J. Turk

Ice water path estimation and characterization using passive microwave radiometry

Abstract Microwave emission emerging from a precipitating cloud top and lying in a radiometers field of view represents the culmination of a complex interaction between emitted microwave radiation and its ongoing extinction through overlapping regions of liquid, melting phase, and ice. The encounter with the ice region represents the final interaction between the upwelling microwave radiation and the cloud constituents. Hence, an ice phase characterization perhaps represents a more inherently retrievable property from a combination of scattering-based channels above 37 GHz than the underlying rainfall. Model computations of top-of-atmospheric microwave brightness temperatures TB from layers of precipitation-sized ice of variable bulk density and ice water content (IWC) are presented. The 85-GHz TB is shown to depend essentially on the ice optical thickness, while the possibility of using the 37- and 85-GHz brightness temperature difference ΔTB to estimate the integrated ice water path (IWP) is investigat...

Active and passive remote sensing of precipitating storms during CaPE. Part II: Intercomparison of precipitation retrievals over land from AMPR radiometer and CP-2 radar

SummaryOne of the recent campaigns devoted to precipitation studies using both active and passive microwave remote sensing systems was the Convection and Precipitation/Electrification Experiment (CaPE), which took place in central Florida during the summer of 1991. During CaPE, the airborne Advanced Microwave Precipitation Radiometer (AMPR), having four channels at 10.7, 19.35, 37.1 and 85.5 GHz and the National Center for Atmospheric Research CP-2 multiparameter radar at S-band (3 GHz) and X-band (10 GHz) were operated simultaneously. In this paper, we compare estimated hydrometeor liquid/ice water contents and surface rainrates, both retrieved from the AMPR radiometer and CP-2 radar measurements, for a case study consisting of a heavy precipitating storm over land near Cape Canaveral on August 12, 1991. The multi-frequency radiometer-based retrieval scheme uses a cloud-precipitation dataset generated from a cloud model and extended by a physically-constrained Monte Carlo procedure, along with a discrete-ordinate radiative transfer model and a principal component statistical technique to help formulate non-linear regression equations for the sought-after hydrometeor quantities. By applying linear discriminant analysis, the algorithm is used to estimate column integrated liquid/ice water contents, as well as the vertical profiles of these quantities to within a specified accuracy. Rainfall rates are estimated either by non-linear regression or by a suitable fallout model. The analysis has confined itself to along-track nadir-looking AMPR measuremets to avoid complications with variable polarization mixing and geometric distortion for off-nadir observations. Considering the different model assumptions used in the two types of retrieval algorithms and the diverse geophysical information content within the two types of measurements, substantial agreement between the radar- and radiometer-derived retrievals has been achieved for the columnar liquid/ice water contents and rainrates.

Microwave radiative transfer studies using combined multiparameter radar and radiometer measurements during Cohmex

Abstract Theoretical calculations of the upwelling microwave radiances from clouds containing layers of rain, ice, and a melting region were performed at frequencies of 18, 37, and 92 GHz. These frequencies coincide with high-resolution microwave radiometer measurements taken aboard the NASA ER-2 high-altitude aircraft during the summer 1936 COHMEX (Cooperative Huntsville Meteorological Experiment) in Alabama. For purposes of brightness temperature computations, the storms were modeled with rain, melting phase, and ice layers. The melting phase region was composed of water-coated ice spheres defined by a “melt index” in terms of the volume fraction of water. Single scatter albedo, scattering, and extinction coefficients were computed at the above frequencies as a function of the rain rate and melt index. In addition, multiparameter radar observations of the storm were mapped into a cartesian space and averaged over regions comparable to the radiometer footprint. Vertical profiles of these data under the E...

