Comparison of Surface Turbulent Flux Products Paul J.

Comparison of Surface Turbulent Flux Products
Paul J. Hughes, Mark A. Bourassa, and Shawn R. Smith
Center for Ocean-Atmospheric Prediction Studies &
Department of Meteorology
Florida State University, Tallahassee, FL 32306-2840

Indian Ocean

Pacific Ocean

Atlantic Ocean

[email protected]

1. Introduction
Monthly averaged surface turbulent fluxes (stress, sensible heat, and latent heat) are compared for nine products: NCEPR2,
JRA25, ERA40, WHOIs OAFLUX, GSSTF2, IFREMER, HOAPS2, FSU3, and NOCSv1.1 (formerly SOC). The common period
of March 1993 through Dec. 2000 is examined. Input data are also compared when available. Each product has been regridded onto
a 1x1 grid. To reduce problems related to land, data within two grid cells of land are not used in this comparison. Some satellite
winds are included in this comparison. There are very large differences in the distribution of zonally averaged heat fluxes (Fig. 1).
Curl of the wind stress (Fig. 2) also shows great differences among these products. In many cases the observing pattern (ships or
TAO buoys; Fig. 3) can be easily identified. In other cases, ringing induced by orography modifies the spatial derivatives.

Zonal
stress

Meridional
stress

2. Flux Product Information
There are many types of products that include turbulent fluxes of latent heat, sensible heat, and stress. Reanalysis products,
created from numerical weather prediction (NWP) models with fixed model physics (albeit with changing data products for
assimilation), are often used because they provide a wide range of additional information at the surface and at various levels in the
atmosphere. However, such models have poor representations of the atmospheric boundary layer, and questionable
parameterizations of surface turbulent stresses. These deficiencies in the lower boundary condition decrease the credibility of these
models for climate studies as well as for ocean forcing. The fluxes from the NWP models are compared to fluxes derived from
satellite measurements and ship and buoy observations, which are not without their own shortcomings (e.g., regional/global biases
and poor/inhomogeneous sampling). Satellite derived products include IFREMER and HOAPS. Products based on ship and buoy
observations include FSU3 and NOC1.1 (see Fig. 3 for in situ coverage). Hybrid NWP model and satellite products include WHOI
and GSSTF2.
Product

LHF

SHF

Stress
(x,y)

NCEPR2

x

x

JRA25

x

ERA40

Wind
Speed

u wind

v wind

Tair

Qair

SST

Product
Type

x

x

x

x

x

x

Reanalysis

Gaussian (T62, 194x94)

x

x

x

x

x

x

x

Reanalysis

(T106L40) ~120km

x

x

x

x

x

x

x

x

Reanalysis

1 1/8 degrees

WHOI

x

x

Hybrid

1 x 1 degree

GSSTF2

x

x

x

x

Hybrid

1 x 1 degree

IFREMER

x

x

x

x

HOAPS2

x

x

FSU3

x

x

x

x

NOC1.1

x

x

x

x

x
x

x

x

x

x

Satellite

1 x 1 degree

x

x

Satellite

0.5 x 0.5 degree

x

x

x

In-situ

1 x 1 degree

x

x

In-situ

1 x 1 degree

x
x

x

Grid Spacing

Sensible
Heat Flux

Latent
Heat Flux

FSU
Figure 1. Distributions (5th, 25th, 50th, 75th, and 95th percentiles) of contributions to zonally averaged fluxes. There
are very large differences among products, and these difference are a function of the percentile examined.

Latent Heat Flux
Mean

3. Conclusions

WHOI

The reanalyses based on numerical weather prediction (NWP) stand out as outliers from the observation-based
products. The various products have differences in zonally averaged latent heat fluxes for all ocean basins. These
differences are large from the point of view of climate modeling, where biases of >10 Wm -2 (including radiation
fluxes) are considered serious problems. Stresses, sea surface temperatures (not shown), and to a lesser extent air
temperatures (not shown) are relatively similar between products. The greatest difference occur with wind speeds
(a seemingly well observed quantity) and atmospheric moisture. These difference both contribute to the large
product to product differences in the latent heat flux. Monthly averaged satellite winds are extremely similar in
magnitude and pattern to the in situ based FSU3 and NOCS winds. The large differences between the latent heat
fluxes of these two products are largely due to differences in the atmospheric moisture.
These results are based on monthly averaged fluxes; therefore, they likely underestimate the issues with fluxes
produced for shorter time scales. The large differences in fluxes, and in the spatial/temporal changes in these
products indicates that there are still serious problems to be resolved in the construction of surface forcing fields
for applications in climate and general oceanography.

GSSTF2

4. Acknowledgements
We thank the many people that made their products available for comparison, and who were involved in
preliminary discussions of how comparisons could be made. We also thank the NOAA Climate Observation
Division and NSF PO for supporting this effort.

HOAPS2

Figure 3. Example VOS observation density for
December, averaged from 1988-1997.

Curl of the Stress
Mean

Curl of the Stress
Standard Deviation

Divergence of the Wind
Mean

Divergence of the Wind
Standard Deviation

NCEPR2

JRA25

ERA40

IFREMER

FSU3

NOCv1.1

Figure 2. Curl of the stress is particularly important for ocean forcing; divergence of
winds is import for atmospheric work. The means show similar patterns, although the
TOA buoy array is easily identified in the ERA40 product, and ship tracks are seen in
the NCEP2, FSU3 and NOC products; however, the ship coverage is much better in
the North Atlantic Ocean, where they are not easily identified in the FSU3 product.
Orographically induced ringing is apparent in the NCEP2 and JRA25 products. The
standard deviations also highlight problems with the objective techniques and the data
assimilation.

Latent Heat Flux
Standard Deviation

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