SAN FRANCISCO STATE UNIVERSITY SPRING 2008

DEPARTMENT OF GEOSCIENCES METR 485/785

In-class Exercise: Estimating Daily/Hourly Precipitation Amounts

14 March 2008

Example of usage of simple proportional factor based upon ratio of mean annual rainfall to estimate daily rainfall at stations in same geographic area. This simple methodology is predicated on the premise that AREAL VARIABILITY of rainfall is low for the given event. It should not be applied in convective situations.

In this case, we'll try to develop an estimate of the 24 hr rainfall at Oakland 1E for December 6, 2003, knowing only what rainfall occurred at the Downtown SF site and the long term annual precipitation at the two locations.

Annual Precip Ratio 24 hr (12/6/03)

San Francisco City 22.38 NA 0.76

Oakland 1E 25.65

Fill in the blanks above as follows: Ratio is the ratio of Oakland's annual precipitation to that for San Francisco. The 24 h total of 0.76 was the officially measured quantity at Duboce Park. Estimate the amount of precipitation you would have expected for the same period at Oakland 1E.

As you can see, your estimate was close enough to what actually was observed to be somewhat meaningful. In general, the longer the time period for which an estimate will be needed, the closer the estimate will be to what actually occurred. For example, San Francisco's monthly rainfall for December 2003 was 7.69".  What would your estimate be for Oakland 1E for the same period? ___________________

The three month rainfall e at San Francisco 16.76". What would your estimate be for Oakland 1E for the same period? _______________