A comparison of surface fluxes from the NCEP Global Operational Analysis/Forecast System and the NCEP Reanalyses

 

Glenn H. White1, K. Campana1 and J. Janowiak2

1Environmental Modeling Center/2Climate Prediction Center

National Centers for Environmental Prediction/National Weather Service

National Oceanic and Atmospheric Administration, U. S. Dept. of Commerce

Washington, D.C., USA

Glenn.White@noaa.gov

 

Introduction

           

            Analysis/forecast systems used in numerical weather prediction (NWP) have matured to the point that further development requires the direct verification of model physics. Changes in operational NWP systems are designed to improve their synoptic forecast performance; their effect on surface fluxes are often not evaluated as fully as their effect on synoptic performance, in part because reliable independent estimates of surface fluxes are more difficult to obtain than measurements of atmospheric fields. 

 

            NCEP conducted two major reanalyses, one from 1948 to the present (Kalnay et al., 1996; Kistler et al., 2000) and a second from 1979 to the present (Kanamitsu et al., 1999).  The long, consistent data sets from reanalysis provided a golden opportunity to evaluate surface fluxes against independent estimates over long periods, and many studies did (Taylor, 2000, sect. 11.4).  However, the operational NCEP global system has developed considerably from the versions used in the reanalyses and it is not clear how applicable the lessons learned from reanalysis are to the operational model.  Even relatively small changes can make large differences in surface fluxes, as can be seen in the differences between the first and second NCEP reanalyses (Taylor, 2000, sect. 11.4). This paper examines the systematic performance of surface fluxes in the current NCEP operational system and compares the fluxes to fluxes for the same period from NCEP’s Climate Data Assimilation System (CDAS), the continuation of the first NCEP/NCAR reanalysis.

 

Global Mean Fluxes

 

 

CDAS

Op

K&T

Range

Precipitation

2.829 mm/day

3.035

2.69

2.69-3.1(2.83,2.83)

Evaporation

2.871

2.98

 

 

Precipitation-Evaporation

-.042

.055

 

 

Sensible heat

15.85 W/m2

12.5

24

16-27(21,21)

Latent heat

83.27

86.43

78

78-90(82,82)

Sfc downward short-wave

206.34

198.59

198

 

Upward short-wave

45.27

28.45

30

 

Net short-wave

161.07

170.14

168

142-174(163, 168)

Downward long-wave

334.44

337.44

324

 

Upward long-wave

395.79

397.47

390

 

Net long-wave

61.36

60.03

66

40-72(59, 63)

Net radiation

99.72

110.11

102

99-119(104,103)

Net heat flux

0.59

11.18

0

 

TOA downward short-wave

341.83

 

342

 

Upward short-wave

116.54

99.18

107

 

Outgoing long-wave

237.9

242.73

235

 

TOA radiation balance

-12.62

-0.09

 

 

Net atmospheric heating

-13.21

-11.27

 

 

Table 1.  Global mean hydrological, surface energy and top of the atmosphere radiation budgets in 0-6 hour forecasts from CDAS and the NCEP operational global system during July 1999-June 2000 compared to Kiehl and Trenberth (1997) (K&T).  The range is the range of global mean estimates examined in K&T, the two numbers in brackets are the mean and the mode of the estimates.


Ocean

 

CDAS

op

COADS

SRB

Precipitation

3.053 mm/day

3.287

 

 

Evaporation

3.322

3.473

 

 

Precipitation-Evaporation

-.269

-.187

 

 

Sensible heat

12.49 W/m2

6.59

10.1

 

Latent heat

96.34

100.73

88

 

Sfc downward short-wave

199.97

195.29

 

 

Upward short-wave

35.58

18.36

 

 

Net short-wave

164.39

176.93

170.4

173.4

Downward long-wave

350.13

352.39

 

 

Upward long-wave

406.7

408.3

 

 

