Precipitation Associated with 500 hPa Closed Cyclones

Anantha Aiyyer

Eyad Atallah

Department of Earth and Atmospheric Sciences, University at Albany

Motivation

• What percentage of precipitation in the Northeast is associated with closed lows?

• What are the regional and seasonal precipitation signals?

• Availability of gridded data sets.

Data and Methodology

• NCEP/NCAR Reanalyses at 500 hPa. • Unified Precipitation Dataset – .250 lat-lon

grid (http://www.cdc.noaa.gov/cdc/data.unified.html).

• Period of interest 1950-1998.• Use an objective algorithm to identify

closed lows.• Define a closed low as a 30 m height rise

from a local min in the height field.

• Mainly zonal distribution of closed lows.

• Genesis area in the NW Atlantic.

• Spring is the most active season.

• Summer is the least active season.

• Possible tropical cyclone contribution in the summer.

• Annual mean precipitation field strongly modulated by topography.

• Upslope and lake effect signatures dominate cool season precip.

• Terrain elevation signatures dominate warm season precip.

• Lows contribute a higher percentage of total precipitation in coastal areas, especially in Spring.

NW NE

SW SE

• Heaviest precip from lows in western quadrants.

• Upslope signatures strongest in western quadrants.

• Precip rates maximized with lows in SW quadrant.

• NE quadrant shows prominent lake effect signature.

• SE quadrant precip rates maximized along eastern most terrain features.

• Probability of occurrence decreases rapidly with increasing precip rates.

• 60% of lows produce mean precip rates of 5 mm or greater over the entire domain.

• Local maximum precipitation has a relatively even distribution over wide range of rates.

• Ideal low position for highest domain-wide precip is west of the NE.

• Ideal low position for highest local max precip is southwest of the NE.

Speed vs. Mean Precip

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Speed vs. Max Precip

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