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  • Author or Editor: Todd M. Crawford x
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Todd M. Crawford
and
Claude E. Duchon

Abstract

An improved parameterization is presented for estimating effective atmospheric emissivity for use in calculating downwelling longwave radiation based on temperature, humidity, pressure, and solar radiation observations. The first improvement is the incorporation of an annual sinusoidal variation in effective clear-sky atmospheric emissivity, based on typical climatological variations in near-surface vapor pressure. The second is the continuous estimation of fractional cloudiness by taking the ratio of observed solar radiation to a modeled clear-sky solar radiation. Previous methods employed observer-estimated fractional cloudiness. Data from the Atmospheric Radiation Measurement (ARM) program were used to develop these improvements. The estimation of cloudiness was then used to modify the effective clear-sky atmospheric emissivity in order to calculate 30-min averages of downwelling longwave radiation. Monthly mean bias errors (mbe’s) of −9 to +4 W m−2 and root-mean-square errors (rmse’s) of 11–22 W m−2 were calculated based on ARM data over a 1-yr period. These mbe’s were smaller overall than any of the six previous methods tested, while the rmse’s were similar to the best previous methods. The improved parameterization was then tested on FIFE data from the summer of 1987. Although the monthly mbe’s were larger, the rmse’s were smaller.

It is also shown that data from upper-air soundings can be used to calculate the effective atmospheric emissivity rather than specifying the aforementioned sinusoidal variation. Using ARM upper-air soundings, this method resulted in larger mbe’s, −7 to +11 W m−2, especially during the summer months, and similar rmse’s. The success of the method suggests that it has application at any observing site within reasonable proximity of an upper-air sounding, while removing the empiricism used to specify the annual sinusoidal variation in emissivity.

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Todd M. Crawford
and
Howard B. Bluestein

Abstract

In recent years, there has been a growing appreciation of the importance of land–atmosphere interactions in determining the state of the boundary layer. To examine this phenomenon in more detail, a new technique has been developed to evaluate the surface energy budget during the daytime from standard meteorological observations. Using only Oklahoma Mesonetwork (Mesonet) data at 5- and 30-min intervals as input, the technique calculates net radiation (R n ), ground heat flux (G), and latent heat flux (LE). The sensible heat flux (H) is calculated as a residual. The R n term is calculated using observed values of downwelling shortwave radiation, an improved method of estimating downwelling longwave radiation, and simple parameterizations of upwelling shortwave and longwave radiation. The modeled values of R n are unbiased and are consistently within 25 W m−2 of observed values. Ground heat flux, which is the combination of a 5-cm soil flux term and a storage term, was difficult to verify without prior knowledge of vegetation height. Latent heat flux is calculated from the Penman–Monteith equation, in which surface resistance is estimated. Using data from the Atmospheric Radiation Measurement Program, simple parameterizations were developed (one each for eastern and western Oklahoma) for this term, based on observations of temperature, relative humidity, solar radiation, soil moisture, and estimates of leaf age.

Net radiation and G are calculated, and then their sum is partitioned into H and LE. Because there were no observations of LE at the Mesonet sites, the preexisting reliable estimates of H were used to verify the new estimates of both H and LE. Because there were problems with the soil moisture data from some of the sites, data from only two Mesonet sites were available for verification. The estimates of H were unbiased and within 60 W m−2 (rmse) at the sites in both eastern and western Oklahoma. Because of the limited verification data currently available, the model results are preliminary and in need of further testing.

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