Tuesday, 9 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Abstract The observational data set for the period of 5th January to 31st December 2010 at Xiaotang in the northern transition zone of the Taklimakan Desert has been used to derive key surface parameters for representing land–atmospheric interactions in the desert climate in northwest China. Flux–based method is used to calculate aerodynamic roughness length (z0m), thermal roughness length (z0h) and excess resistance to heat transfer (kB-1). The bulk momentum transfer coefficient (Cd) and the bulk sensible heat transfer coefficient (Ch) are derived from both eddy correlation and aerodynamic calculations. Results are also compared with observed values in the hinterland of the desert (Tazhong station) to explore their spatial variations. We find surface albedo ( α) and emissivity (ε) are similar to them at Tazhong. They are close to the retrieved α and ε values from remote sensing products. The mean roughness length z0m and z0h are of the same order of magnitude. Both ln(z0h) and kB–1 have weak diurnal but notable seasonal variations, while ln(z0m) shows weak variations.We further find these parameters are strongly influenced by local wind direction. In general, the z0m is 5.6×10–3–6.5×10–3 m in the SW wind direction and 4.9×10–3–5.7×10–3 m in E-NE wind direction; the z0h is 1.7×10–3 m in the NNE wind direction and 2.7×10–3m in SW wind direction. The optimal values for z0m and z0h are 5.858×10–3 m and 2.400×10–3 m in the normally distributed histogram, and the magnitudes of z0m and z0h at Xiaotang are consistent with the results reported by Stull (1991). Monthly z0m vary between 9.51×10–3 (December) and 1.64×10–2 m (July), and the average annual value is 1.29×10–2 m. Monthly z0h vary between 5.11×10–5 (September) to 7.9×10–4 m (April), and the average annual value is 2.7×10–4 m. The ln(z0h) value has obvious two peaks at sunrise and at sunset. The z0h is high in spring and summer, low in winter. The excess resistance to heat transfer, kB–1, varied inversely with ln(z0h). Daily mean kB–1 is between 1.0 and 7.6, and the average annual value is 5.88. This is different from vegetated areas, bare soil, smooth surface, HEIFE gobi and desert, the Qinghai–Tibetan Plateau, and HAPEX-Sahel. The order is Cd > Ch for all months except for February. Daily mean Cd and Ch are 6.34×10–3 and 5.96×10–3, and their neutral values are 4.30×10–3 and 1.70×10–3. The highest and lowest values for Cd are 0.99×10–2 (May) and 0.54×10–2 (March), respectively, while the highest and lowest values for Ch are 0.30×10–2 (February) and 0.27×10–2 (January), respectively. Under its normal prevailing (NNE–ESE) wind, the mean bulk transfer coefficient Cd and Ch are of the same order of magnitude as expected by similarity theory. Using the data under difference wind direction, the relationships between Cd, Ch and wind speed U, and stability parameters z/L are obtained and the results are different. The Cd and Ch decrease rapidly as wind speed approaches below 3.0 m s−1, and reach to their minimum values near 1–2 m s−1. It is also worth noting that the ε value estimated using sensible heat flux (H) that performed well compared to other estimate methods. Comparing these observational results with the ones observed in the central area of the Taklimakan Desert only showed some weak spatial variations of these key parameters and they are close to the products derived from remote sensing. Therefore, our observational results tend to suggest that uncertainty due to model surface parameters is small for modeling error in representing land-atmospheric interactions over the desert in northwest China and central Asia.
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