Jungang Miao

Atmospheric water vapor over Antarctica derived from Special Sensor Microwave/Temperature 2 data

In polar regions, satellite microwave radiometry has not been successful in measuring the total water vapor (TWV) in the atmosphere. The difficulties faced in these regions arise from the very low water vapor burden of the atmosphere and the large and highly variable emissivities of ice surfaces in the microwave frequency range. By exploiting the advantages of the Special Sensor Microwave/Temperature 2 (SSM/T2), a method is developed to retrieve TWV over Antarctica from satellite data. This method shows very low sensitivities to the change of surface emissivity and to the presence of water clouds. However, ice clouds may have considerable effects. Results of radiative transfer model simulation show that they may cause one to underestimate TWV using the proposed method and that the amount of underestimation is proportional to the ice water path of the ice cloud. Validations using radiosonde measurements and numerical model analyzes suggest that SSM/T2 retrievals have a high accuracy (maximum error <10%) as long as TWV is <4.0 kg m−2. Above this value, retrievals show a systematic overestimation. Presumably, this is a result of the seasonal difference between the validation and the training radiosonde data sets. TWV retrievals of 1 years SSM/T2 data show clearly the seasonal variation of water vapor over Antarctica. Throughout the year the mean TWV over West Antarctica is nearly twice as high as that over East Antarctica; the temporal fluctuation of TWV over West Antarctica is also significantly stronger than over East Antarctica. This suggests that precipitation and water vapor transport in West Antarctica are more active than in East Antarctica. Using the same years TWV data, we estimated the mean residence time of atmospheric water vapor over the Antarctica to be merely 3–4 days. This, however, is much shorter than the global mean of 9–10 days.

Signature of clouds over Antarctic sea ice detected by the Special Sensor Microwave/Imager

A method to detect the cloud signature (mainly the cloud liquid water) over the sea ice-covered Weddell Sea in the Austral summer season is presented. By using the polarization differences at the two high frequency channels (i.e., 37 and 85 GHz) of the special sensor microwave/imager (SSM/I), a new quantity called R-factor is defined. Using the R-factor, the atmospheric signal can be easily separated from the surface signal and, more importantly, the surface signal and its variation can be strongly suppressed, especially in regions with low ice concentrations. In regions with high ice concentrations, other sea ice parameters like snow cover play an important role as indicated by simulations using in situ measured sea ice emissivities and observed by the SSM/I. Under the assumption that the sea ice parameters remain sufficiently stable within a short period (e.g., ten days), a method is proposed to determine the background term from SSM/I measurements, allowing the detection of the cloud signature. A comparison with a known SSM/I cloud liquid water algorithm over the open ocean shows a high degree of correlation (0.958) among the cloud signatures detected by the two algorithms. On January 2 and 3, 1996, a low pressure system moved into the sea ice-covered Weddell Sea. Its cloud signature detected using the R-factor method compares well with coincident observations from both visible and infrared sensors.

A Future Millimeter/Sub–Millimeter Radiometer for Satellite Observation of Ice Clouds

The instrument concept of a future spaceborne millimeter/sub-millimeter radiometer is proposed in this paper for the remote sensing of ice clouds from satellite. The proposed radiometer is expected to operate at a series of frequencies within the millimeter and sub-millimeter wave range from 150 to about 900 GHz. Five frequencies are selected in the atmospheric windows, i.e., 150, 220, 463, 683, 874 GHz, and at each frequency there are two orthogonally polarized channels. Three water vapor channels located close to 183.31 GHz are also included in this instrument, since they can provide water vapor information, which is needed for ice cloud parameter retrieval. To simplify system design and test, a modular design philosophy is followed in the receiver frontend design and two antennas are used separately for the millimeter and sub-millimeter channels. Overall, the instrument requirements can be met with todays technology, except for the channels at the highest frequencies, where the radiometric sensitivity can be larger than the required 1.0 K for the 10 km spatial resolution (2.5 ms integration time). However, this situation can be improved by averaging neighboring pixels in data processing if certain compromise in the spatial resolution can be made at these frequencies.

