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 ∼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.
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