Thermodynamic quantities, such as K-index, are commonly used in the nowcasting of thunderstorms and heavy rain. They are derived from upper-air ascent measurements, which are available twice a day. In the tropics, the thermodynamic property of the troposphere normally changes rapidly in rain episodes and the conventional upper-air ascent data alone are not sufficient for nowcasting purpose.

This paper discusses the use of a multi-wavelength, ground-based microwave radiometer to provide frequently updated thermodynamic profiles of the troposphere up to 10 km (once every 5 minutes) for rain nowcasting during a field experiment in Hong Kong in 2004. The accuracy of the radiometer's measurements is first established by comparing with the temperature and humidity profiles of upper-air ascents and the integrated water vapour of Global Positioning System (GPS) receivers. The humidity profile and K-index from the radiometer in a number of rainstorm cases are then studied. They are found to give useful indications of the accumulation of water vapour and the increasing degree of instability of the troposphere before the occurrence of the heavy rain.

The continuous availability of the thermodynamic profiles from the radiometer also makes it possible to study the correlation between K-index and the degree of instability of the troposphere. In this study, the tropospheric instability is expressed in terms of the total number and the rate of lightning strikes within 20 km or so from the radiometer (which is the typical spatial scale of rain bands as shown on radar imageries). It is found to have good correlation with the time-averaged K-index from the radiometer during the heavy rain episodes in the field experiment period.