Retrieval of total water vapor in polar regions using SSM/T2 channels

A method to retrieve the total water vapor in the cloudless atmosphere using SSM/T2 channels is proposed based on a unique property of three contiguous channels situated on the flank of the water vapor absorption line at 183.31 GHz. One of its advantages is its independence to the surface emission, which is the main disturbing factor in retrieving atmospheric parameters from passive satellite measurements. Due to the high sensitivity of the SSM/T2 channels to water vapor, this method is suitable to the total water vapor retrieval for dry polar atmospheres. An algorithm is constructed for austral winter cases through model simulation using radiosonde profiles.

The polarization characteristics of randomly oriented nonspherical ice particles in mm and sub-mm frequency range: Implications to the remote sensing of cirrus clouds using satellite microwave radiometry

The depolarization effects of randomly oriented nonspherical ice particles are studied for frequencies ranging from 90 to /spl sim/900 GHz. It is found that, given the particle shape and the working frequency, the brightness temperature difference between the vertical and the horizontal polarization measured by a space-borne radiometer is only sensitive to ice particles of sizes within a certain range. The center of this range moves from >1000 /spl mu/m to /spl sim/120 /spl mu/m for nearly spherical particles when the frequency changes from 90 to 874 GHz. The particle size given here is the median mass equivalent sphere diameter. This range also changes with the particle shape. Generally, particles with a greater aspect ratio show a stronger depolarization effect by larger sizes. These features suggest that radiometric measurements from satellite on the polarization difference may be useful in determining ice particle size and shape in the cirrus clouds.

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 Administrations 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.

Influence of surface radiation on retrieval of cloud liquid water and precipitable water vapor using AMSR-E data

The Advanced Microwave Scanning Radiometer for EOS (AMSR-E) was launched on May 4, 2002 aboard the NASAs Earth Orbiting System (EOS) Aqua Satellite. This new instrument measures the Earths radiation in two orthogonally polarized channels at 6 frequencies extending from 6.925 to 89.0 GHz. Here we study the influence of surface radiation on retrieval of cloud liquid water (CLW) and precipitable water vapor (PWV) using multifrequency polarized measurements of AMSR-E. Based on this preliminary investigation, a physical method to simultaneously retrieve CLW and PWV is proposed to build.

Single scattering of partly oriented aspherical cloud ice crystals at sub-millimeter wavelengths

The recently suggested sub-millimeter sensors for the global observation of cirrus clouds all bear several channels widely separated from each other and spanning the range from 180 to 880 GHz. While the potential for retrieving the integrated ice water content and the mean effective particle size of ice clouds has been investigated, the quantitative retrieval of particle shape and orientation information is still an active research area. Here we investigate the influence of the parameters of a Gaussian distribution of the horizontal particle orientation on the single scattering cross section. It is only slightly affected, but the polarization signal is reduced to 90% of the completely horizontally oriented case if the standard deviation /spl sigma//sub /spl beta// of the horizontal distribution is 15/spl deg/ and reduced to 50% if /spl sigma//sub /spl beta// = 30/spl deg/.

Signature of clouds over sea ice detected by the Special Sensor Microwave/Imager (SSM/I)

Quantitative determination of cloud parameters over sea ice using the radiometric measurements at SSM/I frequencies has been difficult due to the strong and highly variable surface emissions from sea ice. The authors try to suppress these variations using a ratio of polarisation differences at two frequencies, which is called the R-factor. Using a simple radiative transfer equation for a one-layer atmosphere, they proved that the surface signal and the atmospheric signal can be easily separated in the logarithmic expression of this R factor. If it is valid to assume that the surface parameter changes within two days are not very significant, the day-to-day variation of the cloud parameters (mainly the cloud liquid water) over sea ice can be detected using the SSM/I measurements.

Towards retrieval of Antarctic sea ice using the SSM/I 85.5 GHz polarisation difference

In many sea ice retrieval algorithms the 85.5 GHz channels of the SSM/I (Special Sensor Microwave Imager) are not used because these channels are much more affected by weather systems than the lower frequency channels of the SSM/I. Despite the fact that the 85.5 GHz channels would allow for twice the spatial resolution of the lower ones the latter are used operationally for sea ice retrieval in polar regions. Here we propose an algorithm which uses the polarization at 85.5 GHz, PR(85), for the calculation of the total sea ice concentration in the Antarctic. Using monthly means of the PR(85) and its variation with time provides mean ice concentrations in agreement with ice concentrations calculated with the NASA-Team algorithm for values above 50%.