John F. Galantowicz

A land surface process/radiobrightness model with coupled heat and moisture transport for prairie grassland

The authors present a biophysically based, one-dimensional hydrology/radiobrightness (1dWR) model for prairie grassland that is subject to solar heating, radiant heating and cooling, precipitation, and sensible and latent heat exchanges with the atmosphere. The 1dH/R model consists of two modules, a one-dimensional hydrology (1dH) module that estimates the temperature and moisture profiles of the soil and the canopy and a microwave emission module that predicts radiobrightness (R). The authors validate the 1dH/R model by comparing its predictions with data from a field experiment. The model was forced by meteorological and sky radiance data from their Radiobrightness Energy Balance Experiment (REBEX-1) on prairie grassland near Sioux Falls, SD, during the fall and winter of 1992-1993. Model predictions were compared with 995 consecutive REBEX-1 observations over a 14-day period in October. Average errors (predicted-measured) for canopy temperature are 1.1 K with a variance of 3.72 K/sup 2/, for soil temperatures at 2-, 4-, 8-, 16-, 32-, and 64-cm depths are 2 K with a variance of 4 K/sup 2/, and for H-polarized brightnesses are 0.06 K with a variance of 1.30 K/sup 2/ at 19 GHz and 6.01 K with a variance of 6.04 K/sup 2/ at 37 GHz. The model overestimates the 37-GHz brightness because they have not included scatter darkening within the vegetation canopy in the model. They use the 1dH/R model to simulate a 60-day dry-down of prairie grassland in summer. For grass with a column density of 3.7 kg/m/sup 2/ and soil with an initially uniform moisture content of 38% by volume, the upper 5 mm of soil dries to 27% by the end of the simulation. The corresponding L-band brightness increases from an initial 143 K to a final 163 K. In contrast, none of the special sensor microwave/imager (SSM/I) radiobrightnesses nor the radiobrightness thermal inertia (RTI) technique, either at L-band or at any SSM/I frequency, exhibits significant sensitivity to the soil dry-down.

Freeze/thaw classification for prairie soils using SSM/I radiobrightnesses

Data from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) have been used to classify snow-free soils in the northern Great Plains as either frozen or thawed. The technique is based on differing sensitivities among SMMR radiobrightness frequencies to liquid moisture and volume scattering in the upper few millimeters of bare soil. The SMMR is no longer active. A current near-equivalent is the Special Sensor Microwave/Imager (SSM/I). The authors demonstrate that SSM/I radiobrightnesses also exhibit differential sensitivities to liquid water and volume scattering in frozen soil despite their higher frequencies. They find that the best classification discriminants for SSM/I data are a combination of the 37-GHz V-pol radiobrightnesses and the 19-to-37-GHz V-pol spectral gradients. They also examine the sensitivity of the classification to atmospheric emission and absorption and find little effect.

The radiobrightness thermal inertia measure of soil moisture

Radiobrightness thermal inertia (RTI) is proposed as a method for using day-night differences in satellite-sensed radiobrightness to monitor the moisture of Great Plains soils. Diurnal thermal and radiobrightness models are used to examine the sensitivity of the RTI method. Model predictions favor use of the 37.0 and 85.5 GHz, H-polarized channels of the Special Sensor Microwave/Imager (SSM/I). The model further predicts that overflight times near 2:00 AM/PM would be nearly optimal for RTI, that midnight/noon and 4:00 AM/PM are nearly as good, but that the 6:00 AM/PM overflight times of the current SSM/I are particularly poor. Data from the 37.0 GHz channel of the Scanning Multichannel Microwave Radiometer (SMMR) are used to demonstrate that the method is plausible. >

A volume scattering explanation for the negative spectral gradient of frozen soil

A combination of the 37 GHz radiobrightness (Tb37) and the 10.7 to 37 GHz spectral gradient (S,) has been used to classify prairie soil as either frozen or thawed. Wintertime data from the Scanning Multichannel Microwave Radiometer (SMMR) for northern Great Plains soils tend to fall along the line Sg = -6.42 + 0.026 Tb37 K/GHz for Tb37 < -247 K. Models of emission from homogeneous, frozen soils do not exhibit this behavior. The likely cause of a negative S, is volume scatter darkening like that recognized for microwave emission from snow. While frozen soils are not as transparent to microwaves as is snow, optical thicknesses of frozen soils can become large enough at low temperatures for significant volume scattering to become plausible. We use a radiative transfer version of first order, multiple scattering theory to develop an analytical relationship between brightness temperature and the scattering albedo. This relationship combined with the empirical data for S, yield the approximation to scattering albedo, WO + 6.1 Wo2 = 15.1 (273/T0 - 1) where To is the physical temperature in Kelvin and To < 273.

Seasonal snowpack radiobrightness interpretation using a SVAT‐linked emission model

This study investigates the link between seasonal snowpack radiobrightness dynamics and land-atmosphere energy and moisture fluxes using a 7 month observational record and snowpack simulations. Experimental data are presented from the first Radiobrightness Energy Balance Experiment (REBEX 1) which combined continuous micrometeorological observations with ground-based measurement of terrain radiobrightnesses at Special Sensor Microwave/Imager (SSM/I) frequencies. The REBEX 1 site near Sioux Falls, South Dakota, is characteristic of the northern Great Plains grasslands and climate. The experiment period from October 1992 to April 1993 spanned vegetation senescence, snowpack formation and evolution, and spring thaw. Patterns in the data are analyzed using a snowpack evolution and radiobrightness model driven by REBEX 1 energy and moisture flux measurements. The snowpack model differs from most soil-vegetation-atmosphere transfer (SVAT) schemes in the level of detail used in modeling the near-surface medium. Snowpack temperature and structural details are necessary in order to model both the microwave thermal source and snowpack interactions and the continuing metamorphism of the snow. Modeled radiobrightness variation at 19, 37, and 85 GHz agree with observations to within 29, 16, and 13% (root mean square), respectively, over a 53 day test period. Both observed and modeled radiobrightness dynamics show sensitivity to (1) unfrozen soil moisture content at 19 and 37 GHz, (2) snow grain size and sky brightness variation (primarily at 85 GHz), and (3) snowpack partial melt and refreeze cycles (all frequencies). Furthermore, we find that snowpack stratification does not drive brightness variation in these data even at 19 GHz where snowpack penetration is greatest.

