Demonstration of Gridded Bias Correction
Demonstration of Gridded Bias Correction
Demonstration of Gridded Bias Correction
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<strong>Demonstration</strong> <strong>of</strong> <strong>Gridded</strong><br />
<strong>Bias</strong> <strong>Correction</strong><br />
Daran Rife<br />
National Center for Atmospheric Research<br />
24 February 2009<br />
1
Outline<br />
• What is the gridded bias correction system and<br />
how does it work?<br />
• Examples <strong>of</strong> bias corrected forecasts.<br />
• Discussion: How should bias corrected forecasts<br />
be displayed?<br />
2
Why implement a statistical correction?<br />
Real world<br />
Model representation<br />
3
Why implement a statistical correction?<br />
WSMR 6-h MM5 forecast errors during 2005,<br />
valid 1800 UTC<br />
2-m-AGL temperature<br />
10-m-AGL wind speed<br />
4
Why not use a traditional MOS<br />
approach?<br />
5
One alternative<br />
Running-mean bias correction<br />
6
Schematic <strong>of</strong> running-mean bias<br />
correction<br />
<strong>Bias</strong> correction computed as function <strong>of</strong><br />
station location and time <strong>of</strong> day<br />
7
Estimating and removing errors from<br />
the full model grid<br />
Motivation by analogy: Curve fitting<br />
8
Estimating forecast<br />
biases between and<br />
away from station<br />
locations<br />
9
How does the gridded bias correction<br />
work?<br />
• STEP 1: Measure forecast bias at<br />
observation locations.<br />
• STEP 2: Calculate coefficients <strong>of</strong> regression<br />
that describe the linear relationship<br />
between the running-mean forecast<br />
variables at the obs locations, and those at<br />
every point on the grid.<br />
10
Calculating coefficients <strong>of</strong> regression<br />
11
How does the gridded bias correction<br />
work?<br />
• STEP 1: Measure forecast bias at observation<br />
locations.<br />
• STEP 2: Calculate coefficients <strong>of</strong> regression<br />
that describe the linear relationship between<br />
the running-mean forecast variables at the obs<br />
locations, and those at every point on the grid.<br />
• STEP 3: Subtract bias from “raw” forecast to<br />
obtain a correction at each obs location.<br />
• STEP 4: Use regression coefficients to “map”<br />
to corrections (at obs sites) onto the full grid.<br />
12
Mapping corrections onto grid<br />
Mapping correction onto<br />
grid from bias measured<br />
at single location (S32).<br />
13
Diurnal evolution <strong>of</strong><br />
forecast bias<br />
29 June 2005<br />
14
Improvements realized through bias<br />
correction<br />
Use BC with CAUTION<br />
during regime changes!<br />
MM5 2-m-AGL temperature forecasts WSMR grid 3.<br />
15
<strong>Bias</strong> correction provided for:<br />
• 2-m-AGL temperature<br />
• 2-m-AGL dew point temperature<br />
• 2-m-AGL relative humidity<br />
• 10-m-AGL wind direction<br />
• 10-m-AGL wind speed<br />
16
Displaying bias corrected forecasts<br />
Model forecast table<br />
17
Displaying bias corrected forecasts<br />
<strong>Bias</strong> corrected<br />
BC T2 (°C)<br />
BC Td (°C)<br />
BC RH (%)<br />
BC Wspd10 (m/s)<br />
18
Displaying bias corrected forecasts<br />
19
Displaying bias corrected forecasts<br />
Animation: Uncorrected WRF RTFDDA<br />
Animation: <strong>Bias</strong> corrected WRF RTFDDA<br />
20
Displaying bias corrected forecasts<br />
Animation: Uncorrected WRF RTFDDA<br />
Animation: <strong>Bias</strong> estimate WRF RTFDDA<br />
21
Choosing the bias sampling period<br />
WSMR MM5 grid-3 2-m-AGL temp forecasts 2005<br />
23
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