Computed Brightness Temperatures Research Articles

Deep Convection as Inferred From the C2OMODO Concept of a Tandem of Microwave Radiometers

Probing the atmosphere from space using radiometers is a challenging way to improve our knowledge of deep convection. Exploration of water absorption bands in the 183, 325, and 448 GHz range is promising because of the radiation scattered by icy hydrometeors produced by deep convection. We investigate what properties of deep convection could be inferred from the Convective Core Observations through MicrOwave Derivatives in the trOpics (C2OMODO) concept of a tandem of microwave radiometers separated by several tens of seconds. Two tropical deep convective events (Hector the Convector and a radiative-convective equilibrium case) are simulated with the Meso-NH non-hydrostatic numerical model, the outputs of which are used to compute brightness temperatures (Tbs) using the Radiative Transfer for the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (RTTOV) code. We find different relationships between the ice water path, vertical ice momentum, vertical ice velocity, and the time derivative dTb/dt. They depend on where they are probed after separating the growing convective cores from their surrounding environment with a radiometric point of view. Tb and dTb/dt are highly dependent on the ice water path that depends on horizontal and vertical ice advection and microphysical processes. Looking at deep convection in general, we find that the ice water path increases linearly with decreasing dTb/dt. In the specific case of the core of growing convective cells, the vertical ice momentum and the vertical ice velocity are related to dTb/dt. However, such a relationship breaks down in the anvil because horizontal ice advection can dominate microphysical processes. These results are robust to horizontal resolution and time delay.

Version 4 CALIPSO Imaging Infrared Radiometer ice and liquid water cloud microphysical properties – Part I: The retrieval algorithms

Abstract. Following the release of the version 4 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data products from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, a new version (version 4; V4) of the CALIPSO Imaging Infrared Radiometer (IIR) Level 2 data products has been developed. The IIR Level 2 data products include cloud effective emissivities and cloud microphysical properties such as effective diameter and ice or liquid water path estimates. Dedicated retrievals for water clouds were added in V4, taking advantage of the high sensitivity of the IIR retrieval technique to small particle sizes. This paper (Part I) describes the improvements in the V4 algorithms compared to those used in the version 3 (V3) release, while results will be presented in a companion (Part II) paper. The IIR Level 2 algorithm has been modified in the V4 data release to improve the accuracy of the retrievals in clouds of very small (close to 0) and very large (close to 1) effective emissivities. To reduce biases at very small emissivities that were made evident in V3, the radiative transfer model used to compute clear-sky brightness temperatures over oceans has been updated and tuned for the simulations using Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) data to match IIR observations in clear-sky conditions. Furthermore, the clear-sky mask has been refined compared to V3 by taking advantage of additional information now available in the V4 CALIOP 5 km layer products used as an input to the IIR algorithm. After sea surface emissivity adjustments, observed and computed brightness temperatures differ by less than ±0.2 K at night for the three IIR channels centered at 08.65, 10.6, and 12.05 µm, and inter-channel biases are reduced from several tens of Kelvin in V3 to less than 0.1 K in V4. We have also improved retrievals in ice clouds having large emissivity by refining the determination of the radiative temperature needed for emissivity computation. The initial V3 estimate, namely the cloud centroid temperature derived from CALIOP, is corrected using a parameterized function of temperature difference between cloud base and top altitudes, cloud absorption optical depth, and CALIOP multiple scattering correction factor. As shown in Part II, this improvement reduces the low biases at large optical depths that were seen in V3 and increases the number of retrievals. As in V3, the IIR microphysical retrievals use the concept of microphysical indices applied to the pairs of IIR channels at 12.05 and 10.6 µm and at 12.05 and 08.65 µm. The V4 algorithm uses ice look-up tables (LUTs) built using two ice habit models from the recent “TAMUice2016” database, namely the single-hexagonal-column model and the eight-element column aggregate model, from which bulk properties are synthesized using a gamma size distribution. Four sets of effective diameters derived from a second approach are also reported in V4. Here, the LUTs are analytical functions relating microphysical index applied to IIR channels 12.05 and 10.6 µm and effective diameter as derived from in situ measurements at tropical and midlatitudes during the Tropical Composition, Cloud, and Climate Coupling (TC4) and Small Particles in Cirrus Science and Operations Plan (SPARTICUS) field experiments.