Sources of discrepancies between satellite‐derived and land surface model estimates of latent heat fluxes

Monthly-average estimates of latent heat flux have been derived from a combination of satellite-derived microwave emissivities, day-night differences in land surface temperature (from microwave AMSR-E), downward solar and infrared fluxes from ISCCP cloud analysis, and MODIS visible and near-infrared surface reflectances. The estimates, produced with a neural network, were compared with data from the Noah land surface model, as produced for GLDAS-2, and with two alternative estimates derived from different datasets and methods. Areas with extensive, persistent, substantial discrepancies between the satellite and land surface model fluxes have been analyzed with the aid of data from flux towers. The sources of discrepancies were found to include problems with the model surface roughness length and turbulent exchange coefficients for midlatitude cropland areas in summer, inaccuracies in the precipitation data that were used as forcing for the land surface model, and model underestimation of transpiration in some forests during dry periods. At the tower sites analyzed, agreement with tower data was generally closer for our satellite-derived fluxes than for the land surface model fluxes, in terms of monthly averages.

A volume emission model for the radiobrightness of prairie grass

The radiobrightness of snow-free northern prairie at the SSM/I frequencies of 19.35, 37.0, and 85.5 GHz is dominated by absorption and emission by the grass canopy. The authors measured the radiobrightness at SSM/I frequencies of grassland near Sioux Falls, South Dakota, every 15 minutes from October, 1992, through early April, 1993. The apparent H-polarized emissivities for snow-free periods were generally above 0.95. These high emissivities can only be explained by emission from the grass canopy. The vertical distributions of mass and moisture in prairie grass were used to develop a volume emission model for grass over moist soil. The volume emission model was derived from a formally correct version of the radiative transfer equation where radiant intensities were normalized by the refractive index squared. For the 7 cases that were analyzed, scatter darkening was insignificant at 19.35 GHz and significant in only two cases at the higher SSM/I frequencies.<<ETX>>

Radiobrightness signatures of energy balance processes: melt/freeze cycles in snow and prairie grass covered ground

The authors are developing physically based, one-dimensional heat flow models for snow and soil which, when linked to simple microwave emission models, will predict the temporal radiobrightness response to insolation and atmospheric thermal forcing. The authors use a soil temperature model to analyze the thermal infrared and spectral radiobrightness signatures of snow and grass covered ground undergoing melt/freeze cycles. The data were taken in January and February of 1993 during the first Radiobrightness Energy Balance Experiment (REBEX-1) near Sioux Falls, South Dakota, with horizontally polarized radiometers at 19.35, 37, and 85.5 GHz (SSM/I frequencies). The soil portion of the heat flow model is described and example soil diurnal temperature cycles are compared to those from REBEX-1. It is shown that the diurnal amplitude of the radiobrightness cycle exceeds that of the surface temperature cycle when freeze/thaw transitions occur.<<ETX>>

An AMSR-E Land Surface Microwave Emissivity Database

Accurate knowledge of local surface emissivity is required for lower troposphere microwave remote sensing over land and for land surface parameter retrievals. Ideally, for a stand-alone microwave system (i.e., without an external source of surface temperature), a priori emissivity accuracies of 0.01 or less are needed to minimize the impact of cloud liquid water on temperature and water vapor retrievals and to improve surface temperature retrievals to 2 K or better. We are developing a system for land surface microwave emissivity retrieval and using it to derive emissivities in the AMSR-E channels over a full year. The system now incorporates both AMSR-E and SSM/I brightness temperatures and MODIS-derived land surface temperature (LST) products. We have examined the temporal variability of retrieved local surface emissivities and describe approaches developed to identify and minimize sources of error in the retrieval. The emissivity retrieval system is a precursor for a dynamic emissivity database to be fully implemented for NPOESS CMIS with coincident VIIRS LST observations available up to six times per day. Keywords-AMSR-E; Aqua; MODIS; CMIS; NPOESS; microwave emissivity; microwave radiometry; 1-D VAR.

A SSM/I Radiometer Simulator for Studies of Microwave Emission from Soil

We describe a ground based simulator of the Defence Meteorological Satellite Program’s Special Sensor Microwaveflmager (DMSP SSW) and its integration with micrometeorological instrumentation for an investigation of microwave emission from moist and frozen soils. The simulator consists of three single polarization radiometers at frequencies of 19.35, 37.0 and 85.5 GHz which are capable of both Dicke Radiometer and Total Power Radiometer modes of operation. The radiometers are mounted on a portable, 10 m tower, and are designed for untended operation through a local computer and a daily telephone link to our laboratory. We describe the functional characteristics of the radiometers and their field deployment configuration and give an example of performance parameters. The 19 and 37 GHz radiometers are to be used to verify temporal microwave emission models for freezing and thawing soils [ 11, and the 37 and 85 GHz radiometers are to be used to test our proposed method for estimating soil moisture based upon dayhight radiobrightness differences [2]. The correlative measurements for both investigations include 10 meter wind speed, soil temperature profile with depth, infrared surface temperature, total and net radiation, air temperature and humidity, and rainfall.