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Significant Warm Season Precipitation Events in the Burlington, VT Region By Eyad Atallah, John Gyakum, and Margaret Kimball (McGill) Paul Sisson (NWS-BTV)
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Motivation The goal of this study was to identify large- scale circulation anomalies associated with significant warm season precipitation events. Events were identified using the Unified Precipitation Dataset and then composites were used to compare the events to climatology.
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Data area For every date, the maximum over the grid area was found. This maximum was then used as the precipitation value for that day.
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Unified Precipitation Dataset AvailabilityNOAA CDC online Time period1948-1998 Spatial coverageUnited States gridded.25x.25 degrees Temporal coverageDaily stationsAbout 13,000 after 1992 and 8,000 before that. Includes COOP observers!
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Statistics Mean0.45168 in Standard deviation 0.49831 in Extreme>2 sigma>1.448305 Heavy[+1sigma, +2sigma][0.94995, 1.448305] Moderate[+0.5sigma, +1sigma][0.70084, 0.949995]
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Case Selection RawRestrictedPercentage of total rain events Extreme225531.3% Heavy462943.5% Moderate429615.0% 7 day separation for event independence Restricted to post-1963; used NCEP reanalysis Heavy and Moderate were limited to 53 cases via random elimination
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Data and Methodology Composites and climatologies were developed using the National Centers for Environmental Prediction/National Centers for Atmospheric Research global Reanalysis for the period 1963- 1998 inclusive. Since all events are centered at 00 UTC, diurnal climatologies were developed, and weighted based on the distribution of events in each composite.
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Conclusions Extreme events present a coherent structure dominated by a warm anomaly up to 60 hrs before the event. Troughs in extreme events are more slowly propagating with a positively tilted structure at 00 h. Heavy events have a clear synoptic-scale trough/ridge couplet which develops a bit later, representing stronger, but shorter duration forcing for ascent than extreme cases. Moderate events have little structure- possibly a result of smearing. Precipitable Water shows a greater intensity and geographic extent in the extreme cases then either heavy or moderate. Largest differences in the vertical moisture profile are located between 700-500 hPa.
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