On the interpretation of upper-tropospheric humidity based on a second-order retrieval from infrared radiances

Abstract. We present a novel retrieval for upper-tropospheric humidity (UTH) from High-resolution Infrared Radiation Sounder (HIRS) channel 12 radiances that successfully bridges the wavelength change from 6.7 to 6.5 µm that occurred from HIRS/2 on National Oceanic and Atmospheric Administration satellite NOAA-14 to HIRS/3 on satellite NOAA-15. The jump in average brightness temperature (in the water vapour channel; T12) that this change had caused (about −7 K) could be fixed with a statistical inter-calibration method (Shi and Bates, 2011). Unfortunately, the retrieval of UTHi (upper-tropospheric humidity with respect to ice) based on the inter-calibrated data was not satisfying at the high tail of the distribution of UTHi. Attempts to construct a better inter-calibration in the low T12 range (equivalent to the high UTHi range) were either not successful (Gierens et al., 2018) or required additional statistically determined corrections to the measured brightness temperatures (Gierens and Eleftheratos, 2017). The new method presented here is based on the original one (Soden and Bretherton, 1993; Stephens et al., 1996; Jackson and Bates, 2001), but it extends linearisations in the formulation of water vapour saturation pressure and in the temperature dependence of the Planck function to second order. To achieve the second-order formulation we derive the retrieval from the beginning, and we find that the most influential ingredient is the use of different optical constants for the two involved channel wavelengths (6.7 and 6.5 µm). The result of adapting the optical constant is an almost perfect match between UTH data measured by HIRS/2 on NOAA-14 and HIRS/3 on NOAA-15 on 1004 common days of operation. The method is applied to both UTH and UTHi. For each case retrieval coefficients are derived. We present a number of test applications, e.g. on computed brightness temperatures based on high-resolution radiosonde profiles, on the brightness temperatures measured by the satellites on the mentioned 1004 common days of operation. Further, we present time series of the occurrence frequency of high UTHi cases, and we show the overall probability distribution of UTHi. The two latter applications expose indications of moistening of the upper troposphere over the last 35 years. Finally, we discuss the significance of UTH. We state that UTH algorithms cannot be judged for their correctness or incorrectness, since there is no true UTH. Instead, UTH algorithms should fulfill a number of usefulness postulates, which we suggest and discuss.

Evaluation and Correction of Ground-Based Microwave Radiometer Observations Based on NCEP-FNL Data

Consistency between the brightness temperatures observed with a ground-based microwave radiometer and the brightness temperatures computed by forward modeling is important in many different data applications. Using the National Centers for Environmental Prediction-Final Operational Global Analysis (NCEP-FNL) dataset as a reference, the brightness temperature was obtained through the radiation transfer model for forward calculation. The problem of segmented features in long time of observational data from ground-based microwave radiometers (the so-called “jumping problem”) was identified. By analyzing the deviation and correlation between the observational bright temperature data and the forward calculated data under clear sky conditions, a revised scheme is proposed for the bright temperature observational data. Data obtained with a ground-based microwave radiometer in Beijing from January 1, 2010 to December 31, 2011 around the date of liquid nitrogen calibration show that the correlation between the observed brightness temperatures and the forward computed brightness temperatures is better after correction, especially at 28 and 30 GHz. The “jumping” problem in the observational data for the brightness temperature is eliminated after correction and the time continuity of the observational data and its consistency with the forward calculated data based on the NCEP-FNL dataset are improved. The proposed correction scheme can be used both for real-time data quality control and to improve the accuracy of historical datasets obtained with poorly calibrated microwave radiometers or radiometers working in polluted environments.

