Advanced Microwave Precipitation Radiometer Calibration and Geophysical Retrievals
Abstract
The Advanced Microwave Precipitation Radiometer (AMPR) is a downward-pointing radiometer that has flown on multiple NASA aircraft. AMPR performs cross-track scanning during flight and records mixed-polarization brightness temperatures (Tb) at four frequencies: 10.7, 19.35, 37.1, and 85.5 GHz, which can be converted to true horizontally and vertically polarized Tb via dual-polarization deconvolution using data from the two orthogonal receivers for each AMPR channel.
Scene Tb biases in dual-pol AMPR data have been observed, and are hypothesized to be separable into absolute and cross-scan asymmetry components. The absolute bias is typically a result of uncertainties in the measured calibration load Tb, but the cross-scan asymmetry is a result of inaccurate assumption of the instrument measurement geometry and cross-pol contamination between the polarization channels. We derive a relationship between cross-pol level as well as various instrument and polarization angle errors that seems to explain the observed cross-scan asymmetry. Using a forward model with adjusted instrument geometry, improved polarization deconvolution is achieved. In addition, AMPR deployments on the NASA P-3 aircraft require the use of a radome to protect other instruments sharing the same bomb bay as AMPR. As no radome is 100% transparent, particularly given the multifrequency nature of AMPR, this requires compensation for the transmissivity of the radome as a function of frequency, polarization, scan angle, and radome temperature. These transmissivity compensation factors were developed using a mixture of sky test data as well as flight data over well-described open ocean scenes. Finally, regression equations have been developed to retrieve three geophysical parameters using AMPR Tb: integrated cloud liquid water (CLW), atmospheric water vapor (WV), and ocean-surface wind speed (WS). The regression equations were derived using atmospheric profiles from the National Center for Environmental Protection's Global Data Assimilation System (GDAS) and simulated AMPR Tb values from a radiative transfer model. The retrieval equations were applied to AMPR data to illustrate their capabilities in retrieving these atmospheric properties. To evaluate the performances of the retrieval equations, comparisons were made between the retrieved CLW, WV, and WS values and the values calculated for the same dataset using an independent one-dimensional variational inversion algorithm (1DVAR) developed at Colorado State University. Comparisons were also made between the retrieved values and those obtained using in situ observations from dropsondes and other aircraft. These results indicate marked improvement in the calibration of recent AMPR flight campaign data, and provide a path toward routine provision of geophysical retrievals as part of current and future AMPR campaign datasets. This will be demonstrated using observations from recent field campaigns involving the NASA ER-2 and P-3.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A44D..03L
- Keywords:
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- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSES;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3333 Model calibration;
- ATMOSPHERIC PROCESSES;
- 3354 Precipitation;
- ATMOSPHERIC PROCESSES