Atmospheric Processes Associated with Snow Cover Ablation Events and their Effect on the Flood Hydroclimatology of the Chesapeake Bay Gina Henderson and Daniel J. Leathers Center for Climatic Research University of Delaware

Chesapeake Bay watershed The Susquehanna The Potomac The James

Research Questions 1.How important is the ablation of snow cover to the flood hydroclimatology of the Chesapeake Bay watershed? 2.Are their distinctive types of snow ablation events that contribute to flooding in this area? 3.What are the general atmospheric processes associated with these ablation events and how does this impact the hydrology of the region?

The Susquehanna, Potomac and James rivers account for the majority of Chesapeake discharge The three gaging stations used were: 1.Harrisburg, PA 2.Point of Rocks, MD 3.Bucanan, VA

Data Sources

Snow Depth Data  1 o X 1 o 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.

Chesapeake watershed: 23 grid boxes used to calculate ablation values  Ablation values calculated using: day 1 – day 2  Area of the watershed ~ 165,759 km 2 (64,000 mi 2 )

Methodology 1.Identify major flooding events during the fifty year period for the Chesapeake Bay watershed using stream flow and snow depth criteria. 2.Classify events based on type of snow cover ablation taking place. 3.Identify principle atmospheric features associated with each classification type. 4.Use SNTHERM to model atmosphere snow cover interactions.

Selection of flooding events  Top flooding events were identified from the period  Selection criteria: –> 4247 m 3 s -1 (150,000 cf/s) –> 3.0 cm change in snow depth from the previous day –Only winter months considered Ablation episodes selected from the top 5% of daily discharge values.

Results

Selection of events Three types of ablation events: 1.Ablation 2.Rain on snow 3.Ablation to rain We will look at one of each type of event 1950 to events 8 events

Annual cycle of river discharge: Chesapeake watershed  Discharge based on the total of the Susquehanna, the Potomac and the James Rivers  On average, spring months show highest discharge values  Some maximum discharges occur in summer/autumn  tropical precipitation

Annual cycle of snow depth: Chesapeake watershed  On average, largest snow depth months are January and February  Large decrease in snow depth in March  Maximum daily snow depth shows largest decrease in snow depth from March to April  late season ablation events

1.Ablation event: 16 th March 1978

 Steady ablation for 5 days before flooding event  Decrease in snow depth from approx 28 cm to 5 cm  No significant precipitation events  Discharge peaks at approx 8000 m 3 /s Sea level pressure: 3/15/78 Precipitation rate: 3/14/78

2.Rain on snow: 20 th January 1996

 Rapid loss of snow depth over the 3 days before event  Snow depth decrease from approx 30 cm to almost 0 cm  Large precipitation event the day before event (~4 cm)  Ablation most likely intensified flooding event Sea level pressure: 1/19/96 Precipitation rate: 1/19/96

3.Ablation to rain: 2 th April 1970

 Steady ablation of approx 10 cm of snow  Precipitation event marks the start of a peak discharge event  Discharge peaks at approx 11,800 m 3 /s two days after event Sea level pressure: 4/2/70 Precipitation rate: 4/2/70

Methodology 1.Identify major flooding events during the fifty year period for the Chesapeake Bay watershed using stream flow and snow depth criteria. 2.Classify events based on type of snow cover ablation taking place. 3.Identify principle atmospheric features associated with each classification type. 4.Use SNTHERM to model atmosphere snow cover interactions.

Calculation of energy fluxes during ablation events with SNTHERM snow pack model…. developed by Jordan (1991)

Flux analysis Bingahamton, NY 2.Williamsport, PA 3.Harrisburg, PA 4.Washington, DC

1.Ablation: 16 th March 1978  Net solar flux is largest component affecting snow pack  Precipitation receipt on the 14 th cause sensible and latent heat to spike Sea level pressure: 3/15/78

2.Rain on snow: 20 th January 1996  Huge snow depth amounts. From 1 meter of snow to 0 over 4 days  Huge sensible and latent heat fluxes associated with precipitation event Sea level pressure: 1/19/96

3.Ablation to rain: 2 nd April 1970  Consistent positive fluxes into the pack before event  Snow pack evoulution leads to rapid ablation and precipitaiton Sea level pressure: 4/2/70

Summary of results 1.It is possible to isolate the snow ablation signal for the Chesapeake Bay watershed. 2.A common theme is strong low pressure in the lower Great Lakes Region bringing warm and moist air across the Chesapeake watershed. 3.Large values of sensible and latent heat flux are typically the largest components of the energy budget during the most intense ablation events.

Questions or comments? Contact info: Gina Henderson Department of Geography University of Delaware