Passive microwave observations of South America and surrounding oceans from Russian Meteor-M No. 2 and Japan GCOM-W1 satellites

ABSTRACTA new Russian Meteorological Satellite Meteor-M No. 2 with onboard multichannel microwave radiometer MTVZA-GY was launched on sun synchronous orbit in July 2014. The radiometer receives the outgoing radiation of the Earth at 29 channels at the frequency ranges ν = 10–48, 52–57, 90–93 and 176–190 GHz. Conical scanning is performed at the incidence angle 65°. The data derived over the Amazon basin rain forest and the cloudless calm areas in the South Atlantic and South Pacific Ocean and the computed brightness temperatures (TBs) served for the MTVZA-GY external calibration. A comparison of the TB time series obtained by the MTVZA-GY and well-calibrated AMSR2 radiometer onboard GCOM-W1 was used for assessment of the long-term stability of the MTVZA-GY in flight. The joint analysis of the TB time series at ν = 10.6, 42.0 and 91.6 GHz allowed to reveal the variations associated with the wet and dry seasons in the Amazon forest as well as events caused by the rain and heavy clouds during satellite observations. The multichannel approach was also used to study structure, evolution and fields of the sea surface wind, total water vapour content and total cloud liquid water content in the cyclonic formations over the South Atlantic and South Pacific Oceans.

Solar Prominence Modelling and Plasma Diagnostics at ALMA Wavelengths

Our aim is to test potential solar prominence plasma diagnostics as obtained with the new solar capability of the Atacama Large Millimeter/submillimeter Array (ALMA). We investigate the thermal and plasma diagnostic potential of ALMA for solar prominences through the computation of brightness temperatures at ALMA wavelengths. The brightness temperature, for a chosen line of sight, is calculated using the densities of electrons, hydrogen, and helium obtained from a radiative transfer code under non-local thermodynamic equilibrium (non-LTE) conditions, as well as the input internal parameters of the prominence model in consideration. Two distinct sets of prominence models were used: isothermal-isobaric fine-structure threads, and large-scale structures with radially increasing temperature distributions representing the prominence-to-corona transition region. We compute brightness temperatures over the range of wavelengths in which ALMA is capable of observing (0.32 – 9.6 mm), however, we particularly focus on the bands available to solar observers in ALMA cycles 4 and 5, namely 2.6 – 3.6 mm (Band 3) and 1.1 – 1.4 mm (Band 6). We show how the computed brightness temperatures and optical thicknesses in our models vary with the plasma parameters (temperature and pressure) and the wavelength of observation. We then study how ALMA observables such as the ratio of brightness temperatures at two frequencies can be used to estimate the optical thickness and the emission measure for isothermal and non-isothermal prominences. From this study we conclude that for both sets of models, ALMA presents a strong thermal diagnostic capability, provided that the interpretation of observations is supported by the use of non-LTE simulation results.

Asian dust height and infrared optical depth retrievals over land from hyperspectral longwave infrared radiances

The dust top height and infrared optical depth over land are retrieved from the Atmospheric Infrared Sounder (AIRS) longwave infrared measurements by using a one‐dimensional‐variation retrieval algorithm for different Asian dust storms. By combining particle size measurements from a 10‐year ground observation data set from the Dunhuang Skynet station located to the east of the Taklimakan Desert in China and the Optical Properties of Aerosols and Clouds data set of optical properties, the mineral dust scattering and absorption coefficients are obtained and then used to compute brightness temperatures with RTTOV 9.3. The retrieved dust thermal infrared optical depths are compared with the Ozone Monitoring Instrument and Moderate Resolution Imaging Spectroradiometer (MODIS) products. The retrieved dust top heights are compared against the extinction backscatter profiles obtained from the Cloud‐Aerosol Lidar with Orthogonal Polarization lidar. Infrared optical depths from AIRS correlate favorably with visible optical depths from MODIS, and dust top heights agree reasonably with lidar observations for the single‐layered dust storms over the Taklimakan Desert.

