Synoptic-Scale Atmospheric Processes Associated with Snow Cover Ablation Events Across Eastern

North American Stream Basins

Daniel J. LeathersDaniel J. Leathers

And And

Gina HendersonGina Henderson

Center for Climatic ResearchCenter for Climatic Research

Department of GeographyDepartment of Geography

University of DelawareUniversity of Delaware

Research Questions

1)How important is the ablation of snow cover to the flood hydroclimatology of selected eastern North American stream basins?

2) What are the synoptic-scale atmospheric processes associated with snow cover ablation in eastern North America?

Basins of InterestSusquehanna Ohio

St. LawrenceChesapeake

Data Sources

Snow Depth Data

1o X 1o gridded daily snow depth data set developed by Mote et al.

Utilizes U.S. COOP and Canadian daily surface observations

Extensive quality control routines

Gridded snow cover data used to identify basin or sub-basin wide ablation episodes.

The Spatial Synoptic Classification (SSC)

A statistical methodology to identify Air Mass Types for each day at a given station from hourly meteorological data (Sheridan Inter. J. Climatology, 22, 51-68).

DM – Dry moderate (“Pacific” type air mass)

DP – Dry Polar (cP)

MT – Moist Tropical (mT)

MM – Moist moderate (overrunning situation)

MP – Moist polar (mP)

TR – Transition (air mass transition, frontal passage)

SSC used to identify air mass types during major ablation episodes.

Calculation of Energy fluxes during ablation events with SNTHERM Snow Pack Model….developed by Jordan (1991)

Figure from CRREL website…..

Methodology

1. Examine the record of major flooding events in a basin or sub-basin to ascertain the role of snow cover ablation on the flood hydroclimatology.

2. Identify the major ablation events within a basin that were associated with flooding (using gridded snow cover data).

3. Match ablation events to air mass types using SSC.

4. Examine synoptic-scale atmospheric patterns associated with each air mass type.

5. Use SNTHERM to model atmosphere snow cover interactions under diverse air mass types.

Results:

The Susquehanna River Basin…..

Drains 27,500 square miles, covering half the land area of Pennsylvania and portions of New York and Maryland.

Flows 444 miles from its headwaters at Otsego Lake near Cooperstown, N.Y., to Havre de Grace, Md., where the river meets the Chesapeake Bay.

Is the largest tributary of theChesapeake Bay, providing 90percent of the fresh water flows to the upper half of the bay and 50 percent overall.

Discharge data from Harrisburg, PA

Methodology

1. Examine the record of major flooding events in a basin or sub-basin to ascertain the role of snow cover ablation on the flood hydroclimatology.

2. Identify the major ablation events within a basin that were associated with flooding (using gridded snow cover data).

3. Match ablation events to air mass types using SSC.

4. Examine synoptic-scale atmospheric patterns associated with each air mass type.

5. Use SNTHERM to model atmosphere snow cover interactions under diverse air mass types.

Month

J F M A M J J A S O N DNum

ber

of B

asin

Wid

e F

lood

ing

Eve

nts

0

2

4

6

8

10

31 Basin Wide Flooding Events 1949 - 2000Susquehanna River at Harrisburg

Only three "warm season" events

90% occur during snow cover season (55% during March / April Ablation period)

Agnes June 1972 Eloise Sep. 1975

Nor'easter 1955

Flooding event defined as a discharge at Harrisburg >250,000 ft3/sec

Methodology

1. Examine the record of major flooding events in a basin or sub-basin to ascertain the role of snow cover ablation on the flood hydroclimatology.

2. Identify the major ablation events within a basin that were associated with flooding (using gridded snow cover data).

3. Match ablation events to air mass types using SSC.

4. Examine synoptic-scale atmospheric patterns associated with each air mass type.

5. Use SNTHERM to model atmosphere snow cover interactions under diverse air mass types.

Susquehanna Basin and 12 grid boxes used to calculate ablation values.

Daily ablation calculated using:

day 1 – day 2

Methodology

1. Examine the record of major flooding events in a basin or sub-basin to ascertain the role of snow cover ablation on the flood hydroclimatology.

2. Identify the major ablation events within a basin that were associated with flooding (using gridded snow cover data).

3. Match ablation events to air mass types using SSC.

4. Examine synoptic-scale atmospheric patterns associated with each air mass type.

5. Use SNTHERM to model atmosphere snow cover interactions under diverse air mass types.

Identified the air mass type present (in Williamsport, PA) two days before flood stage was reached in Harrisburg. This two-day response is typical of basin-wide flooding events on the Susquehanna.

