Estimating Regional Surface Heat and Moisture Fluxes above Prairie Cropland from Surface and Upper-Air Measurements

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  • 1 Atmospheric Environment Service, Saskatoon, Saskatchewan, Canada
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Abstract

Upper-air budget methods can be used to estimate the surface sensible and latent heat flux densities on a regional scale. This study assesses the application of radiosonde-based budget methods above homogeneous cropland. Serial daytime soundings were released from Kenaston and Saskatoon, Canada, on fair-weather days between 24 June and 31 July 1991.

Two independent methods were used to establish ground truth: surface-layer Bowen ratio-energy balance and Priestley-Taylor. This study was the fist to extend the surface-layer Bowen ratio method to conventional upper-air soundings. The two ground-truth methods agreed to within 20% at both locations and gave mean daytime Bowen ratios of 0.33.

The upper-air budget surface flux estimates agreed most consistently with ground truth when the budget was integrated over the atmospheric boundary layer (BL) and used parameterized entrainment with a value for the entrainment parameter AR of 0.4. The BL budget with AR of 0.4 closed the daytime surface energy balance to within 4% at Kenaston and 7% at Saskatoon and gave a mean estimate for the Bowen ratio that agreed to within 20% of the mean ground-truth estimates. However, the BL budget estimates for 2-3-h periods were quite variable, and it was necessary to average the budget estimates over periods of 12 days or longer to produce credible values. Random sampling errors and uncertainty in horizontal advection were partly responsible for the high variability of the budget estimates, but these terms averaged to zero over extended periods. More seriously, the BL budget estimates for the surface latent heat flux were quite sensitive to the method for estimating entrainment. Because the authors were unable to establish a preferred entrainment estimate a priori, the BL budget estimates for the surface latent heat flux were considered to be unreliable. Further study is needed to develop a reliable and independent method for specifying the value for AR.

Abstract

Upper-air budget methods can be used to estimate the surface sensible and latent heat flux densities on a regional scale. This study assesses the application of radiosonde-based budget methods above homogeneous cropland. Serial daytime soundings were released from Kenaston and Saskatoon, Canada, on fair-weather days between 24 June and 31 July 1991.

Two independent methods were used to establish ground truth: surface-layer Bowen ratio-energy balance and Priestley-Taylor. This study was the fist to extend the surface-layer Bowen ratio method to conventional upper-air soundings. The two ground-truth methods agreed to within 20% at both locations and gave mean daytime Bowen ratios of 0.33.

The upper-air budget surface flux estimates agreed most consistently with ground truth when the budget was integrated over the atmospheric boundary layer (BL) and used parameterized entrainment with a value for the entrainment parameter AR of 0.4. The BL budget with AR of 0.4 closed the daytime surface energy balance to within 4% at Kenaston and 7% at Saskatoon and gave a mean estimate for the Bowen ratio that agreed to within 20% of the mean ground-truth estimates. However, the BL budget estimates for 2-3-h periods were quite variable, and it was necessary to average the budget estimates over periods of 12 days or longer to produce credible values. Random sampling errors and uncertainty in horizontal advection were partly responsible for the high variability of the budget estimates, but these terms averaged to zero over extended periods. More seriously, the BL budget estimates for the surface latent heat flux were quite sensitive to the method for estimating entrainment. Because the authors were unable to establish a preferred entrainment estimate a priori, the BL budget estimates for the surface latent heat flux were considered to be unreliable. Further study is needed to develop a reliable and independent method for specifying the value for AR.

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