John Plunkett Presentation to The Vermont Systems Planning Committee South Burlington, VT
Significant Warm Season Precipitation Events in the Burlington, VT Region
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Transcript of Significant Warm Season Precipitation Events in the Burlington, VT Region
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Significant Warm Season Precipitation
Events in the Burlington, VT Region
ByEyad 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 areaFor 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
Availability NOAA CDC online
Time period 1948-1998
Spatial coverage United States gridded .25x.25 degrees
Temporal coverage Daily
stations About 13,000 after 1992 and 8,000 before that. Includes COOP observers!
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StatisticsMean 0.45168 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
Raw Restricted Percentage of total rain events
Extreme 225 53 1.3%Heavy 462 94 3.5%Moderate 429 61 5.0%
•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.