(Some) surface data impact studies at ECMWF - · PDF file(Some) surface data impact studies at...

WMO workshop on Impact Studies 9-12 March 2004

(Some) surface data impact studies at ECMWF

Jean-Noël Thépaut, Graeme KellyECMWF


Outline

• Background and rationale• Experimental framework• “Low resolution” OSEs• “High resolution” OSES• Concluding remarks


Background and Rationale

• Request from EUMETSAT to ECMWF• MSG and METOP follow-up: scope of measuring surface

pressure from space?

• Goal: Assess through Observing System Experiments (OSE) the relative merits of the current surface pressure Observing System versus the current surface wind Observing System.

• Further interest expressed internally (and by EUCOS) on the impact of the surface network in a heavily space-based constrained NWP system


Experimental framework (4D-Var)


Elements of 4D-Var (CY25R1)• Temporal window: 12 hour

• Observations binned in hourly slots

• Trajectory run: TL511 (40 km) L60

• Minimization run: TL159 (120 km) L60

• Two minimizations:– 1st minimization: – 30 iterations– 2nd minimization: – 25 iterations (improved physics)


Observations used at ECMWF (CY25R1)

• Conventional:• Synop surface pressure• Ship surface pressure and wind• Buoy surface pressure and wind• PAOB surface pressure• Radiosonde temperature wind and humidity• Pilot wind• Aircraft temperature and wind• American and European profiler winds

• Satellite:• AMSU-A radiances from NOAA-15/16• HIRS-12 radiances from NOAA-14• SSM/I wind speed and TCWV from DMSP-13/14• Seawinds wind vectors from Quikscat• Atmospheric Motion Vectors from GOES/Meteosat/GMS• CSR WV radiances from Meteosat 7• Ozone total column and profiles from GOME/ERS and

SBUV/NOAA16


Observational errors used at ECMWF (CY25R1)

• Synop surface pressure: 0.7 hPa• Ship surface pressure: 1.2 hPa• Buoy surface pressure: 0.8 hPa• PAOB surface pressure: 3.0 hPa

• Ship winds: 2.0 m/s• Buoy winds: 1.8 m/s• Scatterometer winds: 2.0 m/s• SSM/I winds: 2.4 m/s

NB: background errors are:Surface wind: ~ 0.9 m/sSurface pressure:~ 0.8 hPa


“Low resolution” OSES

• Data assimilation system:– Light version of the CY25R1 operational system

• 4D-VAR over 6 hour• Resolution T319/T63• Current operational cycle at that time (data usage, QC,

physics)• More or less consistent with Bouttier and Kelly (1999)

• Period under investigation– 20020201-20020317 (~6-7 weeks) : winter– 20010615-20010801 (~7 weeks) : summer

• Denial type experiments


“Low resolution” OSEs

• The questions the OSEs have been designed to provide guidance to are:1. Merits today of the current surface pressure

Observing System versus the current surface wind Observing System

2. What is the impact of a modification in accuracy of the surface pressure Observing System?

3. Are surface pressure over land+surface winds over sea able to control the surface pressure field?

4. What is the impact of reducing the current surface pressure Observing System over sea?


“Low resolution” OSEs (ctd)

• WINTER OSEs:1. CTL: control (all observations in)2. NOSURF: no surface data3. NOSURFWIND: no surface wind observations (Quikscat,

buoys, Ships, SSMI)4. NOSURFWIND2hPa: no surface wind observations and

all observational errors for surface pressure observations set to 2 hPa

5. NOSP: no surface pressure data6. NOSPSEA: no surface pressure data over sea7. NOSPBUOYSEA: no buoy surface pressure data (only

Ship surface pressure data left)


“Low resolution” OSEs (ctd)

• SUMMER OSEs (PAOBS systematically removed):

1. CTL: control (all observations in)2. NOSURFWIND: no surface wind observations (Quikscat,

buoys, Ships, SSMI)3. NOSPSEA: no surface pressure data over sea4. NOSPBUOYSEA: no buoy surface pressure data (only

Ship surface pressure data left)


Numerical Results: WINTER


Winter Mean Z1000 hPa analysis field

(20020204-20020317)


CTL / NOSURF / NOSPLarge biases in case of no surface pressure observations

Small compensation by surface wind observations

Surface winds alone have a detrimental impact in SH (not shown)


NOSURF versus CTLLarge RMS forecast errors (including large biases) in case of nosurface Observing System.


CTL versus NOSPSEA ?Significant degradation of the scores when surface pressure observations over sea are removed

Sea Surface winds + Land Surface Pressure are not sufficient


NOSURFWIND versus NOSPSEA ?