Net long-wave

56.57

55.91

49.2

41.9

Net radiation

107.82

121.02

 

 

Net heat flux

-1

13.7

23.3

 

TOA downward short-wave

347.38

 

 

 

Upward short-wave

116.58

95.68

 

 

Outgoing long-wave

239.56

245.88

 

 

TOA radiation balance

-8.76

5.83

 

 

Net atmospheric heating

-7.75

-7.87

 

 


Land

 

CDAS

Op

Precipitation

2.272 mm/day

2.388

Evaporation

1.75

1.792

Precipitation-Evaporation

.521

.596

Sensible heat

24.23 W/m2

26.84

Latent heat

50.76

51.97

Sfc downward short-wave

222.18

207.6

Upward short-wave

69.37

53.24

Net short-wave

152.81

154.37

Downward long-wave

295.4

301.27

Upward long-wave

368.67

371.3

Net long-wave

73.27

70.03

Net radiation

79.54

84.34

Net heat flux

4.55

5.54

TOA downward short-wave

328.01

 

Upward short-wave

116.43

108.2

Outgoing long-wave

233.77

235.27

TOA radiation balance

-22.2

-15.46

Net atmospheric heating

-26.74

-21


Table 2.  Global mean hydrological, surface energy, and TOA radiation budgets over ocean and land during July 1999-June 2000.  COADS refers to estimates by da Silva et al. (1994) for 1981-92, SRB to satellite-based surface radiation estimates by Darnell et al. (1992) and Gupta et al. (1992).

 


The operational system has more realistic downward and upward surface short wave radiation, due to changes in the short wave radiation parameterization and new surface albedo.  The CDAS system had too high surface albedo over the ocean.  Both systems, however, had more downward and net short wave surface radiation over the Northern Hemisphere mid-latitude continents during March-May than SRB climatological estimates.  However, the surface energy budget is more out of balance in the operational system than in CDAS, especially over the ocean, and oceanic sensible heat flux in the operational model is lower than other estimates.  The radiation budget at the top of the atmosphere is much closer to balance in the operational system, reflecting compensating errors in the outgoing long wave and short wave radiation. Oceanic evaporation in the first NCEP/NCAR reanalysis was higher than ship-based estimates (Smith et al., 1999) and evaporation in the operational model is higher still; however, decreasing the evaporation would increase the surface energy imbalance.

 

PRECIPITATION

 

            Fig.1 compares precipitation during March-May 2000 from the NCEP operational


system and from CDAS to OPI, an independent estimate based on rain gauges and infrared satellite estimates.  The NCEP1 reanalysis was criticized for not concentrating tropical precipitation enough and that is clearly evident.  Precipitation from the operational analysis/forecast system is clearly more like the OPI estimate in the tropics than CDAS is. 

 

OLR

 

            Fig. 2 compares zonal mean top of the atmosphere outgoing long wave radiation for March-May 2000 from the operational model and CDAS to satellite measurements.  Both have too much OLR in the subtropics, perhaps reflecting the lack of aerosol in either system


and problems with cloudiness in amount and properties.  Over the ocean the operational system is generally further from the observations than CDAS, especially in the subtropics.


 

CLOUDINESS

 

            Fig. 3 displays differences in total cloudiness between the model estimates and Air Force operational nephanalyses for March-May 2000.  Both CDAS and the operational analysis/forecast system show too much cloudiness in the tropics and at high northern latitudes.  CDAS shows a dipole

 


Fig. 1 Precipitation during March-May 2000 from (top) OPI, based on satellite infrared measurements over the ocean and rain gauges over land, and from 0-6 hr forecasts by  (middle) the NCEP operational analysis/forecast system and (bottom) CDAS. Contours 1, 3, 5, 7, 9, 13, 17, 21 mm/day, greater than 5 mm/day shaded.

 


Fig. 2 Zonal mean top of the atmosphere outgoing long wave radiation over (left) land and (right) ocean from satellite measurements and from 0-6 hr forecasts by the NCEP operational global analysis/forecast system and by CDAS in Watts/m2.

