An investigation of the moisture maximization for the probable maximum precipitation

An investigation has been conducted to evaluate the assumptions behind the current methodology for estimating the Probable Maximum Precipitation (PMP) and examine the effects of atmospheric moisture availability on precipitation. Two major assumptions in moisture maximization, a process to scale observed rainfall upward to reflect the maximum moisture conditions, were assessed using mesoscale modeling and raingage, radiosonde, and surface airways observations.; The pseudo-adiabatic atmosphere assumption used to estimate precipitable water from surface 12-hr persisting dewpoint was evaluated 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 both observed extreme rainstorms and the maximum moisture conditions and leads to a larger scaling factor than expected based on the empirical assessment of atmospheric moisture availability. To assess the overestimation of PMP estimates, an exponential relation 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.; The linear scaling assumption that precipitation is proportional to the atmospheric moisture availability was assessed using the PSU/NCAR mesoscale model MM5 for the northeastern Illinois storm of 17--18 July 1996. Three atmospheric moisture adjustment methods were used to adjust the atmospheric moisture availability over a wide range but within the upper limits of the maximum observed precipitable water. The results show that the relationship of precipitation to precipitable water depends on spatial scale. Moreover, the PMP linear scaling model is inadequate on both large and small spatial scales. For large spatial scales, current PMP methodology can be modified to include the effect of increasing wind convergence caused by increasing atmospheric moisture availability. For small spatial scales, deviations are caused by the oversimplified, two-layer convective thunderstorm cell model. Thus, a new storm model is needed to take into account storm dynamics and evolution.