COSMIC Radio Occultation technique for measurement of the tropopause during tropical cyclones

Water vapour transport to the upper troposphere (UT) and lower stratosphere (LS) by deep convective storms affects the radiation balance of the atmosphere and has been proposed as an important component of climate change. The aim of the work presented here is to understand if the GPS Radio Occultation (RO) technique will be useful for characterisation of this process. In the last 25 years, several instruments have become available to study atmospheric water vapour, and many studies show the increase of water vapour in the UT during this period. It is also well known that deep convection from storms increases the relative humidity in the tropics. In this way, Tropical Cyclones (TCs) should play an important role since they lead to deep convective activity. TCs increase the water vapour content of the UT, the integrated water vapour in the vicinity of the storm, they warm the mid troposphere and they cool the tropopause layers. The GPS radio occultation technique is useful for studying severe weather phenomena because the GPS signals penetrate through clouds and allow measurements of atmospheric profiles related to temperature, pressure, and water vapour with high vertical resolution. The assessment presented here is based on an analysis of data from COSMIC satellites. GPS data were analyzed in relation to TC positions for the period 1996-2008 when GPS-MET, SAC-C CHAMP and COSMIC acquisitions where available, identifying several hundred ROs that coincide with TC positions. Whereas no evident variation at the tropopause is seen in the standard RO data products like water vapour pressure and temperature profiles, the GPS signal bending angle profile shows a TC signature in the upper troposphere. The presentation is focused on two particular TCs, the hurricane Bertha, which formed in the Atlantic Basin during July 2008 and reached a maximum intensity of Category 3 and the typhoon Hondo, formed in the south Indian basin during February 2008 with maximum intensity of Category 4. Using measurements from a variety of earth observation satellites (A-Train constellation) and from aircraft together with the best track information, we reconstructed the characteristics of the two TCs and compared them to ROs from COSMIC. Our results suggest that the bending angle of a GPS radio occultation signal contains interesting information on the atmosphere because it consistently shows a pronounced signature at the tropopause above TCs. Parameters such as the temperature and refractivity profiles from the COSMIC mission often show a variation at the same altitudes as the bending angle, but the signature is less pronounced and sometimes not evident. Perturbations to the bending angle are generally caused by temperature or water vapour gradients. The water vapour profile derived from COSMIC data do not show enhancements above TCs at the tropopause, however, the data are model dependent as ECMWF is used in their derivation. Comparisons with A-Train data, on the other hand, show some agreement between RO bending angle profiles and the water vapour mixing ratio profiles in the UT/LS. We are currently investigating to what extent the bending angle signature at the tropopause heights could be caused by the presence of enhanced water vapour content.