 

pattern in the west Pacific, with too little cloudiness over Indonesia and too much to the east, reflecting a failure to concentrate tropical convection enough.  The operational system has too little cloudiness over the eastern subtropical oceans off Baja California and western South America, showing a deficiency in low-level stratus clouds.  CDAS has more realistic low-level stratus clouds, but has too much off Baja California and off Africa and also misplaces them in some regions.  The Global Modeling Branch recently adjusted its low-level clouds in response to complaints of too much low-level cloudiness over the Arabian Sea.  The Global Modeling Branch is currently testing a prognostic cloud liquid water parameterization and is carefully evaluating the resulting cloudiness.

 

Conclusions

 

            Surface fluxes in the NCEP operational global analysis/forecast system are distinctly different from CDAS.  The precipitation pattern is substantially improved and short wave radiation is more realistic.  However, sensible heat flux is now lower than other estimates and the ocean surface energy


Fig. 3 Differences in total cloud cover between 0-6 hr forecasts from (top) the NCEP operational analysis/forecast system and (bottom) CDAS and Air Force nephanalyses during March-May 2000.  Contour interval 10%, zero omitted, values less than –10% shaded.

 

budget is more out of balance.  Radiation and clouds still require substantial improvements. Knowledge of the surface fluxes also requires substantial improvement: for example, the magnitude of evaporation and the hydrological cycle is not well known.  Estimates from ship-based observations imply that both CDAS and the operational model have too much evaporation; however, surface flux estimates from ship observations do not give a reasonable global surface energy balance.  While uncertainties exist in independent estimates of surface fluxes and atmospheric physics in general, they are accurate enough (and errors in NWP systems’ physics are large enough) to point to ways to improve NWP systems substantially.

 

References

 


da Silva, A. M., C.C.  Young and S. Levitus, 1994: Atlas of Surface Marine Data 1994. NOAA Atlas NESDIS 6. Available from: U.S. Dept. Commerce, NODC, User Services Branch, NOAA/NESDIS E/OC21, Washington, D.C. 20233, USA.

Darnell, W.L., W.F. Staylor, S.K. Gupta, N.A. Richey and A.C. Wilber, 1992: Seasonal variation of surface radiation budget derived from International Satellite Cloud Climatology Project C1 Data. J. Geophys. Res., 97, 15741-15760.

Gupta, S.K., W.L. Darnell and A.C. Wilber, 1992: A parameterization for longwave surface radiation from satellite data: Recent improvements. J. Appl. Meteor., 31, 1261-1267.

Kalnay, E., & co-authors, 1996: The NCEP/NACR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437-471.

Kanamitsu, M., W. Ebisuzaki, J. Woolen, J. Potter, and M. Fiorino, 2000: An overview of NCEP/DOE Reanalysis-2. Proc. 2nd Intl. Conf. On Reanalyses, Reading, England, 23-27 Aug. 1999. WCRP-109 (WMO/TD-985) WMO, Geneva, 1-4.

Kiehl, J.T., and K.E. Trenberth, 1997: Earth’s annual global mean energy budget. Bull. Amer. Meteor. Soc., 78, 197-208.

Kistler, R., & co-authors, 2000: The NCEP/NCAR 50-year reanalysis. Submitted to Bull. Amer. Meteor. Soc.

Smith, S.R., D.M. Legler, and K.V. Verzone, 1999: Quantifying uncertainties in NCEP reanalysis using high-quality research vessel observations. CLIMAR-99 WMO Workshop on Advances in Marine Climatology, Vancouver, 8-15 Sept. 1999, 223-230.

Taylor, P.K., ed., 2000: Report of the Working Group on Air Sea Fluxes. To be published by WCRP.  WMO, Geneva. Now available at http://www.soc.soton.ac.uk/JRD/MET/WGASF.