Flash-Flood Producing Mesoscale Convective Systems: A Statistical Analysis of Precipitation Efficiency Patrick S. Market Dept. of Atmospheric Science University of Missouri-Columbia Scott M. Rochette Dept. of the Earth Sciences SUNY, College at Brockport
Support Funding: Outgrowth of a larger 2-year study with NOAA/NESDIS/ORA Precipitation Data: NCEP - Brett McDonald and Michael Baker Analysis: Stacy Allen (UMC) and Brian Pettegrew (CMC)
Problem Larger question For a given PW: higher PE heavier rainfall ? Present question Do MCSs with greater PE values produce greater rainfall totals? OR… Is it necessary for an MCS to have large PE to be a flash flood producer?
Method Data MCS PE values of Market et al. (2002) NCDC online reports of flash flooding Over Missouri During summers: 2000, PE calculations 9 PE calculation periods feature flash flooding
Precipitation Efficiency Ratio of Precipitation (P) to Ingested Moisture (Q) R=6-h gridpoint rainfall Q=gridpoint precipitable water (6-h avg)
Analysis – PE statistics Flash Flood cases N = 9 Mean = 33% St. Dev = 9% Range = 22-48% Non-Flash Flood cases N = 24 Mean = 23% St. Dev = 10% Range = 4%-40%
Analysis – PE statistics Mann Whitney Wilcoxon rank sum test Hypothesis: Median PE of FF sample > Median PE of NFF sample p= =0.05 p< Reject null hypothesis (medians identical) FF PE > NFF PE at 95% confidence
Analysis – PE statistics Median 10 th 25 th 75 th 90 th
MCS Propagation and PE FF PE cases (N=9) Less than half were quasi-stationary or regenerative 4 of 9 All cases (N=32) One third were QS or REGEN 10 of 32
MCS Propagation and PE Therefore, 6 cases were QS or REGEN, but not associated with flash flooding Suspect MCS behavior necessitates neither large PE nor flash flooding
MCS – Maximum PE (2000) 12 September % Vigorous cold front Forward propagation Associated squall line
MCS – Minimum PE (2000) 17 August % Elevated convection North of warm front Scattered convection Forward propagation
Summary MCSs which produce Flash Flooding are more likely to be efficient precipitators Quasi-stationary or regenerative MCS motion does not require large PE or guarantee flash flooding
Questions, Comments, Scorn and Contempt?