Sensitivity Tests of Precipitation and T2m to Soil Moisture Using

NCEP WRF EnsembleJun Du, George Gayno and

Kenneth MitchellEMC/NCEP and SAIC

outline

• Sensitivity study• Basic physical mechanism understanding

• A soil moisture perturbation design• Ensemble system improvement (to be

done)• Summary

Under dispersive issue in warm season precipitation and near-surface variables

such as T2m

Ensemble spread of 12h-apcp from NCEP 21-member SREF (Jun 2006)

Tal agrand Di stri buti on (2m temperature) averagefor 20060808-20060824

0. 000. 050. 100. 150. 200. 250. 300. 350. 400. 450. 50

1 2 3 4 5 6 7 8 9 10 11

NCEP 10 members at T12Z

Prob

abil

ity

18 hours

Tal agrand Di stri buti on (2m temperature)average for 20060808-20060824

0. 000. 050. 100. 150. 200. 250. 300. 350. 400. 450. 500. 550. 60

1 2 3 4 5 6 7 8 9CAMS 8 members at T12Z

Prob

abil

ity

18 hours

Tal agrand Di stri buti on (2m temperature)average for 20060808-20060824

0. 000. 050. 100. 150. 200. 250. 300. 350. 400. 450. 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16CMA 15 members at T12Z

Prob

abil

ity

18 hours

Tal agrand Di stri buti on (2m temperature)average for 20060808-20060824

0. 000. 050. 100. 150. 200. 250. 300. 350. 400. 450. 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17MSC 16 members at T12Z

Prob

abil

ity

18 hours

( from NMC/CMA, Y. Li)

CMA 2M temperatureAverage for 22060808-20060814

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6 12 18 24 30 36Forecast Hours

RMS

erro

r

EPS meanSpread

CAMS 2M temperatureAverage for 22060808-20060814

0

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5

6 12 18 24 30 36Forecast Hours

RMS

erro

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EPS meanSpread

J MA 2M temperatureAverage for 22060808-20060814

0

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4

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6

7

6 12 18 24 30 36Forecast Hours

RMS

erro

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EPS meanSpread

MSC 2m temperatureAverage for 22060808-20060814

0

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6 12 18 24 30 36Forecast Hours

RMS

erro

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EPS meanSpread

NCEP 2m temperatureAverage for 22060808-20060814

0

1

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4

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6 12 18 24 30 36Forecast Hours

RMS

erro

r

EPS meanSpread

(from NMC/CMA, Y. Li)

3 WRF_NMM (NCEP, 40km) and 3 WRF_ARW (NCAR, 45km)

0-10cm Soil moisture diff (nam-gfs)

Nam is wetter than gfs in general (except For SE US and Greenland)

NOAH

NOAH

Diff ~0.1

Sensitivity of 3h-apcp forecast to soil moisture difference within WRF_NMM

With NAM soil moisture (NOAH LSM)

With GFS soil moisture (HOAH LSM)

Difference in forecast caused by different soil moisture IC can not be neglected!

Sensitivity of 3h-apcp forecast to soil moisture difference within WRF_ARW

With NAM soil moisture (NOAH LSM) With GFS soil moisture (NOAH LSM)

Difference in forecast caused by different soil moisture IC can not be neglected!

With nam soil moisture (NMM)

With gfs soil moisture (NMM)

T2m diff (namSM – gfsSM, NMM)

Within WRF_NMM model:Impact on T2m is significant!

With nam soil moisture (ARW)

With gfs soil moisture (ARW)

Within WRF_ARW model:Impact on T2m is significant.

T2m diff (namSM – gfsSM, ARW)

Impact comparison between “IC diff” and “soil moisture diff” within WRF_NMM

Difference caused by different ICs Difference caused by different soil moisture

Impact from diff soil moisture is comparable to that from diff ICs within NMM!

[note: (a) <0.1” contour not plotted, (b) it’s only 3h accumulation]

(3h-apcp)

Impact comparison between “IC diff” and “soil moisture diff” within WRF_ARW

Difference caused by different ICs Difference caused by different soil moisture

Impact from diff soil moisture is comparable to that from diff ICs within ARW!

(3h-apcp)

Impact from soil moisture diff is slightly less than but still comparable in some areas to that even from model diff (NMM vs. ARW)!

Impact comparison between “model difference” and “soil moisture difference”

Difference caused by different models Difference caused by soil moisture diff (nmm)

3h-apcp

Impact comparison between “model difference” and “soil moisture difference”

Difference caused by different models Difference caused by soil moisture diff (arw)

Impact from soil moisture diff is slightly less than but still comparable in some areas to that even from model diff (NMM vs. ARW)!

