Effects of humidity and precipitation on VHF radar vertical beam echoes

We investigate in this paper the variation in power of VHF radar vertical e choes as a function of atmospheric humidity. We find that the echoes are gr eatest in air of moderate (30-70%) humidity, and least in very dry or near- saturated air. Compared to moderate humidities, the standard model for rada r echoes, based on potential refractivity, overpredicts the echo power at h igh relative humidity by similar to 10 dB. We propose that this is due to t he effect of precipitation in suppressing small-scale humidity gradients. A model which assumes complete suppression underpredicts the observed power drastically, showing that precipitation can only remove a part of the small -scale structure in refractivity which is caused by fluctuations in humidit y.