Surface Microwave Emission Research Articles

Further Developments in Estimating Cloud Liquid Water over Land Using Microwave and Infrared Satellite Measurements

Abstract Refinements and improvements of an earlier technique to retrieve the cloud liquid water path (LWP) of nonprecipitating clouds over land surfaces using Special Sensor Microwave/Imager (SSM/I) 85.5-GHz measurements are presented. These techniques require estimates of the microwave surface emissivity, which are derived in clear-sky regions from SSM/I measurements and window infrared measurements from the Visible and Infrared Spin Scan Radiometer on GOES-7. A comparison of forward model calculations with SSM/I measurements in clear regions demonstrates that over a 7-day period the surface emissivities are stable. To overcome limitations in the single-channel retrieval method under certain situations, a new method is developed that uses a normalized polarization difference (NPD) of the brightness temperatures. This method has the advantages of providing estimates of the LWP for low clouds and being extremely insensitive to the surface skin temperature. Radiative transfer simulations also show that the...

Polarimetric brightness temperatures of sea surfaces measured with aircraft K- and Ka-band radiometers

Dual-frequency (19 and 37 GHz), multi-incidence measurements of the Stokes parameters of sea surface microwave emission are reported. A series of aircraft polarimetric radiometer flights were carried out over the National Data Buoy Center (NDBC) moored buoys deployed off the northern California coast in July and August 1994. Measured radiometric temperatures showed a few Kelvin azimuth modulations in all Stokes parameters with respect to the wind direction. Wind directional signals observed in the 37-GHz channel were similar to those in the 19-GHz channel. This indicates that the wind direction signals in sea surface brightness temperatures have a weak frequency dependence in the range of 19-37 GHz. Harmonic coefficients of the wind direction signals were derived from experimental data versus incidence angle. It was found that the first harmonic coefficients, which are caused by the up and downwind asymmetric surface features, had a small increasing trend with the incidence angle. In contrast, the second harmonic coefficients, caused by the up and crosswind asymmetry, showed significant variations in T/sub v/ and U data, with a sign change when the incidence angle increased from 45/spl deg/ to 65/spl deg/. Besides the first three Stokes parameters, the fourth Stokes parameter, V, which had never been measured before for sea surfaces, was measured using our 19-GHz channel. The Stokes parameter V. Has an odd symmetry just like that of the third Stokes parameter U, and increases with increasing incidence angles. In summary, sea surface features created by surface winds are anisotropic in azimuth direction and modulate all Stokes parameters of sea surface microwave brightness temperatures by as large as a few Kelvin in the range of incidence angles from 45/spl deg/ to 65/spl deg/ applicable to spaceborne observations.

The influence of bubble plumes on air‐seawater gas transfer velocities

Laboratory results have demonstrated that bubble plumes are a very efficient air‐water gas transfer mechanism. Because breaking waves generate bubble plumes, it could be possible to correlate the air‐sea gas transport velocity kL with whitecap coverage. This correlation would then allow kL to be predicted from measurements of apparent microwave brightness temperature through the increase in sea surface microwave emissivity associated with breaking waves. In order to develop this remote‐sensing‐based method for predicting air‐sea gas fluxes, a whitecap simulation tank was used to measure evasive and invasive kL values for air‐seawater transfer of carbon dioxide, oxygen, helium, sulfur hexafluoride, and dimethyl sulfide at cleaned and surfactant‐influenced water surfaces. An empirical model has been developed that can predict kL from bubble plume coverage, diffusivity, and solubility. The observed dependence of kL on molecular diffusivity and aqueous‐phase solubility agrees with the predictions of modeling studies of bubble‐driven air‐water gas transfer. It has also been shown that soluble surfactants can decrease kL even in the presence of breaking waves.

Some features observed by the L‐band push broom microwave radiometer over the Konza Prairie during 1985–1989

Airborne L‐band radiometric measurements were conducted over the Konza Prairie near Manhattan, Kansas, in the summers of 1985, 1987, 1988, and 1989 to study the relationship among surface microwave emission, soil moisture, and vegetation cover. The annual surface treatments that were applied to the watersheds in the experimental area appeared to show a significant impact on the surface microwave emission. A watershed that was burned every year showed a better sensitivity to soil moisture variation than those burned less frequently. This feature persisted even though the radiometric measurements were made over those watersheds that were burned in the same year. It was concluded that the burning process might not completely remove a thatch layer of efficient microwave absorption, which was developed through years of accumulation of senescent vegetation. Results from the analysis of these radiometric data sets also suggest the need of an adequate estimation of vegetation biomass in order to obtain a reliable retrieval of surface soil moisture from L‐band radiometric measurements. On the basis of the data acquired from the 1987 and 1989 field campaigns, the push broom microwave radiometer (PBMR) measurements are likely to give errors of the order of ±0.065 g/cm3 in surface soil moisture estimation if there are no measurements of vegetation biomass. Measurements of vegetation biomass to an accuracy of ±0.46 kg/m2 improve the corresponding PBMR estimation of surface soil moisture to an accuracy of ±0.032 g/cm3.