3h-apcp

different IC (NMM; namSM)

different models (nmm vs. arw; namSM)

Diff soil moisture (nam – gfs; NMM)

T2m -- impact comparison (NMM):Soil moisture impact on T2m is bigger Than that of IC and of model difference!

T2m -- impact comparison (ARW):Soil moisture impact on T2m is comparable To that of IC but smaller than model diff

Diff soil moisture (nam – gfs; ARW)

Different IC (ARW, namSM)

Diff models (nmm vs. arw; gfsSM)

Physical mechanism: soil moisture evapotranspiration

partitioning of net available surface thermal energy between latent heat and sensible heat flux convection and local circulation

(a) dry case: increased sensible heat will have an earlier initiation of convection (e.g. afternoon thunderstorm);(b) wet case: in contrast to “dry case”, reduced sensible heat will delay initiation convection (e.g. late night thunderstorm);(c) perturb soil moisture local thermal gradient local circulation precipitation

0-10cm Soil moisture diff (max-min)

Wet ~0.5

Dry ~0.1

Diff ~0.3-0.4

T2m diff (wet – dry, NMM)

Local afternoon Local early morning

T2m diff (wet – dry, NMM)

With WRF_NMM model

(Aligo, Gallus and Segal, 2006 WAF)

Afternoon (dry) Afternoon (wet)

Late night (dry) Late night (wet)

Wet soil moisture

Dry soil moisture

Diff (wet – dry) with ARW

Wet soil moisture more rainfallIn general especially in mid-high Latitudes than dry soil situation

Wet soil moisture condition Dry soil moisture condition

Response of 3h-apcp forecast to IC difference depends on soil moisture content:More sensitive in wet condition than in dry condition (WRF_NMM)

Wet soil moisture condition Dry soil moisture condition

(WRF_NMM model)

T2m is more sensitive to IC in “dry soil” than in “wet soil” condition

Response of 3h-apcp forecast to model difference depends on soil moisture content:More sensitive in wet condition than in dry condition

Wet soil moisture condition Dry soil moisture condition

Wet soil moisture condition Dry soil moisture condition

Similar sensitivity of T2m to model difference is observed under wet vs. dry tests

Sensitivity of T2m to model diff depends on which soil moisture initial states to use

With NAM soil moisture (wetter) With GFS soil moisture (dryer)

Original soil moisture perturbation design for N members

• mem1 = S1 (GFS) • mem2 = S2 (NAM)• Mem3 = (S1+S2)/2• Mem4-N=min(S1,S2)+random(i)*|S1-S2|;

(i=1,N; 0.0<random<1.0) S1(gfs) ----S(i)-----S3-----S(i)-----S2(nam) (for 4 layers: 0-10, 10-40, 40-100, 100-200cm)

Modified soil moisture perturbation design for N members

• mem1 = S1 (NAM-0.05) • mem2 = S2 (NAM+0.05)• Mem3 = (S1+S2)/2=NAM• Mem4-N=min(S1,S2)+random(i)*|S1-S2|; (i=1,N;

0.0<random<1.0)

S1(namL)--S(i)--S3(nam)--S(i)--S2(namU) (constrain: Smin<S(i)<Smax)

(for 4 layers: 0-10, 10-40, 40-100, 100-200cm)

Hopefully an Improved ensemble system after adding soil moisture

perturbations (plan to use pre-operation MIST to do a longer time

period of experiment)

Perturb SM only with 5-member WRF_NMM ensemble (T2m spread)

Perturb SM only with 5-member WRF_ARW ensemble (3h-apcp spread)

Summary

• Impact on precipitation and T2m forecast caused by soil moisture uncertainty cannot be neglected. It might be comparable to that caused by IC uncertainty and to, in a varying degree, that caused by different models in these two cases. Therefore, soil moisture perturbation needs to be considered in mesoscale ensembling although the degree of sensitivity may vary with model used. More careful study is surely needed.

• In general, wet soil moisture produces more rainfall than dry soil moisture especially in high/mid-latitudes. [Others also showed that dry soil leads early afternoon thunderstorms while wet soil late night thunderstorms.] Precipitation becomes more sensitive to IC and physics when soil moisture is wet than dry. (more evapotranspiration more moist air?)

• Soil moisture directly impacts T2m by changing surface thermal energy balance. Perturbing soil moisture has larger impact on T2m than on precip. T2m seems to be more sensitive to IC in “dry soil” than in “wet soil”. (more sensible heat?)

• A simple but realistic soil moisture perturbation generating method is proposed. Hopefully, future test result will show that adding perturbation to soil moisture improves ensemble performance (so keep your finger crossed)