|WORLD METEOROLOGICAL ORGANIZATION||
|Explanation of the criteria for classification and numbering of components||
|Explanation of the dates on the component's description|
TECHNIQUES FOR ESTIMATION OF PROBABLE MAXIMUM PRECIPITATION (PMP)
1. Purpose and objectives
The techniques are those used to approximate "the greatest depth of
precipitation for a given duration meteorologically possible over a given basin at a particular time of year, with no allowance made for long-term climatic trends."
The procedures are based on a meteorological approach, consisting of moisture maximization and transposition of observed storms. Storm transposition involves adjustments for elevation, moisture-inflow barriers, and distance from the moisture source. A 2-dimensional orographic air flow model with limited application to the computation of orographic precipitation is described. Computations are based on a storage equation of water vapour and the continuity equation for air mass. Air flow is assumed to be laminar. Computations are made for single or multiple layers of air passing over the barrier.
Alternative computations of orographic precipitation using subjective valuation of certain meteorological data and analysis are explained. A statistical PMP technique is included, based on general frequency equation in which the maximum rainfall is given as the mean rainfall value plus the standard deviation times a statistical factor.
Methods are also given for determining seasonal variation, and
chronological and areal distribution of PMP. Techniques are suggested for
computing PMP in regions where orographic influences are important. Tables
of precipitable water in a pseudo-adiabatic atmospheric are included, as
are world record and near record rainfalls. Examples from actual studies
are given in the manual; numerous others are available in the references
This is a manual procedure. The quantity and types of data needed are
highly variable, depending on the area to be studied. Most commonly needed
are historical records of precipitation and temperature, as well as meteorological parameters which serve as indicators of, or which affect
Estimates of PMP for points or areas for a given duration.
5. Operational requirements and restrictions
a) A person with knowledge and skills in meteorology and statistics
b) With the above skills, at least 3 months of training is necessary.
c) Limitations: Procedures are applicable in the middle latitudes for
6. Form of presentation
Report in English: Operational Hydrology Report No. 1, Manual for Estimation of Probable Maximum Precipitation, 269 pp. WMO No. 332 (1986)
7. Operational experience
The technique has been used worldwide in temperate climates, and in a limited number of cases in tropical regions.
8. Originator and technical support
Hydrometeorological Design Studies Center.
10. Conditions on use
| Reclassified from K11.2.01 MAY 1987
(First entered: 12 MAY 87
Last updated: APRIL 1998)