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/m^2	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/m^2	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/m^2	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

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