Millimeter-wave Imaging Radiometer Research Articles

Microwave Radiometer Instability Due to Infrequent Calibration

We directly quantify the effect of infrequent calibration on the stability of microwave radiometer temperature measurements (where a power measurement for the unknown source is acquired at a fixed time, but calibration data are acquired at variable earlier times) with robust and nonrobust implementations of a new metric. Based on our new metric, we also determine a component of uncertainty in a single measurement due to infrequent calibration effects. We apply our metric to experimental data acquired from experimental ground-based calibration data acquired from a NASA millimeter-wave imaging radiometer and a NIST radiometer (Noise Figure Radiometer-NFRad). Based on a stochastic model for the NFRad, we determine the random uncertainty of an empirical prediction model of our stability metric by a Monte Carlo method. For comparison purposes, we also present a secondary metric that quantifies stability for the case where calibration data are acquired at a fixed time, but power measurements for the unknown source are acquired at variable later times.

CoSMIR Performance During the GPM OLYMPEX Campaign

The airborne Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) has participated in the Global Precipitation Measurement (GPM) Olympic Mountains Experiment (OLYMPEX) from November to December, 2015, with great success. With similar channels as that of the GPM Microwave Imager (GMI) at 89–183 GHz, CoSMIR served as a proxy for GMI by flying onboard the DC-8 Aircraft for a total of 17 science flights, collecting over 72 h of observations. The high-quality, calibrated brightness temperature data set is the result of several improvements made to CoSMIR prior to OLYMPEX to make the instrument more reliable. This paper describes these improvements and gives a detailed summary of the CoSMIR measurements obtained from OLYMPEX. CoSMIR experienced minor performance issues during the campaign, most of them were not excessive and only resulted in a loss of approximately 4 h of data for the entire campaign. Performance issues are discussed and shown how they were mitigated to achieve a quality data set. Comparisons of CoSMIR and GMI observations are presented to show that the CoSMIR measurements agree well with GMI. The CoSMIR data set is publicly available as a part of the OLYMPEX data suite and can reliably be used in the GPM algorithm development and related studies.

Analysis of the Global Microwave Polarization Data of Clouds

Abstract Information about the characteristics of ice particles in clouds is necessary for improving our understanding of the states, processes, and subsequent modeling of clouds and precipitation for numerical weather prediction and climate analysis. Two NASA passive microwave radiometers, the satellite-borne Global Precipitation Measurement (GPM) Microwave Imager (GMI) and the aircraft-borne Conical Scanning Millimeter-Wave Imaging Radiometer (CoSMIR), measure vertically and horizontally polarized microwaves emitted by clouds (including precipitating particles) and Earth’s surface below. In this paper, GMI (or CoSMIR) data are analyzed with CloudSat (or aircraft-borne radar) data to find polarized difference (PD) signals not affected by the surface, thereby obtaining the information on ice particles. Statistical analysis of 4 years of GMI and CloudSat data, for the first time, reveals that optically thick clouds contribute positively to GMI PD at 166 GHz over all the latitudes and their positive magnitude of 166-GHz GMI PD varies little with latitude. This result suggests that horizontally oriented ice crystals in thick clouds are common from the tropics to high latitudes, which contrasts the result of Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) that horizontally oriented ice crystals are rare in optically thin ice clouds.

The Microwave Radiative Properties of Falling Snow Derived from Nonspherical Ice Particle Models. Part II: Initial Testing Using Radar, Radiometer and In Situ Observations

