Abstract
The aircraft measurements from the HIPLEX-1 weather modification experiment have been examined to determine if the nature of the change of liquid water content (LWC) in the supercooled portion of the clouds can be simply described, Three different data sets were created from the −8 and −5°C aircraft data base. Neither a simple linear nor a simple polynomial fit to the data are suitable for reasons discussed in the text. Two different forms of an exponential model were fit to two of the data sets. When a model for the decay of the maximum 1-km liquid water content (χ) of the form χ=χ0ebt was fit to data set number two, this yielded a cloud liquid water decay constant (τ) of 560 s (9.5 min), with a correlation coefficient r=0.47 and r2=0.22. This reduce the mean first pass χ value of 1.05 g m−3 to e−1 or 0.39 g m−3 in 9.5 min and to (2e)−1 or 0.14 g m−3 in 19 min. The best fit to the observations, however, comes from a calculation of an average rate of change of LWC at a constant altitude (−8°C) in the clouds. This is of the form χ=χ0+bt and gives a lifetime of 15 min for the maximum 1-km average LWC in the 20 HIPLEX-1 clouds. That is, the highest LWC regions in the upper part of the clouds would be expected to completely disappear in about 15 Min. Regions of lower LWC would disappear more quickly. This is a major limitation on both natural and artificial rain forming processes.
