42 ESTIMATION OF MAXIMUM FLOODS

elevation changes. Foothills tend to stimulate convection and increase rainfall, while the windward side of mountain

slopes provides the mechanical lift. These effects in releasing precipitation may more than compensate for the decrease in

precipitable water with higher terrain.

2.3.4.5 In view of these conflicting techniques on precipitation magnitudes by topography, the United States Weather Bureau usually follows these practices in maximum rainfall studies in middle latitudes:

1. Transpositions of large-area storms into generally mountainous areas of less than 1,000 meters elevation from adjacent flat regions are generally made with no elevation adjustments, it being assumed the stimulation of precipitation referred to above offsets the decrease in moisture with _h~gher elevations (31).

2. In regions of high mountains and steep slopes the attempt is made in comprehensive studies to divide storm precipitation into

two parts: that due to orographic effects and that due to the storm processes in the atmosphere that would be about the same without the mountains (25, 32). (The latter component is called "convergence precipitation" in the references.) Only the latter component is

elevation~adjustedfor decreasing moisture. In a recent report (32) an empirical elevation adjustment for this component was adopted which decreases precipitation with elevation about half as fast as

a full precipitable water adjustment would require.

3. Intense loc~l thunderstorms are not elevation-adjusted for transposition to locations where the ground elevation ranges up to about 1500 meters. That violent thunderstorms occur in mountainous

regions with greater frequency than over adjacent valleys is well knovTn. Above 1500 meters the decrease in available moisture becomes over-riding and an elevation adjustment is applied for transposition based on precipitable water. In making such adjustments, the effective elevation of the ground at the place of occurrence of the storm and in the transposed position are employed rather than the precise point elevations, to allow for the fact that a thunderstorm draws in moisture from some distance away. The effective elevation is either

the average ground elevation over some tens of square kilometers surrounding a location, or the average elevation over a specified sector five to ten kilometers long in the downhill direction only.

4. On broad, gradually sloping plains, such as the Plains region extending from Texas and Oklahoma northward, the relocation adjust-ment for transposition is applied as described in paragraph 2.3.4.1 but no explicit additional adjustment for elevation is made. However, elevation change of more than 700 meters is generally avoided.

Local or regional studies of ·available storm precipitation should influence any elevation adjustments. For example, it is not known prior to study of a particular tropical region whether the most intense precipitation from the deep moist air mass occurs at low elevations from ready triggering of convection or at higher elevations from other effects.

2.3.4.6 Climatological adjustments. Other factors besides topography and moisture effect storm magnitudes. The action of these factors is suggested by such climatological charts as mean annual precipitation, maximum observed values of point precipitation, and heavy rainfall frequency charts. An example of the latter would be

44

a six-hour rainfall 'with a mean recurrence interval of five years.

A climatological chart is indicative of geographical variations of prohablemaximum precipit.ation provided both theclimatological chart and the PMP are strongly influenced by the same storm _,type.

For example, ifmonsoon rains contribute most of the annual precipitation and also offer the greatest threat of extreme rain to a basin, then the mean annual precipit'ationand large-area PMP will generally have a similar distribution. On the ether hand,the mean annual precipitation ~may be but slightly correlated with

extreme rain from tropical depressions if these are rare, or with rain from intense thunderstorms over small areas. Frequency maps of point precipitation (such as 6-hr., ID-yr.) should correlate better with the latter.

Often a climatological chart fulfilling the similarity-of-sterm-type requirement will show a larger'percentage variation from point to point than does the PMP because it is influenced by storm frequency as well as storm potential.

2.3.4.7 The hydrometeorologistpreparing estimates of PMP is confronted with the follmving dilemma. Climatological charts of the types referred to may provide the best available guide to the spatial variation of the PMP, especially in regions where moisture variations are slight and are not predominant control on maximum precipi tationmagnitudes. Yet there is no explicit theory available on which to hase a quantitative interrelationship between PMP and the climatological chart. In fact the strength of the climatological chart is that it depicts the real action of the atmosphere in

distributing rainfall independent of theories (except for statistical

theory in constructing frequency maps) and integrates many factors not all of which can be identified.

2.3.4.8 The recommended procedure for using climatological charts as guides to transposition is:

(a) Select the climatological chart that is most strongly influenced by storms of the type to be transposed.

(b) Calculate a tentative transposition adjustment ratio, r', from:

r'

=

IF

where F

l and F

2 are the climatological rainfall values at the location of storm occurrence and the transposed location, respectively.

(c) Regard r' as the outer limit of the transposition adjustment and subjectively adjust to a value, r, closer to 1.0 on the basis of judgment. (Increase r' if less than 1.0, decrease if more than 1.0). The full adjustment, r', would more often be used as a transposition adjustment to develop some lesser category of design storm such as "Standard Project" storm than to develop PMP.

Reference distance procedure for moisture adjustment

2.3.4.9 An alternate procedure for moisture adjustment for relocation to that described in paragraph 2.3.4.1 is to use as an index of the moisture in a storm, not the observed surface dew points at the center of the storm, but rather such dew points at some

distance from the storm as much as several hundred kilometers, in the direction from which the moist air enters the storm. This

procedure is particularly appropriate with winter cyclones. The dew points are from the warm sector of the cyclone regardless of whether the precipitation occurs there or, more typically, north of the