High-latitude moisture structure determined from HIRS water vapour imagery

An atmospheric transmittance model has been applied to Antarctic radiosonde ascents in order to examine the high-latitude normalized weighting functions (NWF) of the water vapour channels on the High Resolution Infrared Radiation Sounder (HIRS). In the Antarctic coastal region the 6·7 μ radiances are shown to be largely atmospheric in origin and imagery created from these data can provide useful diagnostic information on tropospheric water vapour, even during the winter months. The 7·3μm data contain a large surface contribution and have less value as a diagnostic tool. An example is shown of HIRS water vapour imagery of the Antarctic at a time when a cold front was descending from the Antarctic Plateau to the coastal area. The imagery clearly showed the dry air behind the front and provided information that was not available with any other imagery channel. A second example of a North Atlantic polar low shows the contaminating effect of cloud when deep atmospheric systems are being examined.