The Effects of Grid Nudging on
Polar WRF Forecasts in Antarctica
Daniel F. Steinhoff1 and David H. Bromwich1
1Polar
Meteorology Group, Byrd Polar Research Center, and Atmospheric Sciences
Program, Department of Geography, The Ohio State University, Columbus, Ohio
“You’re the Inspiration”
• During my visits to the McMurdo forecasting
office, it was clear that AMPS forecasts
beyond 72 hours are not well trusted
– 72-120 hours: Events generally “pushed back” in
time in subsequent forecasts
– Only taken seriously inside of 72 hours
Last Year
• Aaron Wilson from our group found that grid
nudging of Polar WRF simulations in the Arctic
improves extended simulations compared to
non-nudged simulations.
• Are similar improvements possible in the
Antarctic? Can AMPS forecasts out to 120
hours be more trustworthy?
Grid Nudging
• Grid nudging involves an extra time tendency
term in the forcing equation for a given
variable
• This extra term “nudges” the tendency term
towards the driving analysis/forecast,
weighted by a representative time scale
parameter
• Originally designed for dynamic initialization
of forecasts, but also useful during simulations
Testing Strategy
• Testing Simulations
– Forced by NCEP GFS 1.0° forecasts
– NUDGE: 120 hour simulation, nudged for top 5 model
vertical levels only
– NONUDGE: Same except no nudging
• Validation Simulation
– Forced by NCEP FNL 1.0° analyses
– 48 hour simulations, reinitialized every 24 hours
• So we compare the GFS-forced NUDGE and NONUDGE
runs (24-120 hours) with FNL-forced “validation runs”
(24-48 hours) at corresponding times
Testing Strategy (Cont’d)
• September – November 2009
• Testing simulations initialized at 0000 UTC on
the 5th, 10th, 15th, 20th, 25th, and 30th of each
month (Total of 18 cases)
• Average verification statistics for each forecast
hour from 24-120 hours computed (first 24
hours of each simulation discarded for model
“spin-up”
Model Settings
• Polar WRF 3.1.1
• Identical 20 km
domain to previous
AMPS domain 2
• MP: WSM5
• SW, LW: RRTMG
• Surface, PBL: MYNN
• LSM: Noah
Domain average statistics - PSFC
• Biases rapidly increase after 72 hours, as do
NUDGE and NONUDGE differences
• Small differences between RMSE
Domain average statistics – Z500
• 500 hPa geopotential height statistics similar
to PSFC
Anomaly Correlations - PSFC
Forecast Hour
NUDGE
NONUDGE
24
0.99
0.99
30
0.99
0.99
36
0.99
0.99
42
0.98
0.98
48
0.95
0.95
54
0.94
0.94
60
0.93
0.93
66
0.92
0.92
72
0.90
0.89
78
0.87
0.87
84
0.85
0.84
90
0.82
0.82
96
0.80
0.80
102
0.78
0.78
108
0.76
0.76
114
0.73
0.72
120
0.68
0.68
Bias at 120 hours
NUDGE
NONUDGE
RMSE at 120 hours
NUDGE
NONUDGE
RMSE Differences - PSFC
BLUE: NUDGE BETTER
RED: NUDGE WORSE
• Surface pressure: Scattered differences over ocean, but
large expanse of lower RMSE over Antarctica for NUDGE
RMSE Differences – Z500
BLUE: NUDGE BETTER
RED: NUDGE WORSE
• 500 hPa geopotential height: Scattered differences over ocean, but
large expanse of lower RMSE over Antarctica for NUDGE
RMSE Differences – T2
BLUE: NUDGE BETTER
RED: NUDGE WORSE
• 2 m air temperature: No discernible pattern
RMSE Differences – q850
BLUE: NUDGE BETTER
RED: NUDGE WORSE
• 850 hPa mixing ratio: No discernible pattern
Antarctic Land Only - PSFC
• Taking only the land points:
– Bias and RMSE differences larger in a relative
sense than for entire grid
Ocean Only - PSFC
• Taking only the sea points:
– Bias smaller, RMSE larger than land
– Biases remain constrained longer (almost 96 hours)
– Smaller RMSE differences than land
Conclusions
• Grid nudging improves verification statistics
presented here
– Small improvements on grid average, but greater
improvements over Antarctic land
– Grid nudging can help/hurt over certain ocean areas
(likely placement of synoptic-scale cyclones)
• More work needed:
– Determine causality of changes to forecasts by
nudging over both Antarctica and Southern Ocean
– Verification against station and AWS observations