An Observationally Generated A Priori Database for Microwave Rainfall Retrievals

Abstract The combination of active and passive microwave sensors on board the Tropical Rainfall Measuring Mission (TRMM) satellite have been used to construct observationally constrained databases of precipitation profiles for use in passive microwave rainfall retrieval algorithms over oceans. The method uses a very conservative approach that begins with the operational TRMM precipitation radar algorithm and adjusts its solution only as necessary to simultaneously match the radiometer observations. Where the TRMM precipitation radar (PR) indicates no rain, an optimal estimation procedure using TRMM Microwave Imager (TMI) radiances is used to retrieve nonraining parameters. The optimal estimation methodology ensures that the geophysical parameters are fully consistent with the observed radiances. Within raining fields of view, cloud-resolving model outputs are matched to the liquid and frozen hydrometeor profiles retrieved by the TRMM PR. The profiles constructed in this manner are subsequently used to compute brightness temperatures that are immediately compared to coincident observations from TMI. Adjustments are made to the rainwater and ice concentrations derived by PR in order to achieve agreement at 19 and 85 GHz, vertically polarized brightness temperatures at monthly time scales. The database is generated only in the central 11 pixels of the PR radar scan, and the rain adjustment is performed independently for distinct sea surface temperature (SST) and total precipitable water (TPW) values. Overall, the procedure increases PR rainfall by 4.2%, but the adjustment is not uniform across all SST and TPW regimes. Rainfall differences range from a minimum of −57% for SST of 293 K and TPW of 13 mm to a maximum of +53% for SST of 293 K and TPW of 45 mm. These biases are generally reproduced by a TMI retrieval algorithm that uses the observationally generated database. The algorithm increases rainfall by 5.0% over the PR solution with a minimum of −99% for SST of 293 K and TPW of 14 mm to a maximum of +11.8% for an SST of 294 K and TPW of 50 mm. Some differences are expected because of the algorithm mechanics.

Titan lakes: characterization of physical properties by combined electromagnetic models of SAR and radiometric data

From the analysis derived from the SAR imagery acquired by the Cassini RADAR, Titan reveals a complex surface with dark and bright areas, periodic structures and dark lake-like features, which resemble terrestrial lakes. The abundant methane in Titan’s atmosphere combined with the low temperature, around 89 K, lead scientists to interpret them as lakes of liquid methane or ethane. This paper presents an electromagnetic model of liquid surfaces combined with an inversion algorithm in order to retrieve information on liquid surface. The inferred physical surface parameters are then used as input to a forward radiative transfer model to obtain simulated brightness temperatures. The observed and computed brightness temperatures are compared to address the consistency of the observations from these two different Cassini RADAR modes.

Effects of atmospheric water and surface wind on passive microwave retrievals of sea ice concentration: a simulation study

The atmospheric effects on the retrieval of sea ice concentration from passive microwave sensors were examined using simulated data typical for the Arctic summer. The simulation includes atmospheric contributions of cloud liquid water (CLW), water vapour (WV) and surface wind on the microwave signatures. A plane parallel radiative transfer model was used to compute brightness temperatures at SSM/I frequencies over surfaces that contained open water, first‐year (FY) ice, multi‐year (MY) ice and their combinations. Synthetic retrievals were performed using the NASA Team (NT) algorithm for the estimation of sea ice concentrations. Our results show that if the satellite sensor's field of view is filled with only FY ice, the retrieval is hardly affected by the atmospheric conditions because of the high contrast between emission signals from the FY ice surface and the atmosphere. Pure MY ice concentration is generally underestimated because of the low MY ice surface emissivity, which results in the enhancement of emission signals from the atmosphere. In marginal ice areas, the atmospheric and surface effects tend to degrade the accuracy at low sea ice concentrations. FY ice concentration is overestimated and MY ice concentration is underestimated in the presence of atmospheric water and surface wind at low ice concentration. Moreover, strong surface wind appears to be more important than atmospheric water in contributing to the retrieval errors of total ice concentrations over marginal ice zones.

