10 GHz that agree well with the measurements. On the other hand, the correspondence between the measured and modeled values usually worsens in going to the higher frequency channels at 19 and 34 GHz. In searching for possible reasons for the discrepancies, it is found that changes in the DSD parameter /spl mu/, the radar constants, or the path-integrated attenuation can affect the high frequency channels significantly. In particular, parameters that cause only modest increases in the median mass diameter of the snow, and which have a minor effect on the radar returns or the low frequency brightness temperature, can produce a strong cooling of the 31 GHz brightness temperature.

Microphysical retrievals over stratiform rain using measurements from an airborne dual-wavelength radar-radiometer

Range profiling of the rain rate by an airborne weather radar

Precipitation-rate profiles derived from a single-frequency radar and a radiometer are compared with profiles derived from a dual-frequency radar, using data obtained during the 1985-1986 Communication Research Laboratory/NASA rain measuring experiment over the Atlantic Ocean from a dual-frequency X- and Ka-band radars and an X-band passive radiometer. Profiles were retrieved using a modification of the Hitschfeld-Bordan (1954) algorithm constrained by a measurement of the path integrated extinction derived from passive radiometry. It was found that the two techniques under comparison yielded average rainfall rates that agreed within about 25 percent. It is suggested that, while a single-frequency radar operating in conjunction with multifrequency passive radiometers might provide cost-effective spaceborne measurements of tropical rainfall distributions, a dual-frequency radar might be able to remove some ambiguities that were encountered in this study.

Retrieval of Precipitation Profiles from Airborne Radar and Passive Radiometer Measurements: Comparison with Dual-Frequency Radar Measurements

Airborne microwave measurements of precipitation associated with Super Typhoon Flo in the western North Pacific were conducted during September 16-18, 1990 The sensor package aboard the NASA DC-8 aircraft included a dual-frequency precipitation radar at 10 GHz and 34 GHz and a host of radiometers operating at 10 GHz, 18 GHz, 19 GHz, 34 GHz, and 92 GHz, as well as three frequencies near the strong water vapor absorption line of 1833 GHz The measurements were made during a few passes over the storm center, and active and passive microwave signatures of the rainbands were detected with a fine spatial resolution The relationship between the measured brightness temperature and radar-estimated rain rate is examined at the frequencies between 10-92 GHz At both 34 and 92 GHz this relationship is analyzed with the 10 GHz radar reflectivity factor measured at altitudes above the freezing layer as a further constraint The results show that frozen hydrometeors strongly scatter radiation at these frequencies, especially at 92 GHz It was shown from a close examination of both active and passive microwave signatures that a significant scattering of radiation at frequencies 118 GHz occurred in the inner eyewall at altitudes of 3-8 km This scattering of microwave radiation by hydrometeors in both liquid and frozen forms is discussed under the authors' current understanding of the scattering mechanism >

Airborne active and passive microwave observations of Super Typhoon Flo

Among the methods that have been proposed for estimating precipitation parameters from a down-looking radar are those that use the scattered power from the surface to infer path attenuation. In this paper, an analysis of a dual-wavelength surface reference technique (DSRT) is presented. The principal of the method is that the ratio of the backscattered power from the surface at one wavelength to that at a second wavelength is proportional to the differential attenuation of the signals through the intervening rain. To account for differences in the backscattering cross section of the surface a 0°, and in the radar calibration constant at the two wavelengths, measurements are made of the surface return power during clear air conditions. The characteristics of the method are best understood by comparing it to two closely related methods: the single-wavelength surface reference technique (SRT) and the standard dual-wavelength technique (DWT). A comparison between the DWT and the DSRT shows that the rain rate estimates are identical in form and that the surface reference can be obtained from the standard estimate by replacing the statistics of the rain reflectivity with those of ?°Advantages of the DSRT relative to its single wavelength counterpart occur when the wavelength correlation in ?° is high or when the mean values of ?° at the two wavelengths are nearly equal. To treat the methods under less restrictive assumptions a simulation is used. Error statistics of the rain rate estimates are presented for a space-borne radar operating at an attenuating wavelength.

Analysis of a Dual-Wavelength Surface Reference Radar Technique

This paper describes the retrieval procedure of liquid water content (LWC) in cloud or rainfall distribution by a combination of microwave radiometry and radar and its numerical simulation. The performance of a combined dual-wavelength (35.0 & 9.375 GHz) radar-radiometer system for the remote sensing of cloud LWC and rainfall is given. During the two summers of 1980 and 1981, the preliminary observations of different clouds and storms by the combined system and combined radar-radiometric cloud and rain parameter retrievals are carried out. Research results show that by the use of the combined radarradiometric method, improvement are obtained in accuracy of rms LWC and rainfall over the pure radiometric technique or the radar alone.

Ground-based remote sensing of LWC in cloud and rainfall by a combined dual-wavelength radar-radiometer system

The microwave radiation transfer in a cloudy atmosphere at wavelength 1.35cm and 0.85cm was calculated by using conventional radiosonde data at Guangzhou, China (23°N, 113.3°E) and the Yap Island, a trust territory of the U. S. A. (9.6°N, 138°E). Regressive relationships were established among microwave brightness temperature (Tb λ ), microwave attenuation (τ λ ), atmospheric precipitable water vapor (Q), and liquid water content of clouds (L). A physical-statistical method is modified to retrieve the Q and L from Tb λ and τ λ based on these relationships.

Lin Hai

Papers

Ground-based remote sensing of LWC in cloud and rainfall by a combined dual-wavelength radar-radiometer system

Remote Sensing of Precipitable Water Vapor and Cloud Liquid Water Content Over the Ocean by Ground-Based Dual-Wavelength Radiometer