Applications of Ground-based Microwave Radiation Data to Short-term Rainstorm and Potential Forecast

Microwave radiometer (MWR) can derive the profiles of relative humidity, atmospheric liquid water content, and atmospheric water vapor content with high temporal resolution. Using MWR measurements from June 2008 to August 2012 of Xianning Station in Hubei Province operated by the Institute of Heavy Rain, CMA, based on 523 precipitation cases, the characteristics of MWR measurements before the occurrences of short-term rainstorms with more than 50 mm precipitation in 3 hours or 6 hours and other general rainfalls are analyzed, and the applications of MWR measurements in the forecast of short-term rainstorms are explored. For both short-term rainstorms and general rainfalls, the relative humidity in 0—6 km heights is close to saturation or is saturating in 3 hours before the rainfall occurrence, and no distinct difference is found between these two kinds of precipitations, however, the transforming of relative humidity from unsaturation to saturation always means the occurrence of rainfall process. Within 12 hours before the short-term rainstorm occurrence, the integrated liquid water (ILW) increases sharply from about 1 mm to about 20 mm, and the integrated water vapor (IWV) increases rapidly from 60—70 mm to 90 mm or more, which are helpful for the potential analysis of short-term rainstorm forecast. Compared with the short time rainstorm, amplitude and rate of occurrence of precipitation of ILW and IWV growth will be much smaller.Within 6 hours before the short-term rainstorm occurrence, a temperature inversion layer exists near 800—950 hPa, and the largest K index and TT index are above 35℃ and 40℃, respectively, otherwise, the atmospheric stratification curve is unstable, and the unstable energy may be larger than 1000 J·kg^-1 or maybe small depending on the maintenance of weak precipitation before the rainstorm occurrence.Of all the historical cases, there is a part of general rainfalls also occur with unstable atmosphere and temperature inversion at the bottom level. Therefore, it cannot be determined whether short-time rainstorm will occur only by the atmospheric stratification stability. In conclusion, when the ILW increases sharply from about 1 mm to about 20 mm, and the IWV increases rapidly from 60—70 mm to 90 mm or more, if the relative humidity is close to saturation or is saturating in 0—6 km heights, and a temperature inversion layer exists near 800—950 hPa, and additionally, the largest K and TT index are above 35℃ and 40℃, then the probability of the occurrence of a short-term rainstorm is large in coming 3 hours or 6 hours. The verification with 519 other general rainfall cases shows that, using these criterions, 2 cases matches the above characteristics, and the false alarm rate is about 0.4%.