The Response of the SSM/I to the Marine Environment. Part II: A Parameterization of the Effect of the Sea Surface Slope Distribution on Emission and Reflection

Based on a geometric optics model and the assumption of an isotropic Gaussian surface slope distribution, the component of ocean surface microwave emissivity variation due to large-scale surface roughness is parameterized for the frequencies and approximate viewing angle of the Special Sensor Microwave/Imager. Independent geophysical variables in the parameterization are the effective (microwave frequency dependent) slope variance and the sea surface temperature. Using the same physical model, the change in the effective zenith angle of reflected sky radiation arising from large-scale roughness is also parameterized. Independent geophysical variables in this parameterization are the effective slope variance and the atmospheric optical depth at the frequency in question. Both of the above model-based parameterizations are intended for use in conjunction with empirical parameterizations relating effective slope variance and foam coverage to near-surface wind speed. These empirical parameterizations are the subject of a separate paper.

The information content of data from satellite sounding systems: A simulation study

The potential of the TIROS Operational Vertical Sounder (TOVS) and the Advanced TOYS (ATOVS) for providing information on the vertical profiles of atmospheric temperature and humidity is investigated. the theoretical performances of these systems are examined primarily in terms of their ability to contribute information to a numerical weather prediction (NWP) system. the covariance of retrieval error expected from an optimal nonlinear inversion scheme, which retrieves simultaneously all the atmospheric and surface variables which significantly affect the measured radiances, is calculated theoretically and compared with the covariance of error assumed in the prior information which constrains the inversion. Performance is assessed for different combinations of radiometers, corresponding to present and future operational systems, and for different sets of assumed conditions: in the atmospheric profile, the cloud conditions, the surface characteristics and the assumed prior knowledge of these variables. The potential performance of ATOVS is shown to be a considerable improvement on the present TOVS in many respects. the value of retaining a combined infrared and microwave system is confirmed. It is demonstrated that, although the ability of these sounding systems to improve on an NWP model's analyses of temperature and humidity is rather limited, the potential for improving on the analysis of layer quantities is much greater. Some new ideas are presented for extending current retrieval methods to accommodate the additional problems presented by ATOVS data. Methods are proposed for treating the effects of microwave surface emissivity and cloud liquid water in a simultaneous inversion.

The information content of data from satellite sounding systems: A simulation study

AbstractThe potential of the TIROS Operational Vertical Sounder (TOVS) and the Advanced TOYS (ATOVS) for providing information on the vertical profiles of atmospheric temperature and humidity is investigated. the theoretical performances of these systems are examined primarily in terms of their ability to contribute information to a numerical weather prediction (NWP) system. the covariance of retrieval error expected from an optimal nonlinear inversion scheme, which retrieves simultaneously all the atmospheric and surface variables which significantly affect the measured radiances, is calculated theoretically and compared with the covariance of error assumed in the prior information which constrains the inversion. Performance is assessed for different combinations of radiometers, corresponding to present and future operational systems, and for different sets of assumed conditions: in the atmospheric profile, the cloud conditions, the surface characteristics and the assumed prior knowledge of these variables.The potential performance of ATOVS is shown to be a considerable improvement on the present TOVS in many respects. the value of retaining a combined infrared and microwave system is confirmed. It is demonstrated that, although the ability of these sounding systems to improve on an NWP model's analyses of temperature and humidity is rather limited, the potential for improving on the analysis of layer quantities is much greater.Some new ideas are presented for extending current retrieval methods to accommodate the additional problems presented by ATOVS data. Methods are proposed for treating the effects of microwave surface emissivity and cloud liquid water in a simultaneous inversion.

The RADTRAN microwave surface emission models

The AFGL RADTRAN microwave atmospheric transmission/brightness temperature computer code has been enhanced by a surface emissivity modeling subpackage. This enhancement will provide realistic calculations of frequency-dependent polarized surface emissivity to support simulations of Earth-viewing microwave sensing systems. These models are appropriate for the frequency range 1-40 GHz. Two distinct modeling approaches have been applied: one based on wave theory for random discrete scatterers, and one based on radiative transfer theory for continuous random media. The former approach is used to model the ocean surface, various forms of sea ice, and snow over land, whereas the latter is used to treat soils and vegetation. Surface emissivity values obtained from these models as functions of polarization, frequency, and look angle are illustrated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of SEASAT SMMR wind speed measurements

The SEASAT scanning multichannel microwave radiometer (SMMR) is able to measure the wind speed at the ocean surface through the change in ocean surface microwave emissivity caused by the wind. In this paper we compare the SMMR‐derived wind speeds to the wind speeds derived from an active microwave scatterometer also aboard the SEASAT, the SEASAT A scanning scatterometer (SASS), Four orbits that passed over the severe storm that damaged the Queen Elizabeth II are examined in detail. These orbits and five others were used to investigate effects which degrade the SMMR wind retrievals. When the data are filtered for such effects, we find that the SMMR winds agree with the SASS winds with a scatter (1σ) of less than 2 m/s about a bias of 1 to 2 m/s.