Improvements to Stepped Frequency Microwave Radiometer Real-time Tropical Cyclone Products
Abstract
With the installation of C-band stepped frequency microwave radiometers (SFMR) on Air Force Reserve Command WC-130J hurricane reconnaissance aircraft, the SFMR has assumed a prominent role for operational measurement of surface winds, and thus, hurricane intensity estimation. The current SFMR wind retrieval algorithm was developed from GPS dropwindsonde surface wind measurements, and has been successfully implemented across all SFMR-equipped aircraft. The algorithm improvements were specifically targeted at improving surface wind accuracy at hurricane force conditions (> 65 kts, 33 m/s), especially within the eyewall, although the SFMR surface wind vs. emissivity geophysical model function was developed over a broad range of wind speeds (10-140 kts, 5-70 m/s) with the expectation that the hurricane wind field could be readily measured in general. Due to the significant microwave absorption by precipitation, a by-product of the wind retrieval process is an estimate of the path-averaged rain rate (in actuality, the rain water content). An SFMR surface wind speed high bias in strong precipitation has recently been quantified and is particularly evident at weak-to-moderate wind speeds (<65 kts, 33 m/s) and large rain rates (>20 mm/hr), which has important implications for identifying tropical systems at the depression and storm stages, and additionally for observing significant outer wind radii. A major reason for this wind bias is due to an inaccurate rain absorption model that was used to develop the current surface emissivity vs. wind speed geophysical model function. Observations now suggest that the rain-induced absorption is significantly overestimated by the model, resulting in underestimated rain rate values. With the wind speed bias identified, the rain absorption component of the SFMR geophysical model function is addressed to provide an improved rain rate product. This new absorption model is developed by relating SFMR excess brightness temperature observations to external precipitation information including from WP-3D Droplet Measurement Technologies Precipitation Imaging Probe and airborne tail Doppler (X-band) and lower fuselage (C-band) radar data. Peak rain rate values using this updated model function are typically ~80 mm/hr, which is somewhat higher than maximum values found using the previous model version. This rain rate value equates to a radar reflectivity of ~50 dBZ, consistent with the maximum reflectivity values typically found in tropical cyclones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A23K..06U
- Keywords:
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- 3360 ATMOSPHERIC PROCESSES / Remote sensing;
- 3372 ATMOSPHERIC PROCESSES / Tropical cyclones