Nonspherical and spherical characterization of ice in Hurricane Erin for wideband passive microwave comparisons

In order to better understand the characteristics and physical‐to‐radiative relationships of frozen hydrometeors in hurricane systems, computed brightness temperatures (TB) from 10.7 to 183 ± 10 GHz were compared with radiometric observations of Hurricane Erin (2001) from the NASA ER‐2 aircraft. The focus was on the high frequencies (≥85 GHz) that are particularly sensitive to frozen hydrometeors. In order to initialize the cloud profiles used in the radiative transfer calculations, data from airborne radars, dropsondes, and cloud models were used. Three different ice habit and size parameterizations were used with these cloud profiles to obtain the particle radiative signatures including (1) spherical particles with size distributions derived from in situ observations, (2) spherical “fluffy” snow and graupel particles with modified Marshall‐Palmer size distributions, and (3) a non‐spherical bullet rosette habit where the radiation attributes (scattering, absorption, and asymmetry properties) were computed using the Discrete Dipole Approximation. In addition, three different reflectivity to ice water content (Z‐IWC) relationships were used with the three habit and size parameterizations to provide a measure of the sensitivity of the Z‐IWC relationship. This work showed that both the scattering and asymmetry coefficients, along with the ice water content in each layer, play an important role in determining the resultant high frequency brightness temperatures. All low frequency (<40 GHz) calculations matched the observations with correlation coefficients greater than 0.9. At higher frequencies (> 90 GHz), correlation coefficients ranged from 0.7 to 0.92. Comparing between the three ice habit and size parameterizations showed less than a 0.2 difference in correlation coefficient, while the comparisons between the three Z‐IWC relationships caused changes of up to 0.15 in the correlation coefficients, but they had a significant effect on the mean differences between the observations and the calculations.

Influence of sea surface emissivity model parameters at L-band for the estimation of salinity

In order to prepare the Soil Moisture and Ocean Salinity (SMOS) mission, we present sensitivity studies on a two-scale sea surface emissivity model used to compute brightness temperatures ( T B ) from the Sea Surface Temperature (SST), the wind vector, the incidence angle and the Sea Surface Salinity (SSS). We analyse the impact of uncertainties of the model on T B at 1.4 GHz ( u 0 = 21 cm), namely we determine the influence of the parametrization of the sea surface permittivity and of the cutoff wavelength u d (the limit which separates the short scales of the wave spectrum from the large scales). Using two existing permittivity parametrizations we find differences on T B ranging from 0.4 to 1.1 K. For SST warmer than 12°C and incidence angles smaller than 35°, the difference on T B is a bias independent of SSS and weakly dependent on SST. For these small incidence angles, the choice of the cutoff wavelength does not lead to significant differences on T B . For incidence angles larger than 40°, the permittivity parametrization and the choice of u d are more critical, resulting in a variation of T B of several tenths of a Kelvin.