DM – Dry moderate (“Pacific” type air mass) 5 eventsMM – Moist moderate (overrunning situation) 10 eventsMT – Moist Tropical (mT) 4 eventsMP – Moist polar (mP) 1 eventTR – Transition (air mass transition, frontal passage) 8 events

Methodology

1. Examine the record of major flooding events in a basin or sub-basin to ascertain the role of snow cover ablation on the flood hydroclimatology.

2. Identify the major ablation events within a basin that were associated with flooding (using gridded snow cover data).

3. Match ablation events to air mass types using SSC.

4. Examine synoptic-scale atmospheric patterns associated with each air mass type.

5. Use SNTHERM to model atmosphere snow cover interactions under diverse air mass types.

DM – dry moderate (“Pacific” type air mass) 5 events

LH

SLP (mb) 500 hPa heights

Surf. T (C) R.H. (%)

MM – Moist moderate (overrunning situation) 10 events

L

H

SLP (mb) 500 hPa heights

Surf. T (C) R.H. (%)

MT – Moist Tropical (mT) 4 events

SLP (mb) 500 hPa heights

Surf. T (C) R.H. (%)

L

TR – Transition (air mass transition, frontal passage) 8 events

SLP (mb) 500 hPa heights

Surf. T (C) R.H. (%)

L

Air Mass Type

3-day Ablation (cm/day)

3- day Precipitation

(cm)

3-day Snowfall

(cm)

Mean Temp (C)

DM(5) 5.2 3.2 0.0 7.6

MM(10) 6.0 5.8 1.3 5.9

MT(4) 5.2 3.2 0.0 7.1

Trans(8) 9.3 3.9 1.5 5.6

Three-day meteorological variables associated with basin-wide flooding events

Energy fluxes associated with synoptic-scale atmospheric patterns:

March 2 – 4, 1964

1963-1964 Snow Cover Season

MonthNov. 1 Dec. 1 Jan. 1 Feb. 1 March 1 April 1 May 1

Sn

ow

De

pth

(cm

)

0

10

20

30

40

50

Dis

char

ge (

ft3 /

sec)

0

1e+5

2e+5

3e+5

4e+5

5e+5

Pre

cip

itatio

n (

inch

es)

0.0

0.5

1.0

1.5

2.0

2.5

snow depthdischargePrcp

will examine pre-precipitation ablation March 2-4, 1964

March 2 - 4, 1964 Ablation EventEnergy Budget Components

Day

March 2 00 March 3 00 March 4 00 March 5 00

Ene

rgy

Flu

x (W

/m2 )

-100

0

100

200

300

400

Sno

w d

epth

(m

)

0.0

0.1

0.2

0.3

0.4

0.5sensible heat fluxlatent heat flux0 W/m2

snow depth net solar net longwave

H

L L

DM MT MM

Initial Findings

1. More than 80% of the major flooding events within the Susquehanna River Basin are associated in some way with snow cover ablation.

Month

J F M A M J J A S O N DNum

ber

of B

asin

Wid

e F

lood

ing

Eve

nts

0

2

4

6

8

10

31 Basin Wide Flooding Events 1949 - 2000Susquehanna River at Harrisburg

Only three "warm season" events

90% occur during snow cover season (55% during March / April Ablation period)

Agnes June 1972 Eloise Sep. 1975

Nor'easter 1955

2. Although several different synoptic patterns can lead to ablation, a common theme is strong low pressure in the lower Great Lakes Region bringing warm and moist air across the Susquehanna Basin (also precipitation).

L

HLH

LL

DM MM

MT Tran

3. Large values of sensible and latent heat flux are typically the largest components of the energy budget during the most

intense ablation events across the Susquehanna River Basin (net solar and net longwave are not as large). Latent heat flux can be particularly important in some instances.

March 2 - 4, 1964 Ablation EventEnergy Budget Components

Day

March 2 00 March 3 00 March 4 00 March 5 00

Ene

rgy

Flu

x (W

/m2

)

-100

0

100

200

300

400

Sno

w d

epth

(m

)

0.0

0.1

0.2

0.3

0.4

0.5sensible heat fluxlatent heat flux0 W/m2

snow depth net solar net longwave

Future Work

1. Continue similar methodology for the other three eastern North American basins.

2. Study the role of basin size in regard to the importance of snow cover ablation in the flood hydroclimatology of these basins.