J.R. Wang

Precipitation and Latent Heating Distributions from Satellite Passive Microwave Radiometry. Part II: Evaluation of Estimates Using Independent Data

Rainfall rate estimates from spaceborne microwave radiometers are generally accepted as reliable by a majority of the atmospheric science community. One of the Tropical Rainfall Measuring Mission (TRMM) facility rain-rate algorithms is based upon passive microwave observations from the TRMM Microwave Imager (TMI). In Part I of this series, improvements of the TMI algorithm that are required to introduce latent heating as an additional algorithm product are described. Here, estimates of surface rain rate, convective proportion, and latent heating are evaluated using independent ground-based estimates and satellite products. Instantaneous, 0.5°-resolution estimates of surface rain rate over ocean from the improved TMI algorithm are well correlated with independent radar estimate s( r 0.88 over the Tropics), but bias reduction is the most significant improvement over earlier algorithms. The bias reduction is attributed to the greater breadth of cloud-resolving model simulations that support the improved algorithm and the more consistent and specific convective/stratiform rain separation method utilized. The bias of monthly 2.5°-resolution estimates is similarly reduced, with comparable correlations to radar estimates. Although the amount of independent latent heating data is limited, TMI-estimated latent heating profiles compare favorably with instantaneous estimates based upon dual-Doppler radar observations, and time series of surface rain-rate and heating profiles are generally consistent with those derived from rawinsonde analyses. Still, some biases in profile shape are evident, and these may be resolved with (a) additional contextual information brought to the estimation problem and/or (b) physically consistent and representative databases supporting the algorithm. A model of the random error in instantaneous 0.5°-resolution rain-rate estimates appears to be consistent with the levels of error determined from TMI comparisons with collocated radar. Error model modifications for nonraining situations will be required, however. Sampling error represents only a portion of the total error in monthly 2.5°-resolution TMI estimates; the remaining error is attributed to random and systematic algorithm errors arising from the physical inconsistency and/or nonrepresentativeness of cloud-resolving-model-simulated profiles that support the algorithm.

Water Vapor Profiling From CoSSIR Radiometric Measurements

Previous water vapor profiling by millimeterwave radiometry using the 183-GHz absorption line is generally limited to an altitude range of 0-11 km. The additional measurements at the frequencies of 380.2 0.8, 380.2 1.8, 380.2 3.3, and 380.2 6.2 GHz by the new airborne compact scanning submillimeterwave imaging radiometer (CoSSIR) reported in this paper can extend this profiling capability up to an altitude of about 15 km. This is demonstrated by recent CoSSIR measurements onboard the NASA WB-57 aircraft in a flight from Texas to Costa Rica on January 14, 2006. Retrievals of water vapor mixing ratio were performed at eight altitudes of 1, 3, 5, 7, 9, 11, 13, and 15 km from the CoSSIR data set acquired at observational angles of 0 and 53.4. The results were compared with other available measurements from near-concurrent satellites. A very good agreement was found between the collocated values of total precipitable water (TPW) derived from the CoSSIR-retrieved water vapor profiles and those estimated from Tropical Rainfall Measuring Mission Microwave Imager; the average TPW differences range between 0.30 and 0.64 cm, depending on CoSSIRs observational angles. The accuracy of the retrievals was inferred from an analysis of inflight CoSSIR radiometric signal fluctuations.

The airborne Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR)

Results of the first science flight of the airborne Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) for high-altitude observations from the NASA ER-2 is discussed. Imagery collected from the flight demonstrates CoSMIRs unique conical/cross-track imaging mode and provides comparison of CoSMIR measurements to those of the SSM/T-2 satellite radiometer.

Airborne millimeter-wave radiometric observations of cirrus clouds

This paper reports the first radiometric measurements of cirrus clouds in the frequency range of 89-325 GHz from a high-altitude aircraft flight. The measurements are conducted with a Millimeter-wave Imaging Radiometer (MIR) on board the NASA ER-2 aircraft over a region in northern Oklahoma. Aboard the same aircraft are a cloud lidar system and a multichannel radiometer operating at the visible and infrared wavelengths. The instrument ensemble is well suited for identifying cirrus clouds. It is shown that the depressions in brightness temperatures associated with a few intense cirrus clouds occur at all frequency channels of the MIR. Estimates of total ice water path of the cirrus clouds are derived from comparisons of radiative transfer calculations and observed brightness depressions.

Analysis of snow cover in Alaska using aircraft microwave data (April 1995)

During April of 1995, a field and aircraft experiment was conducted in central and northern Alaska. A millimeter-wave imaging radiometer (MIR), and other sensors, were flown on-board a NASA ER-2 aircraft in a grid pattern centered over Fairbanks. Resulting MIR data show brightness temperature patterns that are related to land cover and snowmelt patterns.

Profiling of atmospheric water vapor from the SSM/T-2 radiometric measurements

An advantage of using the millimeter-wave measurements for water vapor profiling is the ability to probe beyond a moderate cloud cover. J. R. Wang et al. (1997) demonstrated such a capability from an airborne MIR (Millimeter-wave Imaging Radiometer) flight over the Pacific Ocean during an intense observation period of TOGA/COARE (Tropical Ocean Global Atmosphere/Couple Ocean Atmospheric Response Experiment) in early 1993. A Cloud Lidar System (CLS) and MODIS Airborne Simulator (MAS) were on board the same aircraft to identify the presence of clouds and cloud type. The retrieval algorithm not only provides output of a water vapor profile, but also the cloud liquid water and approximate cloud altitude required to satisfy convergence of the retrieval. The validity of these cloud parameters has not been verified previously. In this article these cloud parameters are compared with those derived from concurrent measurements from the CLS and AMPR (Advanced Microwave Precipitation Radiometer).

Simultaneous measurements of water vapor profiles from airborne MIR and LASE

A NASA ER-2 aircraft flight with both Millimeter-wave Imaging radiometer (MIR) and Lidar Atmospheric Sensing Experiment (LASE) was made over ocean areas in the eastern United States on September 25, 1995. The water vapor profiles derived from both instruments under both clear and cloudy conditions are compared. It is shown that good agreement is found between the MIR-derived and the LASE-measured water vapor profiles over the areas of clear-sky conditions. In the cloudy areas, the MIR-retrieved values at the altitudes of the cloud layers and below are generally higher than those measured by the LASE.

Observations and modelling of radiometric signatures of storms in the frequency range of 90-220 GHz

During the past three years, a number of observations have been made with an airborne millimeter-wave imaging radiometer (MIR) over the storms in the western Pacific Ocean and in the coastal region of the eastern United States. MIR measured radiometric signatures of these storms at six frequencies of 89, 150, 183.3/spl plusmn/1, 183.3/spl plusmn/3, 183.3/spl plusmn/7, and 220 GHz. Analyses of these measurements show that brightness temperatures (T/sub b/) at all frequencies are strongly depressed. In some cases the T/sub b/ depression displays a strong frequency dependence of scattering by hydrometeors. In other cases there is a leveling off of scattering at high frequencies, i.e., Tb values at 150 and 220 GHz are quite comparable. A few high-towering scattering cells are found to display unique signatures at the three water vapor channels near 183.3 GHz which in turn could be used to identify these cells. A series of calculations with a backward Monte Carlo technique, using profiles of hydrometeors generated by a cloud model, are performed to simulate some of these observed features. Results from these calculations are compared with observations and their implications on the storm structures are discussed.