Demonstration of Gridded Bias Correction

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Demonstration of Gridded Bias Correction

Demonstration of Gridded

Bias Correction

Daran Rife

National Center for Atmospheric Research

24 February 2009

1


Outline

• What is the gridded bias correction system and

how does it work?

• Examples of bias corrected forecasts.

• Discussion: How should bias corrected forecasts

be displayed?

2


Why implement a statistical correction?

Real world

Model representation

3


Why implement a statistical correction?

WSMR 6-h MM5 forecast errors during 2005,

valid 1800 UTC

2-m-AGL temperature

10-m-AGL wind speed

4


Why not use a traditional MOS

approach?

5


One alternative

Running-mean bias correction

6


Schematic of running-mean bias

correction

Bias correction computed as function of

station location and time of day

7


Estimating and removing errors from

the full model grid

Motivation by analogy: Curve fitting

8


Estimating forecast

biases between and

away from station

locations

9


How does the gridded bias correction

work?

• STEP 1: Measure forecast bias at

observation locations.

• STEP 2: Calculate coefficients of regression

that describe the linear relationship

between the running-mean forecast

variables at the obs locations, and those at

every point on the grid.

10


Calculating coefficients of regression

11


How does the gridded bias correction

work?

• STEP 1: Measure forecast bias at observation

locations.

• STEP 2: Calculate coefficients of regression

that describe the linear relationship between

the running-mean forecast variables at the obs

locations, and those at every point on the grid.

• STEP 3: Subtract bias from “raw” forecast to

obtain a correction at each obs location.

• STEP 4: Use regression coefficients to “map”

to corrections (at obs sites) onto the full grid.

12


Mapping corrections onto grid

Mapping correction onto

grid from bias measured

at single location (S32).

13


Diurnal evolution of

forecast bias

29 June 2005

14


Improvements realized through bias

correction

Use BC with CAUTION

during regime changes!

MM5 2-m-AGL temperature forecasts WSMR grid 3.

15


Bias correction provided for:

• 2-m-AGL temperature

• 2-m-AGL dew point temperature

• 2-m-AGL relative humidity

• 10-m-AGL wind direction

• 10-m-AGL wind speed

16


Displaying bias corrected forecasts

Model forecast table

17


Displaying bias corrected forecasts

Bias corrected

BC T2 (°C)

BC Td (°C)

BC RH (%)

BC Wspd10 (m/s)

18


Displaying bias corrected forecasts

19


Displaying bias corrected forecasts

Animation: Uncorrected WRF RTFDDA

Animation: Bias corrected WRF RTFDDA

20


Displaying bias corrected forecasts

Animation: Uncorrected WRF RTFDDA

Animation: Bias estimate WRF RTFDDA

21


Choosing the bias sampling period

WSMR MM5 grid-3 2-m-AGL temp forecasts 2005

23

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