Effects of humidity and precipitation on VHF radar vertical beam echoes

We investigate in this paper the variation in power of VHF radar vertical echoes as a function of atmospheric humidity. We find that the echoes are greatest in air of moderate (30-70%) humidity, and least in very dry or near-saturated air. Compared to moderate humidities, the standard model for radar echoes, based on potential refractivity, overpredicts the echo power at high relative humidity by similar to 10 dB. We propose that this is due to the 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 humidity.