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

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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

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