Using 925 mb Temperatures to Improve Operational River Forecasts Ronald S. W. Horwood Meteorologist National Weather Service Northeast River Forecast Center.
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Using 925 mb Temperatures to Improve Operational River Forecasts Ronald S. W. Horwood Meteorologist National Weather Service Northeast River Forecast Center Northeast Regional Operational Workshop 7-8 November 2007
Overview Background on Northeast River Forecast Center ( NERFC ) river forecast operations Collaborative Project with NCEP – HPC Case Simulations Summary/Future Work Questions/Comments
Background on NERFC Operations The NERFC produces daily river stage forecasts for approximately 200 forecast points across New England and New York State.
Background on NERFC Operations - cont River forecasts are produced on a 6-hourly time step. –Observed precipitation and temperature as well as forecast precipitation and temperature is input to the model as 6-hourly basin averages.
NERFC operations - cont Since 2000…the NERFC has been transitioning from the Antecedent Precipitation Index ( API ) model to the Sacramento Soil Moisture Accounting model ( SACSMA ). –The API model is an index based event type model while the SACSMA model is a continuous model –SACSMA can be broken up into lower and upper elevations zones…each having different model states ( e.g. temperature…snow water equivalent…soil moisture etc ). Zones delineated ~ 2000 feet. Currently about 75% of NERFC forecast basins have transitioned to the SACSMA model.
NERFC Operations - cont Temperatures for the upper zones in the SACSMA model were being determined by applying a constant lapse rate that varied monthly across river basins. –During the winter season…problems were quickly noted during elevation snow events. Calculated lapse rates were typically 3-5 degrees Farenheit between the upper and lower basins so when valley locations ( where the data is ) were 37-40 F and raining…upper zones would still be at or above the 34 F threshold for frozen precipitation and rain would be forecast. We would also notice problems in freezing rain events since the lapse rate was always forecast to decrease with height.
Collaborative Project with HPC NERFC Hydrometeorological Analysis and Support forecasters proposed a scheme to initialize upper zone forecast temperatures with 925 mb temperatures. –For most NERFC forecast basins with significant elevation above 2000 feet…there is little if any real- time temperature data available. –A constant monthly lapse rate between the lower and upper basins is not a valid assumption. –925 mb ~ 3000 feet which is a nice approximation for the mean elevation for most upper zones in the NERFC service area and model data is readily available.
Collaborative Project with HPC HPC agreed to provide the NERFC with 48 hours of 925 mb forecast temperatures 4 times per day from the 40 km NAM-WRF model. –Originally…the 12 km NAM-WRF was used for test purposes but problems arose when gridding the dataset. HPC generates vgf files which are compatible with NMAP software which the NERFC uses to generate QPF and temperature forecasts. –The files are gridded by NERFC HAS forecasters and upper zone basin average temperatures are sent directly into NWSRFS. The program became operational at the NERFC during March 2007.
Case Simulations 2 case simulations will be shown for the NERFC forecast point at Au Sable Forks on the East Branch of the Au Sable River in northern New York State. –The East Branch of the Au Sable River is a 198 square mile basin draining the north slopes of the Adirondack mountain range. The headwaters of the basin are steep and mountainous and drain terrain well over 4000 feet including the eastern slopes of New York State’s highest peak…Mount Marcy. The outlet of the basin is a broad valley with gently rolling hills and elevation around 600 feet.
CASE Simulations - cont We will examine two precipitation events across the basin…both with 2.00 inches of melted equivalent precipitation in 12 hours with the following initial conditions: –1 inch of snow water equivalent across the upper elevation zone and 0.5 inches of water equivalent across the lower zone. Simulations of constant lapse rate (old forecast method – 4F in Jan) will be compared to 925 mb temperature forecasts for the following two cases: –Lower elevation rain with an average temperature of 38F ( cold rain ) with cold air aloft. –Lower elevation freezing rain with an average temperature of 32F with warm air aloft.
Simulation 1 Lower elevation rain at 38F with cold air aloft results in an all rain simulation using the constant lapse rate (old forecast method). Using 925 mb forecast temperatures…snow is predicted in the upper basin which reduces runoff and lowers the river stage forecast.
Using the old forecast method of a constant lapse rate between the lower and upper zones…the entire basin would be above the melt threshold and all rain would be generated by the river model…in this case with a forecast for flooding.
Inserting the 925 mb temperatures for this simulation keeps areas in the upper zone (> 2000 feet) below freezing…allowing frozen precipitation to accumulate and decrease runoff. The resultant forecast is now below flood stage.
Simulation 2 Lower elevation freezing rain results in cold air through both elevation zones and no runoff using the constant lapse rate (old forecast method). Using 925 mb forecast temperatures…warm air aloft results in runoff and a rise in forecast river stage.
Using the old forecast method of a constant lapse rate between the lower and upper zones…the entire basin would be below the melt threshold and all precipitation would be frozen. This would produce a forecast with minimal rise.
Inserting the 925 mb temperatures for this simulation keeps areas in the upper zone (> 2000 feet) above freezing…allowing rainfall and snowmelt to occur. This results in a marked rise at the forecast point.
Summary/Future Work By more accurately modeling temperatures in the upper zones (>2000 ft) of NERFC forecast basins...we believe resultant river forecasts will be improved. –This is especially true for elevation snow events in fall and spring. A case study is planned from the winter of 2007/2008 with results published during FY 2009.