The 37 GHz Cyan Ring and Tropical Cyclone Rapid Intensification: What Does the Cyan Color Truly Represent?

A distinctive ring pattern in the inner core area of tropical cyclones (TCs) was found in the 37 GHz composite passive microwave images to be associated with rapid intensification (RI) (Kieper 2008). By reviewing the 37 GHz color composite (hereafter 37color) product developed by the Naval Research Laboratory (NRL, Lee et al. 2002) for 2003-2007 TCs in the Atlantic (ATL) basin, it was found that the first appearance of a cyan color ring around the TC center could be an early indicator of RI when environmental conditions are favorable. Kieper (2009) subsequently developed a subjective forecast method to forecast RI during the next 24 hours based on this finding. This method was tested in real time for the 2008 hurricane season in the ATL basin by emailing forecasts to the Tropical Storms (tstorms) email-list. It turns out to be a successful method. However, quantitative information is sacrificed in the NRL 37 GHz color product, although it provides unique qualitative information. It has been a mystery for the TC community what the cyan color truly represents.

The Tropical Rain Measuring Mission (TRMM) satellite has a unique suite of instruments, including Precipitation Radar (PR) at 13 GHz and TRMM Microwave Imager (TMI) with both 37 GHz and 85 GHz channels. Over 0.1 million Tropical Cyclone related Precipitation Features (TCPFs) have been identified from the 13 year (1998-2010) Florida International University/University of Utah TRMM Precipitation Feature (PF) database (Jiang et al. 2011). Two TC cases with a good 37 GHz ring pattern and a direct PR coverage in the inner core are examined in this study using the TRMM TCPF database. They are Hurricane Danielle (2004) 15:27Z Aug. 14, 2004 TRMM overpass and Hurricane Epsilon (2005) 19:24Z Dec. 2, 2005 TRMM overpass. Both overpasses were used in Kieper (2008)'s hindcast study. Dannielle was a successfully-hindcasted RI case, while Epsilon was a correct rejection due to unfavorable environment conditions indicated by the SHIPS RI index.

A preliminary examination of the TRMM PR reflectivities inside the 37 GHz cyan ring pattern through PR vertical cross-sections of the Danielle overpass shows that the cyan ring mainly includes warm rain or shallow convection with echo height less than 5-7 km and the maximum near surface reflectivity less than 35 dBZ. The Epsilon case will be compared with Danielle to see if there is any difference between the reflectivity feature of the ring for RI and non-RI cases. A statistics of the vertical profiles of PR reflectivity in the 37 GHz ring for both cases will be generated and compared. The PR 2A25 rain rates and 85 GHz brightness temperatures will also be examined. The results should answer the question of what the cyan color truly represents in the 37color images. In addition, some new RI cases during 2009 and 2010 will be presented as an evaluation of the 37 GHz ring pattern forecast method.