Retrieval of cloud base heights from passive microwave and cloudtop temperature data

Water vapor profiling algorithms that treat liquid clouds explicitly yield a cloud base height as a byproduct. A single case of a water vapor profile retrieval using a combination of the SSM/T-2 on the DMSP satellite and cloud parameters from the AVHRR on the NOAA satellite retrieved a reasonable cloud base. While hardly definitive, this case is suggestive. The authors examine the cloud base signal in a combination of the SSM/1 and SSM/T-2 on the DMSP satellite from a theoretical point of view. It is shown that the signal is strong enough for a useful retrieval only over the ocean. For low altitudes, a cloud top temperature (CTT) constraint, as could be provided from an infrared radiometer, is required. While difficult with the DMSP-NOAA satellite combination, this has become much easier with the recent launch of NOAA-K with the AMSU-B and AVHRR. It is shown that the signal is acceptable over the relevant range of cloud liquid water content values. To achieve useful results, some local tuning of the algorithm will be necessary. This tuning could take the form of water vapor profile covariance matrices, climatological estimates of the cloud liquid water density, or purely empirical methods. Broken and multilayer clouds provide additional complications to the problem     

A Time-Varying Radiometric Bias Correction for the TRMM Microwave Imager

Recent intersatellite radiometric comparisons of the Tropical Rainfall Measurement Mission Microwave Imager (TMI) with polar orbiting satellite radiometer data and modeled clear-sky radiances have uncovered a time-variable radiometric bias in the TMI brightness temperatures. The bias is consistent with a source that generally cools during orbit night and warms during sunlight exposure. The likely primary source has been identified as a slightly emissive parabolic antenna reflector. This paper presents an empirical brightness temperature correction to TMI based on the position around each orbit and the Sun elevation above the orbit plane. The results of radiometric intercomparisons with WindSat and special sensor microwave imager are presented, which demonstrate the effectiveness of the recommended correction approach based on four years of data.     

NASDARainfall algorithms for AMSR-E

Three related algorithms have been developed to retrieve rainfall from the Advanced Microwave Scanning Radiometer-Earth Observing System observations. First, land and ocean backgrounds must be treated separately. The highly reflective ocean background is ideal for observing atmospheric constituents including rainfall. The high and variable emissivity of the land background limits the usefulness of the signal from the liquid hydrometeors under most conditions. For an algorithm with global applicability, it is more reasonable to use the signal generated by scattering of microwaves from frozen hydrometeors. Unfortunately, this scattering is related to rainfall only indirectly. This dichotomy is reinforced by the differing availability of ground truth in the two environments. Thus, two distinct approaches must be used in generating the algorithms for the two cases. The oceanic algorithm is based on physical models, and the land algorithm is generated empirically. Functionally, the two algorithms are merged into a common structure for retrievals on a pixel-by-pixel (Level-2) basis, but the distinct philosophies remain. The third element is the monthly total product (Level-3) which could be generated by simply summing the Level-2 products. However, subtle biases, unimportant on a Level-2 basis, could introduce serious errors into the totals. Therefore, we have chosen to generate a separate product over the oceans consisting of monthly averaged rain rates over 5/spl deg/ /spl times/5/spl deg/ boxes. The Level-3 ocean algorithm assumes that the rainfall intensity is distributed log-normally and also absorbs small instrumental calibration errors and some modeling errors.     

Observations of Oceanic Surface-Wind Fields from the Nimbus-7 Microwave Radiometer

Brightness temperatures from the fiIve-frequency (6.6, 10.7, 18, 21, and 37 GHz) dual-polarized scanning multichannel microwave radiometer (SMMR) on Nimbus 7 have been used to obtain surface wind fields over the ocean. The sateilite-derived wind field for 1200Z, February 19, 1979, in the eastern North Pacific has been compared with an operationally generated surface-wind analysis field. Previous point comparisons at selected locations have indicated that satellite winds are accurate to 3 ms-1. The results here, although of a preliminary nature, indicate that SMMR-derived winds may be used to determine large-scale wind fields over the ocean, particularly in areas of strong wind gradients such as found in cyclonic systems.     

Water vapour profile retrievals from SSM/T-2 data constrained by infrared-based cloud parameters

An algorithm has been developed for the retrieval of water vapour profiles from passive microwave observations near 183GHz. The algorithm treats clouds explicitly and can incorporate cloud information from other sources. As a test, the algorithm has been configured to use data from the SSM /T-2 on the DMSP polar orbiting satellite series. Cloud top temperature and phase information have been derived from the AVHRR on the NOAA polar orbiting satellite series. Obtaining suitable coincidences between the SSM /T-2 and the AVHRR is challenging but two reasonable cases have been found. In one case, a liquid cloud injected into the retrieval results in significant improvement in the accuracy and vertical resolution in the neighbourhood of the cloud top. A cloud base is a by-product of the algorithm, but its validity has yet to be established. In the other case, a cirrus cloud injected into the algorithm resulted in little change in the retrieval which indicates that the algorithm was performing well in the cloud top vicinity (400mb).     

Airborne active and passive microwave observations of Super TyphoonFlo

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 183.3 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     

Microwave radiometric determination of wind speed at the surface of the ocean during BESEX

Microwave radiometric measurements were made at wavelengths ranging from 0.8 to 2.8 cm at altitudes from 0.16 to 11 km under well documented meteorological conditions over the Bering Sea. It is shown that determinations of wind speed at the ocean surface and liquid water content of the clouds may be made from such data. Determinations were made from two simultaneous but independent sets of radiometric measurements. The wind speeds and liquid water contents made from these two sets showed remarkable agreement. Independent estimates of these parameters made from in situ measurements showed reasonable agreement as well.     

Retrieval of Ocean Surface Parameters from the Scanning Multifrequency Microwave Radiometer(SMMR) on the Nimbus-7 Satellite

Sea-surface temperature retrievals have been tested on 2 months of Nimbus-7 scanning multichannel microwave radiometer data. Using the prelaunch versions of the instrument calibration and geophysical parameter retrieval algorithms the initial results were poor. Improved algorithms produced substantially better results. It appears that at least for the night-Southern Hemisphere portion of the Nimbus-7 orbit, a rms measurement accuracy of 1.45°C has been achieved. Similar tests with wind speed retrievals yield an accuracy of 2.7 m/s rms with no substantial differences between day and night measurements but limited by availability of surface observations to the Northern Hemisphere. Moreover, it appears that the retrieved wind speed is more nearly related to the square of the wind observed at the surface than to the wind itself.