,TRANSPOSED
• STORM CENTER
, "sTORM CENTER
~(IN PLACE)
.,/REFERENCE.-l""'"
r DISTANCE
MAXIMUM DEW POINT
~l./(READFROM MAXIMUM
'V
DEW POINT MAPS)~STORMDEW POINT (READ FROM
WEATHER CHARTS)
Figure 2.10 - Use of reference distance in storm transposition
warm front with cold air at the surface. In this circumstance the surface dew points near the storm are not representative
of the moisture flowing into the storm. At the transposed location the same referenced distance is laid out on the same bearing from the transposition point. This indicates where to scale the maximum dew points from the maximum dew point chart for calculating the transposition and maximization adjustment. See Figure 2.10.
Examples of transposition
2.3.5 Figures 2.11 and 2.12 illustrate transDostion limits applied to storms in the course of studies in the United States. Included are notes as to the reasons for establishing
the indicated transposition limits. In the study of a particular basin, i t is of course not necessary to establish transposition limits completely around a storm but only in the direction of the basin.
2.4 Storm Rainfall Maximization 2.4.1 Introduction
2.4.1.1 There are three princiDal methods of storm rain-fall maximization: statistical, physical and composite. Statistical methods are discussed in chapters 5 and 6.
2.4.1.2 Physical Method. rne physical method of maximization is applied to individual storms and is used in combination with trans-position and envelopment. References 3, 10, 12, 15, 18, 20 dnd 36 are survey papers which describe this method or some aspect of it.
The physical method is based on the model described in section 2.1.
In that model, the most vital element of a rainstorm is a cloud system into which air converges radially at lower levels, rises to
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Figure 2.11 _ Transposition of July 9-13, 1951 storm rainfall. X's show location of synoptically similar storms. Heavy dashes show transposition limits.
Light dashed lines are curves of maximum sea-level dew point labeled in OF. and in corresponding moisture adjustment in percent. Shading denotes a study basin to which storm was transposed.
Explanation of Vigure 2.11
Study Basin
Transposition limits
Northern: Encloses storms' of similar type and region of high. frequency of nocturnal thunderstorms.
Eastern: Extent to which inflow can come to
region without crossing higher Appalachian Hountains.
Hestern: Generalized lOOOmeter elevation contour -above which isohyets would reflect
topographic effects
o Center of July 9-13,1951 -storm isohyetal pattern in place of occurrence.
/ X . Center of major summer storms with all of the following synoptic and rainfall ch2.racteristics similar to
July 9-13, 1951:
1. East-west frontal and rainfall patterns.
2. No marked wave action or occulsions during and after rain period.
3. Duration of rain equal or greater than 2 days.
4. Rain at storm center equal" or greater than 7 inches.
5. Polar High to north or rain center during rain period.
6. Southward movement of frontal system .after rain.
( F)---Percent(%):o Transposition adjustment iso1ines labeledlvith enveloping mid-July deH points (OF) and percent (%)
of storm rainfall values of storm in place of occurrence.
50
ESTIMATION OF ~~XIMUM FLOODS-' \.-
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Figure 2.12 - Transposition of July 13-17, 1916 hurricane rainfall.
Transposable to shaded area. Line with spikes is 2000-ft. elevation contour. Light dashed line is ridge crest. Maximum dew point isopleths labeled as in figure 2.11.
Explanation of Figure 2.12
The storm
A hurricane approaching the D.S. from the southeast crossed the Coast on the night of the 13th. On the relatively flat
coastal region 13.5 inches of rain over 1000 square miles was observed in 24 hours. The storm continued in a northwesterly
direction and a day later came against the slopes of the Appalachian Mountains which rise to about 5000 feet in this region. Here the
rainfall intensified due to orographic lifting resulting in a second rain center with 15.0 inches in 24 hours over 1000 square miles. This is one of the most severe rainstorms of record for the Appalachian Mountains.
Transposition
Hurricanes affect all of the eastern D.S. seaboard and have been responsible for floods throughout the eastern Appalachians.
The storm under consideration occurred along some of the steepest and highest eastern slopes and the central isohyets have orientation and shape that conform to terrain features. Because of this
orographic control, transposition of the storm center is limited to a rather narrow strip along the eastern slopes.
52
ESTIMATION OF ~~XIMUM FLOODSTransposition limits
Northern and southern: Brackets the generalized ridge line that is approximately 2500 ft. in elevation or higher.
Western: The generalized ridge line.
Eastern: The generalized 2000-ft. contour.
Generalized ridge line of Appalachians, 2500 to 5000-ft.
,
/elevation
Generalized 2000-ft. contour.
Maximu~persistingl2-hr. 1900-mb. dew points, mid July, labeled with of. and adjustments (in percent) based on precipitable water.
Path of hurricane with 8 a.m. positions on days indicated.
July 13-17, 1916 storm isohyets in inches.
Transposition limits for storm center.
some greater elevation with adiabatic expansion and cooling and the consequent release of precipitation, and diverges at upper levels.
Within the cloud the temperature variation with height is at the saturation adiabatic lapse rate.
2.4.1.3 The physical maximization is by ratio. The moisture maximization ratio is the ratio of the rate of precipitation produced by ascent of air along the warmest saturated adiabat that might be
expected in the region near the time of the year of the storm to the rate of production along a moist adiabat of the existing storm.
The ,.,ind maximization ratio is the ratio of the maximum speed of the low-level inflow wind to the estimated speed of the inflow
wind in the existing storm. The hypothesis of the wind maximization is that the convergence of the wind - the essential element for rain production - is proportional to the wind speed. This hypothesis is most nearly valid for convergence averaged over large areas and long durations and is patently of no value for individual thunder-storms. The moisture adjustment is the primary one in the physical maximization of rainstorms because of its greater reliability and definiteness. The wind maximization is rarely used alon~ but occasionally in combination with the moisture maximization. Wind maximization is more readily applied to orographic rainstorms than
to other types.
2.4.1.4 Composite method. The composite method of maximization is comprised of sequential maximization and spatial maximization. Reducing the observed time interval between storms over a basin, or between bursts within a storm, is seouential
maximization. Rearrangement of storms or bursts within the seauence