Active and passive microwave remote sensing of precipitating storms during CaPE. Part I: Advanced microwave precipitation radiometer and polarimetric radar measurements and models

SummaryThe Advanced Microwave Precipitation Radiometer (AMPR), an across-track scanning, four-channel (10.7, 19.35, 37.1, 85.5 GHz) total-power radiometer system, was instrumented aboard a NASA ER-2 aircraft during the 1991 CaPE (Convection and Precipitation/Electrification) project in central Florida. At a 20 km flight altitude, the AMPR provides fine-scale microwave imagery of Earth surfaces and its atmosphere, and is well-suited for diverse hydrological applications. During overflights of precipitation, coincident ground-based radar measurements were taken with the NCAR CP-2 dual-frequency, dual-polarization radar system. After remapping the radar data into a format compatible with the AMPR scanning geometry, the radar-derived profiles of rain, melting, and frozen hydrometeors are compared against the AMPR equivalent blackbody brightness temperature (TB) imagery. Microwave radiative transfer modeling procedures incorporating the radar-derived hydrometeor profiles were used to simulated the multifrequency AMPR imagery over both land and ocean background ER-2 flights. Within storm cores over land, columnar ice water paths up to 20 kgm−2 gradually depressed the 85 GHzTB as low as 100 K. The presence of tall vertical reflectivity columns encompassing > 20 kgm−2 columnar ice water path often produced 37 GHzTB<85 GHzTB directly over the core. Over ocean, the 10 GHz channel provided the clearest correlation with the rainfall amounts, whereas the 19 GHz channel saturated near 260 K past 10–15 mm hr−1 rain rate as determined by radar. Scattering by ice and melting ice at 37 GHz producedTB ambiguities over both raining and clear-ocean regions. Sensitivity to the columnar mixed phase region via the intermediate frequencies (19 and 37 GHz) is demonstrated and explained with the radar-derivedTB modeling. By superimposing vertical profiles of cloud liquid water (which this radar cannot measure) upon the radarinferred hydrometeor structure, additional information on the location of the peak cloud water and its amount relative to the vertical ice structure can be noted, along with a possible inference of the dominant ice particle size within the upper storm core.These results suggest that as the resolution of passive radiometric measurements approaches dimensions where the antenna beams become increasingly filled by the cloud, precipitation retrieval via multifrequencyTB input is well-suited to a vertical profiling-type algorithm. This is further examined in Part II of this manuscript, where the radarderived vertical hydrometeor profiles are used to test the applicability of a multispectral cloud model-based approach to passive microwave precipitation retrieval from space.

Active and passive remote sensing of precipitation over ocean surfaces

Describes the remote sensing capability of an airborne radiometry system. The Advanced Microwave Precipitation Radiometer is an across-track, four-channel, total power radiometer specifically designed to mount and operate within the NASA ER-2 aircraft. Frequencies of 10.7, 37.1 and 85.5 GHz are used. The authors describe observations made near the Kennedy Space Center in Florida. The radiometric observations are compared to radar.<<ETX>>

High resolution microwave radiometric signatures of mid-latitude and tropical rainfall

Combinations of both microwave and IR sensors, including a 14 GHz radar, are included on the upcoming Tropical Rainfall Measuring Mission (TRMM) satellite. This project is spurring advancements in precipitation profiling retrieval algorithms, which exploit the information contained in a set of multispectral equivalent blackbody brightness temperature observations along with ancillary information to account for cloud microphysical variability. Two NASA high-resolution imaging radiometers, the Advanced Microwave Precipitation Radiometer (AMPR; 10, 19, 37, 85 GHz), and the Millimeter Wave Imaging Radiometer (MIR; 89, 150, 183.3, 220 GHz) were flown in early 1993 aboard an ER-2 aircraft during TOGA-COARE over the tropical western south Pacific warm pool. The AMPR and MIR instruments are airborne simulators of the SSM/I and the recent SSM/T2 polar orbiting satellite radiometer systems. During 1991, the AMPRwas also operated during CaPE field experiments in central Florida. Observations from the two different climatic regimes show marked differences in the brightness temperature behavior associated with precipitation. Notably weak 85 GHz scattering T, depressions were characteristic of the tropical TOGA-COARE environment, where heavy rainfall can be generated from dominantly warm rain clouds containing very little ice content. The variable cloud microphysics characteristic of both warm and cold rain precipitation processes must be accounted for in precipitation profiling algorithms if reliable estimates of the latent heat are to be inferred from satellite-based rainfall measurements.<<ETX>>

