precipitation associated with 500 hpa closed cyclones anantha aiyyer eyad atallah department of...

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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 40 50

Speed (m/s)

Are

a a

vg

pre

cip

(m

m/d

ay)

Avg precip

Speed vs. Max Precip

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Speed (m/s)

Ma

x p

reci

p (

mm

/da

y)

max precip