Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2005
Publication Date: 12/5/2005
Citation: Kustas, W.P., Prueger, J.H., MacPherson, I., Wolde, M., Li, F. 2005. Effects of landuse and meteorological conditions on local and regional momentum transport and roughness for midwestern cropping systems. Journal of Hydrometeorology. 6:825-839. Interpretive Summary: Land surface aerodynamic roughness is a key boundary condition used in hydrologic and atmospheric modeling schemes for estimating aerodynamic resistance. Measurements from the Soil Moisture Atmosphere Coupling Experiment provided local (field scale) estimates of aerodynamic roughness from towers and regional scale aerodynamic roughness from aircraft. Procedures to estimate regional or effective aerodynamic roughness from local measurements and land use information gave significantly lower estimates than the aircraft-based values. Factors causing differences in regional roughness values included meteorological conditions as well as not accounting for the effects of isolated obstacles (i.e., trees and houses and farm buildings), and the interaction of adjacent corn and soybean fields not accounted for in existing algorithms. The effect of the underestimation in aerodynamic roughness on heat flux estimation, however, was not found to be very significant because the magnitude of the resistance to heat exchange (called excess resistance term) was significantly larger than the aerodynamic resistance. Underestimation of effective aerodynamic roughness for other landscapes will have a more pronounced impact on heat flux estimation in areas with homogeneous tall dense vegetation, where the excess resistance is term is significantly less than the aerodynamic term. However, landscapes are typically heterogeneous, and numerical studies indicate that heat fluxes are not significantly affected by underestimation of the effective aerodynamic roughness length. This observational study (one of the few available) supports these numerical results.
Technical Abstract: Eddy covariance measurements of wind speed, u, and shear velocity, , from tower and aircraft-based systems collected over rapidly developing corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) fields were used in determining local and regional (effective) surface roughness length, zo, and < zo >, respectively. For corn, canopy height increased from ~1 to 2 m and leaf area index changed from ~1 to 4 during the study period, while for soybean canopy height increased from ~0.1 m to 0.5 m and leaf area index increased from ~0.5 to 2. A procedure for aggregation of local roughness values from the different land cover types based on blending height concepts yielded effective surface roughness values that were from ~1/2 to 1/4 the magnitude estimated with the aircraft data. This indicated additional kinematic stress caused by form drag from isolated obstacles (i.e., trees and houses and farm buildings), and the interaction of adjacent corn and soybean fields were probably important factors influencing the effective surface roughness length for this landscape. Comparison of measurements from the towers versus the aircraft indicated that from aircraft was 20% to 30% higher on average, and that over corn was 10% to 30% higher than over soybean, depending on stability. These results provide further evidence for the likely sources of additional kinematic stress. Although there was an increase in zo, and < zo > over time as the crops rapidly developed, particularly for corn, there was a more significant trend of increasing roughness length with decreasing wind speed at wind speed thresholds of around 5 m s-1 for the aircraft and 3 m s-1 for the tower measurements. Other studies have recently reported such a trend. The impact on computed sensible heat flux, H, using derived from aggregation of zo from the different land cover types using the blending height scheme and that estimated from the aircraft observations was evaluated using a calibrated single-source/bulk resistance approach with surface-air temperature differences from the aircraft observations. An underestimate of by 50% and 75% resulted in a bias in the H estimates of approximately 10% and 15%, respectively. This is a relatively minor error when considering that the root-mean-square-error (RMSE) value between single source estimates and the aircraft observations of H was 15 W m-2 using the aircraft-derived , and only increased to approximately 20 and 25 W m-2 using the 1/2 and 1/4 value, as estimated from the blending height scheme. The magnitude of the excess resistance relative to the aerodynamic resistance to heat transfer was a major contributing factor in minimizing the error in heat flux calculations due to these underestimations of .