Using Surface Humidity Measurement to Estimate Atmospheric Moisture Availability for Extreme Rainstorms

Surface humidity measurements are used to estimate atmospheric moisture availability for Probable Maximum Precipitation (PMP) estimation. In particular, a pseudo-adiabatic dewpoint profile is assumed to estimate precipitable water from surface 12-hour persisting dewpoint. This assumption is employed in estimating both the precipitable water for observed extreme rainstorms, and the maximum possible precipitable water used for scaling observed rainfall to reflect maximum conditions (a concept known as moisture maximization). This assumption was reevaluated using pairs of radiosonde and surface airways observations for the central United States. The results show the deficiencies in estimating atmospheric moisture availability using surface humidity measurements with this approach. The pseudo-adiabatic assumption systematically overestimates precipitable water for observed extreme rainstorms; the overestimation of maximum moisture conditions is even greater. As a result, the PMP estimate based on the moisture maximization concept is larger than expected based on the empirical assessment of atmospheric moisture availability. To assess the overestimation of PMP estimates, a natural logarithm formula was derived from a 23-year climatology of maximum precipitable water and 12-hr persisting dewpoint, to better represent moisture conditions for extreme conditions. Using the proposed formula for moisture maximization suggests that the pseudo-adiabatic assumption overestimates PMP by about 6% on average.