NumericalModelling of Extreme Precipitation Events
ReportNo WSAA 131
Theproject investigates whether numerical models of the atmosphere may be used astools for quantitative precipitation forecasting, over catchment size regions,for extreme precipitation events.
Theconcept of PMP, or Probable Maximum Precipitation, is used by hydrologists andmeteorologists involved in the design of structures, such as dams or bridges,where the need is to compute extreme rainfall (and hence flood) events. Theconcept is that the PMP is an upper bound for rainfall at a site. It is thusseen as a single deterministic number (governed by physical principles) thatwould never be exceeded. However, there have been several occasions on whichobserved rainfalls have exceeded PMP estimates valid at the time; theoccurrence of such events led to a re-evaluation of the PMP methodologies andresulted in the development of the generalised techniques that are now usedthroughout Australia.
Fiveconclusions with implications for the estimation of PMP may be drawn from thework presented in this report.
1. As the moistureavailability is increased the precipitation efficiency of the storms does notchange significantly. For each case study presented, the production of heavyrainfall (rainfall rates greater than 25 mm hr-1) is between 80% and100% efficient. This supports the simple model that assumes implicitly thatextreme precipitation storms have the highest efficiency.
2. As moistureavailability is increased the duration of the heavy rainfall increases, i.e. itbegins earlier and is more continuous. Life cycles are not considered in thesimple model; however, the results presented here and the recent paper of Zhao et al. (1997) suggest that the durationof the storm increases as the moisture availability increases. An increase inthe duration of heavy rainfall will result in higher total rainfall.
3. As moistureavailability changes, the spatial distribution of the area over which more than50% of the total rainfall occurs as heavy rainfall changes. Zhao et al. (1997) also found that the arealcoverage of rainfall varies nonlinearly with the precipitable water.
4. The controlsimulation may be thought of as giving the depth-area curve for the actualstorm, while the enhanced-moisture simulation provides the depth-area curvesfor a storm maximised by the moisture while conserving its dynamic integrity.The enhanced moisture storm is associated with a moisture-adjustment factor andthe current PMP methodology would multiply the depth-area curves of the controlsimulation by this factor. If the depth-area curve for the increased moisturesimulation lies above that of the control simulation, then the maximisationrelationship of the current PMP technique under-estimates the precipitationsimulated by the model. The simulations reported here indicate that this mayoccur and hence the precipitation is not linearly related to the precipitablewater. Where this was the case, for the case studies presented in Section 3,the model produces between 15% and 35% more precipitation than the currentstorm maximisation technique for areas of 50 to 70 km2. For areas of500 km2, the model produces between 5% and 15% more precipitationthan the current storm maximisation technique.
5. The topographyaffects the distribution of the “convergence component” of the precipitationdue to feedback effects to the dynamics of the storm system.
Despitethese deficiencies in the assumptions used to estimate PMP, we believe thatthere is no operational replacement available at present for the current PMPmethodology. However, improvements in the estimation of PMP may soon bepossible if increased effort is placed on (amongst other things) the numericalmodelling of extreme rainfall events. These improvements are only possible ifthe results of these efforts are communicated to, and accepted by, thehydrological community.
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