FLOOD07101

• County: Statewide
• Basin: Statewide
• Title: A Numerical Modeling Study to Investigate the Assumptions Used in the Calculation of Probable Maximum Precipitation
• Date: 3/1/1999
• Prepared By: Water Resources Research
• Floodplain - Doc Type: Educational/Technical/Reference Information

79(1

ABBS: INVESTIGATION OF PROBABLE MAXIMUM PRECIPITATION ASSUMPTIONS

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Figure.3. (a) Observed rainfall (mm) for tbe 24 hours ending
2>00 UTC, August 5 [from Me/nnes and Hess, 19921 and (b)
simulated rainfall (mOl) on grid 3 for the 24 hO'Jrs ending 2300
UTe. August 5. Shading indicates terrain higher than 250 m
with a contour interval of 500 m. Black contours are from 50 to
150 mm with a contour interval of 50 mm, Overlaid dark
~hading is for rainfalls >200 mOl.

the mnJcl from the ECMWF analyses as hefore, but in this
ral;;c the moisture values have been increase:d by uniformly
increasing the temperatures of the atmosphere everywhere
while maintaining the relative humidities. In Ihis way the sys-
tl'l11 is still in dynamic balance, but the specific humidity, and
hellce precipitable water, has been increased. The maximiza-
tion faelor is defined as the ",ti(l..Onhe precipitable water for
the increased moisture simulation to the precipitable water for
thL' control simulation. The maximization factor is determined
fpr those portions of the storm for which the atmosphere is
saturated. This method is comparable to the technique used to
maxim:zc storms in the current approach.
Two simulntions with higher moisture vnlucs have heen per~
\prllH:d: the nrst where the temperatures have been increased

by 30e and the second where temperatures have been in-
creased by 5'e. Henceforth these simulations will be referred
to as EC86_3 and EC86_5, respectively.
Under both sets of conditions the development of the C!l!\t
coast low proceeded in a similar manner to that of the control
simulation. The main differences related to the intensity of the
low, which in these cases experienced a deepening of the cen~
tral pressure of 3 and 4 hPa, respectively, more than that of the
control simulation. The position of the low is similar in both
cases.
The precipitation predicted by the model for these two sim-
ulations is presented in Figure 4. The distribution of the pre-
cipitation for both cases is similar to that of the control simu~
lation, but the amounts are greater, particularly in the regions
of maximum rainfall that occurred along the IIlawarra eS~'lrp.

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Figure 4. The predicted precipitation on grid 3 for the 24
hOllrs ending 2300 UTe, August 5, for (a) EC86_3 and (b)
EC86_5. Shading indicates terrain higher than 250 m with a
contour intervnl of 500 m. Contours and overlaid shading arc
as for Figure 3b.

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