Measurements and applications of combined radar-passive microwave rainfall profiling during TOGA-COARE

The NASA Tropical Rainfall Measuring Mission (TRMM) represents the first opportunity to incorporate both multifrequency passive microwave and active radar measurements in a satellite precipitation retrieval algorithm. The TRMM microwave imager (TMI) includes many of the same passive channels as the Special Sensor Microwave Imager (SSMI), with the addition of a 10.7 GHz channel and a 14 GHz incoherent precipitation radar (PR). During the 1992-1993 TOGA-COARE experiment in the western Pacific Ocean, coordinated radar and microwave radiometer data were gathered data over a multitude of precipitating storm regions including the forming and mature stages of a tropical cyclone. Although the TRMM radiometer and radar fields of view will differ, the radar data provide clues to the underlying cloud vertical structure that can be used to mitigate ambiguities in profile-based retrieval algorithms that rely upon the passive T/sub B/ alone.

Measurements and implications for combined radar-passive microwave rainfall profiling techniques

An upcoming NASA satellite platform, the Tropical Rainfall Measuring Mission (TRMM), represents the first opportunity to incorporate both multifrequency passive microwave and active radar measurements in its retrieval algorithms. The TRMM microwave imager (TMI) includes many of the same passive channels as the current Special Sensor Microwave Imager (SSMI), with the addition of a 10.7 GHz channel. The 14 GHz incoherent precipitation radar (PR) scans an across-track swath width about one-third the width of the forward-view conical TMI scan. Therefore, the radar data arrives about a minute after the TMI scan, and the PR and TMI beams view significantly different profiles in the atmosphere for a given on-Earth pixel location. During the 1992-1993 TOGA-COARE experiment in the western Pacific Ocean, a DC-8-based precipitation radar and an ER-a-based 4-channel microwave radiometer gathered data over a multitude of precipitating storm regions including the forming and mature stages of a tropical cyclone. Example imagery and vertical radar profiles are presented. These data are currently being used by the authors for vertical profiling algorithms which exploit the information contained in both the radar and radiometer. The radar data provide clues to the underlying cloud vertical structure that can be used to mitigate ambiguities in profile-based retrieval algorithms that rely upon the passive T/sub B/ alone.<<ETX>>

Ka-band propagation measurements from the Advanced Communications Technology Satellite (ACTS)

In September 1993, the NASA Advanced Communications Technology Satellite (ACTS) was deployed into stationary orbit, near 100/spl deg/W longitude by the Space Shuttle Discovery. ACTS supports both communication and propagation experiments using two beacons, one at 20.185 GHz and another at 27.505 GHz, where rain attenuation and tropospheric scintillations will significantly affect new technologies proposed for this spectrum. Heavy rain at Ka-band can easily produce 20 dB of attenuation along the propagation path. Beacon attenuation and attenuation derived from radiometric sky noise brightness temperatures are presented from early 1994. Concurrent polarimetric weather radar measurements are shown to characterize the attenuation not only from rain, but melting, wet snow.<<ETX>>

Multiparameter Radar and Advanced Microwave Precipitation Radiometer Observations of Tropical Convection

Combinations of both active and passive microwave sensors have been proposed for experiments such as the tropical rainfall measuring mission (TRMM). During the summer 1991 Convection and Precipitation/Electrification Experiment (CaPE) in central Florida, both cold- and warm-rain precipitation processes were studied with the NCAR CP-2 multiparameter radar, operating at 3 and 10 GHz. The Advanced Microwave Precipitation Radiometer (AMPR), which operates at 10.7, 19.35, 37.1, and 85.5 GHz, was flown aboard NASAs ER-2 high-altitude aircraft over storms being scanned by the CP-2 radar. Top-of-atmosphere multifrequency TB from the AMPR is presented along with coincident CP-2 radar observations. Joint analysis of both radar and radiometer data sets allows refinement of new and existing precipitation retrieval techniques which will utilize the multifrequency TB from an integrated spaceborne microwave radiometer/radar system.