A combined radiative transfer model for sea ice, open ocean, and atmosphere

A radiative transfer model to compute brightness temperatures in the microwave frequency range for polar regions including sea ice, open ocean, and atmosphere has been developed and applied to sensitivity studies and retrieval algorithm development. The radiative transfer within sea ice is incorporated according to the “many layer strong fluctuation theory” of Stogryn [1986, 1987] and T. Grenfell [Winebrenner et al., 1992]. The reflectivity of the open water is computed with the three‐scale model of Schrader [1995]. Both surface models supply the bistatic scattering coefficients, which define the lower boundary for the atmospheric model. The atmospheric model computes the gaseous absorption by the Liebe et al. [1993] model. Scattering by hydrometeors is determined by Mie or Rayleigh theory. Simulated brightness temperatures have been compared with special sensor microwave imager (SSM/I) observations. The comparison exhibits shortcomings of the ice model for 37 GHz. Applying a simple ad hoc correction at this frequency gives consistent comparison results within the range of observational accuracy. The simulated brightness temperatures show the strong influence of clouds and variations of wind speed over the open ocean, which will affect the sea ice retrieval even for an ice‐covered ocean. Simulated brightness temperatures have been used to train a neural network algorithm for the total sea ice concentration, which accounts for these effects. Sea ice concentrations sensed from the SSM/I data using the network and the NASA sea ice algorithm show systematic differences in dependence on cloudiness.

Polarization Observations of 1720 MHz OH Masers toward the Three Supernova Remnants W28, W44, and IC 443

We present arcsecond resolution observations from the VLA with full Stokes polarimetry of the ground-state satellite line of the hydroxyl molecule (OH) at 1720.53 MHz (2Π3/2, J = 3/2, F = 2 → 1) toward three Galactic supernova remnants: W28, W44, and IC 443. The total number of individual OH (1720 MHz) spots we detect in each of these three remnants is 41, 25, and 6, respectively. The OH (1720 MHz) features appear to lie along the edge of radio continuum emission from the supernova remnants, but they are displaced behind the leading edge of the shock as traced by the synchrotron emission. The brightness temperatures of the OH (1720 MHz) emission features range from 2 × 104 to 108 K, convincingly demonstrating the maser nature of the OH (1720 MHz) features. We argue that the partially resolved angular diameters that we measure for the masers are neither intrinsic sizes nor scattering disks, but result from a blend of several unresolved maser features near the same velocity. Thus, our computed brightness temperatures are lower limits to the true values. The characteristic antisymmetric S profile, indicative of Zeeman splitting in the weak-field case, is identified in the Stokes V spectrum of several of the brighter maser spots. The derived line-of-sight magnetic fields are of order 0.2 mG and are remarkably constant in both direction and magnitude over regions several parsecs apart. These are the first measurements of postshock magnetic fields in supernova remnants and demonstrate the importance of magnetic pressure in these molecular shocks. The velocity dispersion of the maser features is typically less than a few km s-1, and, except in the special case of W28, the mean maser velocity is equal to the systemic velocity of the remnant. We suggest that the maximum amplification of the maser transition will occur when the acceleration produced by the shock is transverse to the line of sight. Additional support for this point comes from the location of the masers in IC 443, and from molecular observations that allow the shock geometry to be determined. All of our observations are consistent with a model in which the OH (1720 MHz) is collisionally excited by H2 molecules in the postshock gas heated by a nondissociative shock. Finally, we end with a discussion of the importance of supernova remnants with OH (1720 MHz) maser emission as promising candidates to conduct high-energy searches for the sites of cosmic-ray acceleration.

Comparison of 6.7‐μm radiances computed from aircraft soundings and observed from GOES

Radiances observed by the GOES 6.7‐μm channel are compared to computations from simultaneous aircraft soundings of the temperature and moisture profiles. The soundings were measured using high precision instruments aboard the National Center for Atmospheric Research Sabreliner. An advanced line‐by‐line radiative transfer model is used to simulate the satellite observations. This comparison shows that the computed brightness temperatures overestimate the observed brightness temperatures but reproduce the full range in observed brightness temperatures. The computed brightness temperatures are, on average, 2.6 K higher than observed. Possible sources for this bias are investigated, such as transmission through a cloudy lower boundary, scattering by thin high‐level cirrus, and water vapor continuum absorption. It is likely that a combination of uncertainties in the radiative properties of water and unaccountable ice or aerosol absorption contributes to the disparity.