AbstractIn this study, two different particle models describing the structure and electromagnetic properties of snow are developed and evaluated for potential use in satellite combined radar–radiometer precipitation estimation algorithms. In the first model, snow particles are assumed to be homogeneous ice–air spheres with single-scattering properties derived from Mie theory. In the second model, snow particles are created by simulating the self-collection of pristine ice crystals into aggregate particles of different sizes, using different numbers and habits of the collected component crystals. Single-scattering properties of the resulting nonspherical snow particles are determined using the discrete dipole approximation. The size-distribution-integrated scattering properties of the spherical and nonspherical snow particles are incorporated into a dual-wavelength radar profiling algorithm that is applied to 14- and 34-GHz observations of stratiform precipitation from the ER-2 aircraftborne High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) radar. The retrieved ice precipitation profiles are then input to a forward radiative transfer calculation in an attempt to simulate coincident radiance observations from the Conical Scanning Millimeter-Wave Imaging Radiometer (CoSMIR). Much greater consistency between the simulated and observed CoSMIR radiances is obtained using estimated profiles that are based upon the nonspherical crystal/aggregate snow particle model. Despite this greater consistency, there remain some discrepancies between the higher moments of the HIWRAP-retrieved precipitation size distributions and in situ distributions derived from microphysics probe observations obtained from Citation aircraft underflights of the ER-2. These discrepancies can only be eliminated if a subset of lower-density crystal/aggregate snow particles is assumed in the radar algorithm and in the interpretation of the in situ data.

Signatures of Hydrometeor Species from Airborne Passive Microwave Data for Frequencies 10–183 GHz

AbstractPassive microwave brightness temperatures (BTs) collected above severe thunderstorms using the Advanced Microwave Precipitation Radiometer and Conical Scanning Millimeter-Wave Imaging Radiometer during the Midlatitude Continental Convective Clouds Experiment are compared with a hydrometeor identification applied to dual-polarimetric Weather Surveillance Radar-1988 Doppler radar data collected at Vance Air Force Base, Oklahoma (KVNX). The goal of this work is to determine the signatures of various hydrometeor species in terms of BTs measured at frequencies used by the Global Precipitation Measurement mission Microwave Imager. Results indicate that hail is associated with an ice-scattering signature at all frequencies examined, including 10.7 GHz. However, it appears that frequencies ≤ 37.1 GHz are most useful for identifying hail. Low-level (below 2.5 km) hail becomes probable for a BT below 240 K at 19.4 GHz, 170 K at 37.1 GHz, 90 K at 85.5 GHz, 80 K at 89.0 GHz, 100 K at 165.5 GHz, and 100 K at 183.3 ± 7 GHz. Graupel may be distinguished from hail and profiles without any hydrometeor species by its strong scattering signature at higher frequencies (e.g., 165.5 GHz) and its relative lack of scattering at frequencies ≤ 19.4 GHz. There is a clearer distinction between profiles that contain liquid precipitation and profiles without any hydrometeors when the liquid is associated above with hail and/or graupel (i.e., a hydrometeor category with a strong scattering signature) than when the liquid is associated with smaller ice. Near-surface precipitation is much more likely for a 19.4-GHz BT < 250 K, 37.1-GHz BT < 240 K, 89.0-GHz BT < 220 K, and 165.5-GHz BT < 140 K.

Observations of Storm Signatures by the Recently Modified Conical Scanning Millimeter-Wave Imaging Radiometer

The Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) is an airborne total-power radiometer that, after the recent modification, measures radiation at the nine frequencies of 50.3, 52.8, 89 (dual-polarized), 165 (dual-polarized), 183.3 ± 1, 183.3 ± 3, and 183.3 ± 7 GHz. During the Mid-latitude Continental Convective Clouds Experiment of April 22-June 1, 2011, it is programmed to acquire radiometric measurements in both conical and cross-track scans nearly simultaneously. Its new capability of measuring scattering signatures from storm-associated hydrometeors in dual polarization at both 89 and 165 GHz is illustrated and reported in this paper. We find that, from all seven flights over stratiform rain and convective storms, the polarization index ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PI</i> ), is small but definitively positive at both frequencies, and generally (89 GHz) ≤ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PI</i> (165 GHz). When brightness temperatures <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tbp</i> s are ≥ 240 K, there is a significant correlation between <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PI</i> and the brightness difference between the two frequencies ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dTbp</i> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tbp</i> (89) - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tbp</i> (165) , where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> is either vertical V or horizontal H polarization); linear regression between these two parameters gives positive slopes for all seven events, with 165-GHz slopes generally larger the 89-GHz ones. Observations from five special sensor microwave imager/sounder passes in near concurrence with the CoSMIR measurements are examined for the relation between <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PI</i> (91.665 GHz) and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dTbh</i> (91.665 GHz-150 GHz). The regression slopes are again found to be positive, and their magnitudes show some correspondence to those of CoSMIR. The significance of these findings to improvement in the parameter retrievals of hydrometeors is briefly discussed.

