1. THE POTENTIAL FOR FLOOD AND DROUGHT PREDICTION
N. Voisin, D.P. Lettenmaier and A.W. Wood
Department of Civil and Environmental Engineering
University of Washington 4
Downscaling from 2.5 to 0.5 degree 1 3
GFS reforecast Bias Correction
Introduction
While weather and climate forecast methods have advanced greatly over the last two decades, this capability has yet to be evidenced in mitigation of water-related natural hazards (primarily floods and droughts), especially in the developing world (Lettenmaier et al, 2006). For instance, Mozambique experienced major droughts in 2005 and 2002 which resulted in widespread food shortages and major floods in 2000 and 2001 which affected large parts of the country. In Southeast Asia, early monsoon rains that began in July 2000 resulted in flooding of the Mekong River and its tributaries in Cambodia, Vietnam, Laos and Thailand. It was the worst flooding in several decades and affected more than 4.5 million people and killed several hundreds. Mitigation of these events through advance warning was at best modest; despite the above noted improvement in weather and climate forecasts, there is at present no system for forecasting of floods and droughts globally, notwithstanding that the potential clearly exists. We describe development of a methodology that is eventually intended to generate global flood and drought predictions routinely. It draws heavily from the experimental North American Land Data Assimilation System (NLDAS) and the companion Global Land Data Assimilation System (GLDAS) for development of nowcasts, and the University of Washington Experimental Hydrologic Prediction System to produce ensemble hydrologic forecasts based on the NCEP Global Forecast System for lead times from seven days to six months using the University of Washington/Princeton University Variable Infiltration Capacity (VIC) macroscale hydrology model. In the prototype (tested using retrospective data), VIC is driven globally up to the time of forecast with daily ERA40 precipitation (rescaled on a monthly basis to a station-based global climatology), ERA40 wind, and ERA40 average surface air temperature (with temperature ranges adjusted to a station-based climatology). In the retrospective forecasting mode, VIC is driven by global NCEP ensemble 15-day reforecasts provided by Tom Hamill (NOAA/ERL), bias corrected with respect to the adjusted ERA40 data and further downscaled spatially using higher spatial resolution Global Precipitation Climatology Project (GPCP) 1dd daily precipitation. Downward solar and longwave radiation, surface relative humidity, and other model forcings are derived from relationships with the daily temperature range during both the retrospective (spinup) and forecast period. The initial system is implemented globally at one-half degree spatial resolution. We evaluate model performance retrospectively for predictions of major floods for the Rhine and Meuse in 1995.
Precipitation is downscaled using the Schaake Shuffle (Clark et al. 2004) with the satellite product GPCP1dd (Huffman et al. 2001) used as observations. Observations are not sorted yet, see future work section. GFS reforecast precipitation is scaled using the ratio of GPCP1dd values at 0.5 degree over the value at 2.5 degree:
The daily temperature average is downscaled via an inverse square distance interpolation. The daily temperature range is assigned using a Schaake Shuffle type of selection using Adam et al. (2006) global temperature dataset:
The similar quantile-quantile method as used in (Wood et al. (2005) is applied directly to the GFS reforecast values.
ERA40 CDF
GFS reforecast CDF
GFS control run
bias corrected GFS reforecast value
GFS reforecast value
degree Tmin as a function of Tmax
Daily average 2.5 degree bias corrected GFS reforecast temperature
0.5 daily temperature average ( interpolated)
degree cells GPCP 1dd value used for downscaling the corresponding bias corrected 0.5 degree GFS reforecast
2.5 degree GPCP 1dd precipitation value used for downscaling the corresponding bias corrected 2.5 degree GFS reforecast
Bias correction applied to cell (50oN,2.5oW) for the GFS reforecast of January 20th, 1995
Precipitation Daily Average Temperature
GFS reforecast of January 20th, 1995 , for one ensemble, at cell (50oN,2.5oW)
Precipitation downscaling Temperature downscaling 2
Approach 5
January 1995 Rhine and Meuse Floods
GFS reforecast as of January 20th, 1995
Ongoing Improvements
Need to remove bias (generally downward) in monthly ERA40 precipitation with respect to gridded observations Adam et al. (2006), and improve VIC calibration (presently essentially uncalibrated)
Sort observed GPCP1dd events and interpolate them before downscaling
Apply to more events: Mozambique floods 2000 and 2001, Danube floods 2000, Mississippi floods 1993, Mekong floods 2000, Yangtze floods 1998, Oder floods 1997.
5-day accumulation Runoff
5-day change in soil moisture
local scale (1/2 degree) weather inputs
streamflow, soil moisture,
snow water equivalent, runoff
5-day accumulation Precipitation 6
GFS ens. Avg GFS det. Fcst ERA40
GFS ens. Avg GFS det. Fcst ERA40
GFS ens. Avg GFS det. Fcst ERA40
INITIAL STATE
Hydrologic model spin up
Hydrologic forecast simulation
ECMWF ERA40
(or
Analysis)
NCEP Reforecasts
(Hamill 2006)
bias corrected w/r to ERA40
downscaled to 0.5 degree
( NCEP GFS, ECMWF ESP)
Later on:
Update of initial conditions
GFS ens. Avg : average of the 15 GFS reforecast ensembles
GFS det. Fcst : GFS reforecast control run – GFS deterministic forecast
ERA40 : ECMWF 40 year reanalysis, surrogate for observations
Climatology daily flow
Climatology monthly flow
References
Adam, J.C., E.A. Clark, D.P. Lettenmaier, and E.F. Wood, 2005: Correction of Global Precipitation Products for Orographic Effects .J. Climate,19 (1),
Clark, M.P., S. Gangopadhyay, L.E. Hay, B. Rajagopalan, and R.L. Wilby (2004): The Schaake Shuffle: A method to reconstruct the space-time variability of forecasated precipitation and temperature fields. Journal of Hydrometeorology, 5,
Huffman, G.J., R.F. Adler, M. Morrissey, D.T. Bolvin, S. Curtis, R. Joyce, B McGavock, J. Susskind, 2001: Global Precipitation at One-Degree Daily Resolution from Multi-Satellite Observations. J. Hydrometeor., 2,
Lettenmaier, D.P., A. de Roo, and R. Lawford, Towards a capability for global flood forecasting, WMO Bulletin 55,
Wood, A.W. and D.P. Lettenmaier, 2005, A testbed for new seasonal hydrologic forecasting approaches in the western U.S., Bulletin of the American Meteorological Society (in press).
SHORT TERM FORECASTS (flood)
Several years back
NOWCASTS
Simulated
forecasted discharges
This schematic is similar experimental procedure as used by Wood et al (2005) West-wide seasonal hydrologic forecast system. Hydrologic simulations are performed using the Variable Infiltration Capacity model developed at the University of Washington and Princeton University.