Radio emission from coronal streamers

Different models of coronal streamers are used to calculate the radio brightness temperature at the wavelengths of observation of the Nancays Radioheliograph. Calculation are performed assuming the location of the streamer both on the disk and at the limb. Their comparison with observations show that a satisfactory agreement with a particular model can be found in the shape and in the relative enhacement of the streamer with respect to the quiet Sun, although the absolute values of the computed brightness temperatures are much higher than the observed ones.

Numerical modeling of passive microwave O2 observations over precipitation

The microwave scattering and emission properties of precipitation cells are investigated by comparing 118‐GHz radiometric observations with a planar‐stratified numerical radiative transfer model. Liquid and frozen hydrometeors are modeled as spherical Marshall‐Palmer and Sekhon‐Srivastava distributed Mie scattering polydispersions, respectively, with Henyey‐Greenstein phase functions. Comparisons are made between computed brightness temperatures based on weather radar observations of a convective precipitation cell couplet during the Cooperative Huntsville Meteorological Experiment (COHMEX), 1986, and brightnesses observed coincidentally by an airborne imaging spectrometer. Agreement between observed and computed brightness perturbations is within 10% over the radiometrically opaque, mature regions of the cell, although the model brightnesses over the partially transparent anvil region are highly sensitive to the assumed mean ice particle size. The sensitivity of the 118‐GHz channels to temperature at various levels in the troposphere is exhibited, in the presence of precipitation, through the perturbed temperature weighting functions. Calculations using the perturbed and nonscattering weighting functions suggest 118‐GHz transparent‐channel cell top reflectivities of up to 50% in the convective core region and 6% in the anvil region. A rain cell model parameterized by cell top altitude and total (liquid and ice) water density is used to analyze the information content of 118‐GHz precipitation cell spectra. The model suggests that at 118 GHz, cells with uniform water densities greater than 0.5 g m−3 are opaque, and cells with uniform densities less than 0.1 g m−3 are transparent. The dominant 118‐GHz spectral modes are found to contain useful information on the cell top altitude and can be used to detect transparent anvil regions. The physical retrieval of cell water density is shown to be facilitated by coincident observations using similar weighting‐function channels at 53 and 118 GHz, although the retrieval is sensitive to the assumed mean ice particle size.

Determination of snowpack properties from satellite passive microwave measurements

The use of satellite microwave data to determine snowpack properties is investigated through calculation of theoretical microwave brightness temperatures and comparison of the computed brightness temperatures with actual satellite microwave measurements. Archived data from the Nimbus-5 and Nimbus-6 Electrically Scanning Microwave Radiometers (ESMR), as well as data from the Nimbus-7 Scanning Multifrequency Microwave Radiometer (SMMR), are analyzed for a study area in the north-central United States. The results of the investigation indicate that snow boundaries can usually be defined by the 37-GHz or 18 (19) GHz data because of the sharp decrease in brightness temperature when going from land to a snow surface. For dry snow conditions, the 37 GHz data display a decrease in brightness temperature with snow depth due to the stronger volume scattering effect of the deeper snow; at 18–19 GHz, the sensitivity to snow depth is not as significant. The onset of snowmelt can be determined at both microwave frequencies investigated (18–19 GHz and 37 GHz) because of the significant increase in the brightness temperature with melting due to the decrease in volume scattering that occurs in the presence of free water.

A two-scale scattering model for foam-free sea microwave brightness temperatures

A two-scale surface comprised of small irregularities superimposed on large undulations is defined from an empirical sea spectrum. The scattering of microwave radiation from the surface is modeled by combining geometric-optics and small-scale perturbation theory in a manner consistent with the conservation of energy. The two-scale scattering model includes multiple reflections and shadowing effects. Foam-free sea brightness temperatures are computed and show that the presence of small-scale roughness increases the brightness temperature at nadir. Comparisons with experimental data show better agreement for the two-scale model than for the geometric-optics model. Changing the small-scale perturbation parameter from 0.125 to 0.25 slightly affects the computed brightness temperatures.