A Comparison of Near-Concurrent Measurements From the SSMIS and CoSMIR for Some Selected Channels Over the Frequency Range of 50–183 GHz

Ten underflights of the Special Sensor Microwave Imager/Sounder (SSMIS) with Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) onboard the NASA ER-2 aircraft were conducted over the coastal region of California between March 2004 and March 2005. The measured brightness temperature (Tb) values from both sensors are collocated and compared at frequencies of 50.3, 52.8, 53.6, 91.655, 150, 183.3 plusmn 1,183.3 plusmn 3, and 183.3 plusmn 6.6 GHz. The more transparent channels at 50.3, 91.655, and 150 GHz are strongly affected by the changes in surface emission and low-level liquid clouds. Thus, the average differences in T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> values (deltaT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> ), measured by the two sensors, and their changes from flight to flight are difficult to assess. For the remaining opaque channels, using the CoSMIR measurements as reference, the lowest SSMIS T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> values occur when the SSMIS is completely under the Earth's shadow. As the satellite moves out of the Earth's shadow in the ascending passes, the SSMIS T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> values are found to gradually increase with more exposure to the sun. The magnitudes of these Tb changes are about 4-5 K for the three 183.3-GHz channels and about 2 K for the 52.8- and 53.6-GHz channels.

Physical Validation of Microwave Properties of Winter Precipitation Over the Sea of Japan

In order to evaluate variations in the physical properties of winter precipitation over the Sea of Japan, we analyzed the Wakasa Bay Experiment 2003 datasets. The results show that synoptic scale differences in freezing-level height (FLH) were observed between extra-tropical low cases and winter monsoon and upper cold low cases, and that, frozen-particle density and cloud liquid water content (CLWC) observed at the surface showed great variation independent of the precipitation rate, although positive correlation may exist between these two properties. The radiative transfer model (RTM) simulation and the validation using the advanced microwave scanning radiometer on EOS (AMSR-E) and millimeter-wave imaging radiometer brightness temperatures (TBs) indicate that sensitivity to frozen precipitation rate (0-5 mm ldr h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) was found in 89-GHz polarization-corrected temperature (PCT89), 89-GHz polarization difference (PD89), and 150-GHz TBs and that this sensitivity decreased with FLH due to emission from mixed- and liquid-phase particles. The RTM simulation, however, overestimated the AMSR-E PCT89 depressions for nonzero FLH cases. This indicates that microwave retrieval for winter precipitation requires a priori information about FLH and an efficient mixed-phase-particle model for RTM. The relationship between surface precipitation rate and TBs was also influenced by the observed variation of CLWC. The relationship between PCT89 and PD89 is greatly altered in terms of CLWC. Relationship between AMSR-E PCT89 precipitation sensitivity and CLWC estimated from PCT89 and PD89 was qualitatively consistent with those of the RTM simulations. This suggests that CLWC, another important retrieval parameter, can be estimated from PCT89 and PD89.

Airborne CoSMIR Observations Between 50 and 183 GHz Over Snow-Covered Sierra Mountains

An airborne Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) was developed recently for calibration/validation of the new-generation DMSP F-series microwave radiometer, the Special Sensor Microwave/Imager/Sounder. The CoSMIR is a total-power radiometer that measures radiation at nine channels over the frequency range of 50-183 GHz. The instrument employs a two-axis gimbaled mechanism to generate the conical scan with periodic calibration. Its scan geometry is software programmable and can be designed to serve the scientific requirements of an experiment. A series of CoSMIR flights was conducted over the coastal regions of California in March and December of 2004, in which the instrument was programmed to acquire both conical and across-track scan data sets simultaneously. Two of these flights on March 25 and December 2 contained segments over the snow-covered Sierra Mountain Range and were selected to demonstrate the novel features of this new instrument

Potential to Estimate the Canting Angle of Tilted Structures in Clouds From Microwave Radiances Around 183 GHz

