© Imperial College LondonPage 1 Deriving clear-sky fluxes for GERB Jo Futyan Gist 21, CERES-GERB meeting 02/04/04 Boulder, Colorado

© Imperial College LondonPage 1

Deriving clear-sky fluxes for GERB

Jo FutyanGist 21, CERES-GERB meeting02/04/04 Boulder, Colorado


Why do we want clear sky flux data?

• Calculation of radiative forcing (e.g. clouds/ aerosol) – reference state

• Study of clear sky processes • Model comparison – separate out

uncertainties due to cloud representation• Consistency with existing ERB datasets

– ERBE/ CERES


GERB and SEVIRI data

ARCH data (3x3 SEV pix)

Instantaneous clear-sky for all times

(days with at least 1 obs.) Monthly time-step

mean (times with obs on at least 1 day)

Daily mean clear-sky fluxes

Instantaneous clear-sky ARCH

Threshold choice

BARG data Instantaneous

clear-sky BARG (missing data)

ARCH scale cloud mask

Cloud cover data(MPEF, RMIB, other)

BARG scale cloud mask

Threshold choice

Spatial Average

and integrate

Process clear pix

Monthly mean clear-sky fluxes

Monthly mean diurnal cycle

interpolate

Collect into time bins

Average ?

Interpolate through individual days

average ?


Choice of cloud flag…• RMIB – SW only flag, used for ADM selection• MPEF - LW only flag, produced by

EUMETSAT (CLM product)• Need 24h cloud flag - ? Use MPEF?

RMIB MPEF

11:45 3rd Dec 2003


Cloud flags and fluxes


Cloud flag comparison

• RMIB:– Sun-glint – fixed for new data– Desert, due to vegetation

changes - fix in progress

• MPEF :– misses some cloud – Problem over Sahara at night

• Compare overall performance with ISCCP– no concurrent SEVIRI/ ISCCP

data


Comparison with ISCCP cloud fraction

1 2 3 4 5

6 7

20

-40-40

0 60

40

0

sunglint desert • RMIB finds more cloud than ISCCP vis

• MPEF – low fraction at 1200 UTC


ISCCP comparison continued…

• MPEF lies mostly within ISCCP range at 0000 & 0600 UTC

• Flags look reasonable– MPEF fraction

only low in middle of day?


Impact on clear-sky flux/ albedo

• Look for biases due to cloud contamination• Choose flag → clear sky fluxes/ albedo• Form time-step mean at each pixel

RMIB 0%MPEF 0%

24/12/03 – 13/01/04 (15 days with data)


Impact on clear-sky flux/ albedo

• RMIB/ combined flag for SW, MPEF for LW?– Differences in sampling between channels– Small amount of cloud contamination in LW-clear fluxes at all times

• Combined flag for day, MPEF only at night?– Best possible at all times– Introduces small diurnal bias in LW clear-sky fluxes

• significant bias in albedo using MPEF – not suitable in SW


Data Quantities• Limited data if require 0% cloud esp for RMIB

• Relax threshold at BARG scale or use ARCH data• How much data do we gain?• how big are the errors?

– Cloud contamination– Spatial variability


Example – MPEF flag

Using ARCH – relax number of clear pixels required

Relaxing % cloud threshold

ARCH 25 clear pix














Errors in using ARCH data?

• Use average over clear ARCH pix to estimate GERB scale flux/ albedo

• Estimate error by sub-sampling clear footprints

1 GERB footprint

1 ARCH pixel

• Select n pixels at random + compare mean flux with the true value when all 25 pixels used

• Use albedo – convert to flux using true ISW• Consider scene types separately


Errors in using ARCH data cont…• Read off 95% (2 sd) spread at

each footprint• Find value including 95% of

footprints of given surf type

1200 UTC – other times similar/ smaller errors


Errors…• Errors in LW small - <10Wm-2 for 4 pix

– Good - fewer alternatives in LW

• Large errors in SW for land (desert/coast)– 10% error achieved at ~9 pix (except for coast)

• In average products these errors will cancel!• Does benefit (more data) outweigh loss of accuracy?

• Temporally sub-sample

• Estimate error if only have n days of data– Problem – lack of clear

data to sub-sample – Use 14 day period

• This is error in mean


Errors…• Estimate error in time-step mean at each footprint

with at least 1 day of data– Combine these estimates + knowledge of gain in number of days

with data• In LW always best to use few ARCH pix – for SW…

ocean dark veg dark des

• Errors small – doesn’t matter!• For instantaneous data reduce npix until error ~ that from filling data

– eg with time-step mean or via knowledge of diurnal cycle…


Effect on time-step mean fluxes – 1200 UTC


Effect on time-step mean fluxes 2.

• Estimate ‘error’ in monthly time-step mean

• See shift/ tail to lower LW (higher SW) flux– Expect symmetrical distribn from spatial variability– Don’t expect to introduce clouds…


Effect on time-step mean fluxes 3.

• Larger differences often where have little data– Initial estimate may not represent mean well

• Regions with large error not same for SW/LW– SW - cloud contamination? – LW effect in part due to humidity? – Less of a dry bias?


To do…• Investigate errors in using CERES ADMs to fill

missing data in SW – Effects on average & instantaneous data

• Possible interpolation in LW - sinusoids?– Associated errors?

• Compare monthly mean clear sky flux and cloud forcing products with CERES– ERBE like & Geo-enhanced

• Study of radiative forcing of tropical convection over Africa!

Documents

© Imperial College LondonPage 1 Deriving clear-sky fluxes for GERB Jo Futyan Gist 21, CERES-GERB meeting 02/04/04 Boulder, Colorado