NOSURFWIND versus NOSPSEA ?

surface pressure observations (ships, buoys, paobs) have a (marginally) larger impact than surface winds (ships, buoys, SSM/I and QuikScat) in the Northern Hemisphere (large scatter)

Similar importance in Southern Hemisphere (1000 hPa)


NOSURFWIND versus NOSPSEA?

NH

SH

Fc+48 Fc+96


NOSURFWIND versus NOSPSEA?


NOSURFWIND versus NOSURFWIND2hPa

• Proxy to a degraded surface pressure Observing System in Northern Hemisphere, and improved in the Southern Hemisphere



NH

SH

Fc+48 Fc+96



• Conclusions:

• Northern Hemisphere: Specification of 2 hPa observation errors for Surface Pressure Data has a negative impact (degradation of the Observing system)

• Overall, impact of surface pressure observations smaller than in Bouttier and Kelly (2002)

– Sign of an improved and better constrained assimilation system?


CTL versus NOSPBUOYSEA?

NH

SH

Fc+48 Fc+96



Europe

N. America

Fc+48 Fc+96



Northern Hemisphere: Slight degradation of the scores when surface pressure buoys are removed

Southern Hemisphere: neutral impact (PAOB+SHIP sp + surface winds able to compensate?)


NOSPSEA versus NOSPBUOYSEA?

Surface pressure Ships (+ surface winds) manage to recover a substantial part of the quality of the CTL


Concluding remarks (1)

• With the precautions required due to the limits of the OSEs, the main findings of the WINTER OSEs (44 cases) are:

• Surface data are an essential element of the current Observing System

• Surface pressure observations are absolutely essential to anchor the surface pressure field

• Surface winds assimilated in isolation can have a detrimental impact



• More specifically, the WINTER OSEs provide some guidance to the original questions:

1. In isolation, surface wind Observing System (1/3 buoys, Ships, QuikScat and SSM/I) have a similar impact as surface pressure Observing System (buoys, Ships, PAOBs) in the Southern Hemisphere

2. Larger impact of surface pressure Observing System in the Northern Hemisphere

3. A degradation of the accuracy of the current surface pressure observing system has a clear detrimental impact



4. In presence of surface winds, a limited number of (accurate) surface pressure observations – Ships -manage to recover most of the forecast error

SUMMER OSES (48 cases):1. In boring anticyclonic regimes, the scatter of the impact

from surface pressure observations is very large2. Impact for the forecaster questionable?3. If at all, positive impact of surface pressure observation

from buoys (but it is hard to be conclusive)!


“High resolution” OSES

• Data assimilation system:– CY25R4 operational system

• 4D-VAR over 12 hour• Resolution T511/T159

– Consistent with Kelly et al. (2004)

• Period under investigation– Winter 2002-2003 (2 months)– Summer 2002 (2 months)

• Reduced set of denial type experiments– NOSHIPBUOY (both seasons)– NOSHIP (summer)– NOBUOY (summer)


ECMWF system (25R4)

• 3xAMSUA (NOAA 15/16/17) - radiances• 3 SSMI (F-13/14/15) - radiances• 2xHIRS (NOAA-16/17) - radiances• Radiances from 2xGEOS (Met-7 GOES-10)• Winds from 3xGEOS (Met-7/5 GOES-10) and

MODIS/TERRA • SeaWinds on QuiKSCAT• ERS-2 Altimeter / SAR• SBUV (NOAA-16)

Do surface observations still matter?


The answer is yes!NH

SH

Europe

N.Am


Winter

Summer

Larger impact in Winter than in Summer


NH

EUROPE

SH

60 summer cases

- positive impact of both buoys and ships


Northern Hemisphere: Forecast error day 1/2/3


Southern Hemisphere: Forecast error day 1/2/3


Europe: Forecast error day 1/2/3


Case study: 20020809 12Z

NOSHIPBUOY

3day forecast

CONTRL

3day forecast

NOSHIPBUOY 3day

forecast diff.

Verifying

Analysis

(ops)


Where does it come from?

buoys

Ships

NOSHIPBUOY – CONTRL MSL diff


Time evolution of the MSL difference


NOBUOY - CONTRL

NH SH

EUROPE


NH SH

EUROPE

NOSHIP - CONTRL


Conclusions from “high resolution” OSEs

• The Sea surface observation network is important despite the growing impact of satellite observations– Results are statistically significant (not shown)– Larger impact in the Northern Hemisphere (specially in

winter)

• Buoys and Ships seem to be equally important– Perhaps a slight advantage to the Ships

• Buoys and Ships can have a crucial impact synoptically• Complementarity of the Space and Terrestrial network

for global NWP

(Some) surface data impact studies at ECMWF - · PDF file(Some) surface data impact studies at...

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