The effects of cloud structures on microwave radiances at frequencies from 89-190 GHz are investigated by simulations using the Goddard cumulus ensemble model data as input for a radiative transfer model. It was found that the brightness temperatures at these frequencies have different sensitivities to clouds with a tilted structure. The different sensitivities to altitude and amount of hydrometeors allow the estimation of the canting angle and tilt direction of tilted clouds using brightness temperatures at the water vapor channels at 183.3 /spl plusmn/ 1 and 183.3 /spl plusmn/ 7 GHz. The estimated canting angle and tilt direction are in agreement with the model situation. This method provides a potential to estimate tilted convective structures from microwave radiometric observations at 183.3 /spl plusmn/ 1 and 183.3 /spl plusmn/ 7 GHz. It is applied to a tilted storm observed from the National Aeronautics and Space Administration's ER-2 aircraft flying at about 20 km on August 26, 1998 during the third Convection and Moisture Experiment using the observed downlooking brightness temperatures at the water vapor channels of a Millimeter-wave Imaging Radiometer. The estimated results are in good agreement with the realistic storm situation obtained from the simultaneous observations of the ER-2 Doppler radar. This method also provides information about the vertical displacement of cloud structure and thereby to estimate the accurate location of surface rainfall. This is important when validating precipitation retrieval based on observations of the ice scattering above surface rainfall against surface rain observations using the microwave frequencies sensitive to high altitudes.

Intercomparison of Deep Convective Cloud Fractions From Passive Infrared and Microwave Radiance Measurements

The common method to detect deep convective clouds is from satellite infrared (IR) measurements, which is based on thresholds of cloud-top temperatures. However, thick cirrus clouds with high cloud tops are difficult to screen out using IR methods, resulting in an overestimation of deep convective cloud fractions. Two aircraft cases with simultaneous Millimeter-wave Imaging Radiometer, Multispectral Atmospheric Mapping Sensor, and ER-2 Doppler radar measurements during the Convection and Moisture Experiment 3 in August 1998 are analyzed to investigate the influence of high thick cirrus clouds on two previously developed IR methods. In contrast, a microwave method based on the brightness temperature differences between the three water vapor channels around 183.3 GHz of the Advanced Microwave Sounding Unit-B (AMSU-B) (183.3/spl plusmn/1,183.3/spl plusmn/3, and 183.3/spl plusmn/7 GHz) can screen out high thick cirrus clouds efficiently. The tropical deep convective cloud fractions (30/spl deg/S-30/spl deg/N) estimated by the IR methods and the AMSU-B method are compared. Although their geographical distributions are in well agreement with each other, the total fractions detected by the IR methods are about 2-3.5 times greater than that detected by the AMSU-B method. Moreover, the overestimation of deep convective cloud fractions by the IR method (11-/spl mu/m brightness temperature less than 215 K) can result in a displacement in the detected location of the deep convective clouds. The average thick cirrus clouds cover 2.5 times the area of the deep convective clouds that generates them.

Near concurrent MIR, SSM/T-2, and SSM/I observations over snow-covered surfaces

The airborne Millimeter-wave Imaging Radiometer (MIR) and MODIS Airborne Simulator (MAS) measurements over the Arctic region and the Midwest region of the US are used to derive surface emissivities ξ( ν) for three frequencies, ν=89, 150, and 220 GHz, as well as Normalized Difference Snow Index (NDSI) and R87 (0.87-μm reflectance). These derived parameters are compared with parameters estimated from near concurrent measurements made by the SSMI and SSM/T-2 over snow-covered areas. It is shown that the MIR-estimated ξ( ν) values at ν=89 and 150 GHz agree well with those estimated from the SSM/T-2 at ν=91 and 150 GHz, respectively. Low MIR-estimated ξ( ν) values are generally associated with high NDSI and R87 over the snow-covered areas. Over forested areas, more fluctuations in the values of MIR-estimated ξ( ν), NDSI and R87, as well as a reduction in polarization index (PI) at 37 and 85 GHz are observed. Both observations and results from radiative transfer calculations show a change in the difference between brightness temperatures ( T b) at 19 and 37 GHz, as well as PI at 37 and 85 GHz, when measured at satellite altitudes and at the surface. The amplitude of the T b difference and PI is reduced by about 10–15% from surface to high altitudes when integrated water vapor is ≤1.5 g/cm 2. This effect is readily correctable and requires consideration when validating satellite retrieval algorithms based on surface and low-elevation aircraft measurements.

