NDSWC 20100106
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Presentation Transcript
North Central River Forecast Center:An Overview of Operations and Mission North Dakota State Water Commission January 6, 2010
Outline • NCRFC Mission and Forecast Products • Introduction to NWS Hydrologic Models • Model Calibration • Deterministic Forecast Process • Meteorological driving data • Hydrologic model analysis • Other factors • Forecast preparation
Mission of NOAA's NWSHydrologic Services Program To provide river and flood forecasts and warnings for protection of life and property Provide basic hydrologic forecast information for the nation's economic and environmental well being.
Office and Staff • One of 13 River Forecast Centers in the USA • Staff of 19 forecasters and administrative personnel • Operates 365 days a year, 18-24 hours a day 13 NWS River Forecast Centers
Forecast Services • Flash Flood Guidance • Time Series Forecasts • Daily Stage Forecasts • Flood Forecasts • Long Range Forecasts • Inter-agency guidance • Long Range Probabilistic Products
Products based on models • All forecast products are based on model runs using current model states. • Time series forecasts incorporate additional information: • Analogous historical events • Rules-of-thumb • Field notes & measurements • Un-modeled phenomena • & more!
NCRFC Forecasting NWSRFS Models
Lumped Model Conceptual Model National Weather Service River Forecast System CHPS is coming Snowpack Models Runoff Models Unit Hydrograph Model River routing Models Reservoir Models
NCRFC Area of Responsibility • 1117 sub-basins in 33 forecast groups • 33 forecast groups • 3 Major Basins • Mississippi River • Great Lakes • Hudson Bay
North Dakota (& Canada) • 132 sub-basins in North Dakota & Canada • Sub-basin Statistics • Mean Area = 316 mi2 • Min Area = 14 mi2 • Max Area = 3061 mi2
NWSRFS Models Model inputs
Meteorological Driving Data Models run on a 6-hr time step
PrecipitationSurface observations Point precipitation Mean areal precipitation • Hourly to daily accumulations • Manual and automated observers • Quality controlled
PrecipitationRadar Estimates Gridded precipitation Mean areal precipitation • 27 USA radars & 8 Canadian radars • Hourly grids • Bias corrected based on surface observations • Quality controlled
PrecipitationForecast • Gridded • 6-hour accumulation • 5 days available • Collaborative
Air TemperatureObserved & Forecast • Observed Period • Daily max/min derived from 3-hr time series • Forecast Period • Daily max/min • 7 day gridded
NWSRFS Models Snow model (snow-17)
SNOW-17 Model Overview • Input time series • Air Temperature • Precipitation • Output time series • Rain + Melt • Simulated Water Equivalent • Simulated Areal Extent of Snow Cover • Simulated Snow Depth
SNOW-17 Model calculations • Determine precipitation type • Accumulation of snow cover • Snowpack energy budget • Surface energy exchange • Snowpack temperature • Liquid water content • Snowmelt
Model Schematics • Override precip type • Change snowpack water equivalent • Change areal extent of snow cover • Modify melt rate Snow Model Run-time modifications
Example of SNOW-17 OutputWild Rice River at Abercrombie Minimum Temperatures above freezing during ripe pack Rain on Snow March 31 causes rest of pack to melt
Average Daily Temps Above Freezing Most Melt Occurred In 2 days! Areal Extent of Snow Cover NOHRSC SNODAS v. SNOW-17 Depths
SWE Updating • SNOW-17 simulation • Manual snow msmts • NOHRSC simulation • NOHRSC Gamma snow surveys • Point precip accumulation • Special Sensor Microwave/Imager
Forecaster Adjustments • MFC is a multiplier on “normal” melt • MFC 03100612Z – 03260612Z FGROUP RED 0.1 • Used annually to mimic runoff stuck in ditches, field ponding, etc. • Ice break-up on rivers & streams • Usually once runoff shows up in river gaging, MFC set to default to 1.0 Example: reduce melt rate
Melt Factor Correction Mod • 2006 Melt rate was faster than “normal” requiring a larger MFC value (1.5 – 3.0!!) • .MFC 03260618Z – 04050612Z FGON8 2.0 • During time of active melt • Meteorological conditions causing abnormal snowmelt • High dewpoints • High windspeeds • Difficult to forecast MFC into future • Watch meteorological forecasts closely • Possible temperature bias • Rain on snow
Current SNOW-17 Model • Other observations from the field • Water flowing in ditches • Water stuck in ditches • Runoff under snowpack • Ice jams, ice out, river ice conditions, etc. • Overland flow freezing • Distribution of snow cover • Drain tiles running? • Anything regarding the movement of water during snowmelt
NWSRFS Models Sacramento Soil moisture accounting model (SAC-SMA)
Model Schematics Soil Moisture/Runoff Model • Spatially-lumped, continuous soil moisture accounting model • Ideal model for the simulation of medium-large-scale (hundreds of mi2) basins • Takes areal averaged rain+melt and temperature time series as input • 19 parameters
Forecaster Interaction • Change the contents of any “bucket” in the model • Modify the input rain+melt time series • Modify the output runoff time series • Change the frozen ground index
NWSRFS Models SAC-SMA Frozen Ground Option
The Frozen Ground Scenario • Soils are wet as freeze-up begins • There is little snow cover to insulate the soil • Hard freeze occurs making the soil less permeable to subsequent snowmelt events • Somewhat rare, but responsible for big floods Mankato, Minnesota, 1965
Modeling Frozen Ground Runoff The frozen ground model reduces the percolation and interflow rates. From Center for Hydrometeorology and Remote Sensing, University of California, Irvine
Percolation & Interflow Reduction Deeper frost = Less percolation WET DRY
SAC-SMA Frozen Ground Option • Frozen ground parameters are calibrated • The computed value for FGIX is dependent on temperatures and snow cover – a running history of calculations all winter long… • The Frozen Ground option is very sensitive
Example: Goose River at Hillsboro, March 2004 Frozen Ground Index -20C Observed Stage = 6.5 feet Simulated Crest 4.1 feet (1500cfs)
SAC-SMA Model States Frost Index = 0.0 Frost Index = -20
SAC-SMA Model States Frost Index = 0.0 Frost Index = -50.0