Synoptic-Scale Atmospheric Processes Associated with Snow Cover Ablation Events Across Eastern North American Stream Basins Daniel J. Leathers And Gina Henderson Center for Climatic Research Department of Geography University 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 Interest Susquehanna Ohio St. Lawrence Chesapeake

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.

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 the Chesapeake Bay, providing 90 percent 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.

Flooding event defined as a discharge at Harrisburg >250,000 ft 3 /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 events MM – Moist moderate (overrunning situation) 10 events MT – Moist Tropical (mT) 4 events MP – Moist polar (mP) 1 event TR – 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 L H 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) MM(10) MT(4) Trans(8) Three-day meteorological variables associated with basin-wide flooding events

Energy fluxes associated with synoptic- scale atmospheric patterns: March 2 – 4, 1964

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

H LL DMMT 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.

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 H L H L L DM MM MTTran

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.

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.