Profiling of atmospheric water vapor with MIR and LASE

Concurrent measurements of atmospheric water vapor profiles were conducted over the Atlantic Ocean on September 25, 1995 with both the Millimeter-wave Imaging Radiometer (MIR) and Lidar Atmospheric Sounding Experiment (LASE) on board the NASA ER-2 aircraft. LASE provides high precision measurements of both aerosol backscatter and water vapor profiles; aerosol backscatter has a vertical resolution of 60 m while the water vapor profiles have a resolution of 330 m in the low-to-mid troposphere and 550 m in the upper troposphere. Therefore, LASE measurements provide an excellent resource for assessing the capabilities and limitations of MIR as a water vapor profiler. Previously, the water vapor profiles retrieved from the MIR measurements have been compared with those of rawinsonde and Raman lidar observations at point locations. The frequency and extent of the comparisons made in that fashion were largely constrained by the requirement of near coincidence in time and space. The data acquired concurrently by MIR and LASE from this ER-2 aircraft flight enable the comparison of MIR-retrieved and LASE-measured moisture profiles over a long stretch of time and space. In addition, the LASE-measured profiles of aerosol backscatter provide a resource to assess the impact of clouds on the retrieval of water vapor profiles from the MIR measurements. It is shown that profiles of water vapor mixing ratio retrieved from the MIR data generally conform to those measured by the LASE; however, differences in the values of mixing ratio at individual altitude levels are quite often not small. The standard deviations of these differences are found to be about. /spl plusmn/0.98, /spl plusmn/0.84, /spl plusmn/0.95, /spl plusmn/0.42, and /spl plusmn/0.06 g/kg at altitudes of 1.25, 2.75, 4.75, 7.25, and 10.25 km. It is demonstrated that a substantial portion of these differences are due to the poor vertical resolution inherent in the profile retrieval using the MIR radiometric measurements. Additionally, MIR water vapor profiling under cloudy conditions is demonstrated, and it is shown that location and height of the low-altitude clouds estimated from the retrieval process were generally consistent with those observed by the LASE. For study cases where cirrus clouds are present, retrievals from the MIR data over-estimate the mixing ratio; this over-estimate is provoked by brightness temperature decreases that occur at 183-220 GHz within these regions. Undoubtedly, the retrieval method needs an additional procedure to account for the mm-wave scattering by cloud ice particles so that water vapor profiling can be improved within regions where cirrus clouds are present.

A comparison of the MIR-estimated and model-calculated fresh water surface emissivities at 89, 150, and 220 GHz

The airborne millimeter-wave imaging radiometer (MIR) measurements over three lakes (surface temperature /spl sim/273 K) in the Midwest region of the USA during February 1997 were used to estimate surface emissivities at 89, 150, and 220 GHz and the results were compared with those calculated from three different dielectric permittivity models for fresh water. The measurements were during clear and dry atmospheric conditions so that the column water vapor could be accurately retrieved and its effect on the MIR measurements predicted. The standard deviations of the estimated emissivities were found to be about 0.003, 0.004, and 0.008 for 89, 150, and 220 GHz, respectively. The errors of the estimation were calculated to be /spl plusmn/0.005, /spl plusmn/0.006, and /spl plusmn/0.011 in the same order of frequency, respectively, based on the MIR measurement accuracy of /spl plusmn/1 K in the brightness temperature range of 190-290 K. The estimated emissivities at normal incidence, under the assumption of a calm water surface, compare quite well with values generated by the model of P. Stogryn et al. (1995). These estimated values are slightly lower than those calculated from the model of H. J. Liebe et al. (1991) at both 89 and 150 GHz. The estimated 89 GHz emissivity is higher than that calculated from the model of W. Ellison et al. (1998). Additionally, the retrievals using different models of atmospheric absorption as well as off-nadir measurements of the MIR are explored. The impact of these retrievals on the comparison of estimated and calculated emissivities is discussed.

Retrievals of column water vapor using millimeter-wave radiometric measurements

The airborne Millimeter-wave Imaging Radiometer (MIR) measurements conducted over the Midwest region of the continental United States during January/February 1997 and over the Alaska-Arctic region during May 1998 are used to estimate column water vapor W<0.8 g/cm/sup 2/ under a clear sky. On board the same aircraft are two other instruments, the Cloud Lidar System (CLS) and MODerate-resolution Imaging Spectrometer (MODIS) Airborne Simulator (MAS), which provide cloud cover information and independent measurements of W, respectively. The MIR-estimated W values are compared and found to be in very good agreement with those measured by rawinsondes at near concurrence. A close correlation is found between the MIR-estimated W and that estimated from the MAS near-IR reflectance ratios. Water surface emissivities at several MIR frequencies are obtained in the process of the W retrieval from several flights over the Midwest lakes. These estimated emissivities compared favorably with values calculated for a calm water surface, which are based on a di-electric permittivity model and MAS-measured surface temperatures. The results from all comparisons strongly demonstrate the soundness of the technique for estimating W.

A robust retrieval of water vapor column in dry Arctic conditions using the rotating shadowband spectroradiometer

A method to retrieve water vapor column using the 940‐nm water vapor absorption band in dry Arctic conditions is presented. The retrievals with this method are stable with respect to uncertainties in instrument radiometric calibration, air pressure, solar source function, and aerosols. The water vapor column was retrieved with this method using spectra obtained with the rotating shadowband spectroradiometer (RSS) that was deployed during an intensive observation period near Barrow, Alaska, in March 1999. A line‐by‐line radiative transfer model was used to compute water vapor transmittance. The retrievals with this method are compared with retrievals obtained from three independent measurements with microwave radiometers. All four measurements show the same pattern of temporal variations. The RSS results agree most closely with retrievals obtained with the millimeter‐wave imaging radiometer (MIR) at its 183 GHz±7 double‐side band channel. Their correlation over a period of 7 days when water vapor column varied between 0.75 mm and 3.6 mm (according to RSS) is 0.968 with MIR readings 0.12 mm higher on average.

Observations and retrievals of cirrus cloud parameters using multichannel millimeter‐wave radiometric measurements

The May 26, 1998, measurements by millimeter‐wave imaging radiometer (MIR) onboard the NASA ER‐2 aircraft over the arctic region north of Alaska are analyzed to study the characteristics of cirrus clouds. The brightness from the 1.88 and 10.4 μm channels of the MODIS airborne simulator (MAS) and the 1.064 μm backscatter from the cloud lidar system (CLS) are used to identify these clouds. The brightness temperature depressions derived from the 340 and 183.3±7 GHz measurements at 43° incidence are compared with the results of radiative transfer calculations to arrive at an estimation of ice water path (IWP) and median equivalent mass sphere diameter (Dme). The measurements at a high incidence angle of 43° are used to minimize the effect of surface emission. Two different particle size distributions are assumed in these calculations. The estimated average Dme values are ∼270 and ∼241 μm, and the average IWPs are ∼174 and ∼86 g/m2 for the two distributions. The large difference in the estimated IWP values occurs in the domain of radiative transfer calculations at Dme ≤ 100 μm. There are also a number of pairs of 340 and 183.3±7 GHz brightness temperature depressions that occur in the same domain, and the estimation of IWP and Dme cannot be made. These difficulties are probably caused by the low sensitivity to cirrus cloud detection at 183.3±7 GHz.

Retrieval of precipitable water vapor by the millimeter-wave imaging radiometer in the arctic region during FIRE-ACE

Millimeter-wave radiometric measurements obtained from the NASA ER-2 aircraft over the arctic region on May 20, 1998, were used to estimate precipitable water (PW) in the range/spl les/0.60 g/cm/sup 2/. The approach is a modified version of the recent work by J. Miao (1998), which utilized the radiometric measurements at 150, 183.3/spl plusmn/3, and 183.3/spl plusmn/7 GHz of the SSM/T-2 sensor to retrieve PW over the antarctic region. However, Miao has implicitly assumed a surface emissivity that is frequency independent over the 150-183 GHz range. This assumption turns out not to be a good one based on the airborne measurements described below and the errors introduced in the PW estimation were substantial in many cases. It is shown below that four-frequency radiometric measurements in the frequency range of 150-220 GHz provided a robust retrieval of PW, while allowing for a surface emissivity that varied linearly with frequency. The retrieved PW compared favorably with that calculated from rawinsonde data at two widely separated locations. The differences between the retrieved and calculated values are not more than /spl plusmn/0.02 g/cm/sup 2/, which is smaller than errors associated with measurement uncertainty. It is found necessary to account for the double side-band nature of the 183.3 GHz measurements in the radiative transfer calculations for development of the retrieval algorithm. The PW values estimated from the algorithm developed from single side band, 183.3 GHz radiative transfer calculations could be in error by as much as /spl plusmn/0.10 g/cm/sup 2/. Finally, the effect of surface temperature variations is shown to introduce only a small error in the estimation of PW.

Determination of Ice Water Path and Mass Median Particle Size Using Multichannel Microwave Measurements

The method of simultaneously retrieving ice water path and mass median diameter using microwave data at two frequencies is examined and implemented for tropical nonprecipitating clouds. To develop the retrieval algorithm, the authors first derived a bulk mass–size relation for ice particles in tropical clouds based on microphysical data collected during the Central Equatorial Pacific Experiment. This relation effectively allows ice particle density to decrease with particle size. In implementing the retrieval algorithm, 150- and 220-GHz Millimeter-Wave Imaging Radiometer data collected during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment were used. Ice water path and mass median diameter are determined based on a lookup table generated by a radiative transfer model. The lookup table depends on cloud type, cloud liquid water path, and atmospheric temperature and humidity profiles. Only nonprecipitating clouds are studied in this paper. Error analyses were performed by a Monte Carlo procedure in which atmospheric profiles, ice cloud height, liquid water content, surface temperature, and instrument noise vary randomly within their uncertainty range through a Latin hypercube sampling scheme. The rms error in the retrievals is then assessed and presented in a two-dimensional diagram of ice water path and mass median diameter. It is shown that the simultaneous retrieval method using 150 and 220 GHz may be used for clouds with ice water path larger than 200 g m−2 and mass median diameter larger than 200 μm. To obtain meaningful retrievals for “thinner” clouds, higher microwave frequencies are needed. It is also shown that liquid water clouds that are at the same altitude as ice clouds interfere with the retrievals to a significant degree. To obtain reasonable ice water path and mass median size retrievals, it is necessary first to group clouds into several classes, then to apply separate algorithms to the different classes. The accuracy of the retrievals also depends on cloud type, with the best accuracy for cirrus and the worst for the midtop mixed-phase cloud among the clouds investigated in this study.

A Novel Ice-Cloud Retrieval Algorithm Based on the Millimeter-Wave Imaging Radiometer (MIR) 150- and 220-GHz Channels

Abstract A novel microwave technique for simultaneously retrieving cirrus ice water path (IWP) and characteristic ice particle size is described. The retrieval algorithm exploits radiance measurements made at 150 and 220 GHz by the airborne Millimeter-Wave Imaging Radiometer (MIR). Other MIR channels additionally are used to test for the presence of liquid clouds and precipitation, which otherwise would have a contaminating effect on the retrievals. Forward radiative transfer modeling was used to generate a two-dimensional retrieval table in which brightness-temperature depressions (relative to clear-sky values) for both microwave channels were recorded as functions of IWP and characteristic particle size for gamma distributions of ice particles. Retrieval errors due to particle shape, size distribution, clear-sky water vapor variability, cirrus-cloud altitude variability, and instrument noise were estimated using Monte Carlo analysis. Particle shape uncertainty is believed to be the dominant source of retrieval error. The technique is demonstrated using MIR data recorded on the National Aeronautics and Space Administration ER-2 aircraft during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment experiment in the tropical western Pacific Ocean in 1993. The retrieval technique with MIR data is suited only to high-IWP clouds with large ice particles, such as thick frontal cirrus and convective anvils. The general methodology, however, is applicable to higher frequencies that have greatly increased sensitivity to thinner cirrus.