Ali Mamtimin

Improving the CoLM in Taklimakan Desert hinterland with accurate key parameters and an appropriate parameterization scheme

Improving and validating land surface models based on integrated observations in deserts is one of the challenges in land modeling. Particularly, key parameters and parameterization schemes in desert regions need to be evaluated in-situ to improve the models. In this study, we calibrated the land-surface key parameters and evaluated several formulations or schemes for thermal roughness length (z0h) in the common land model (CoLM). Our parameter calibration and scheme evaluation were based on the observed data during a torrid summer (29 July to 11 September 2009) over the Taklimakan Desert hinterland. First, the importance of the key parameters in the experiment was evaluated based on their physics principles and the significance of these key parameters were further validated using sensitivity test. Second, difference schemes (or physics-based formulas) of z0h were adopted to simulate the variations of energy-related variables (e.g., sensible heat flux and surface skin temperature) and the simulated variations were then compared with the observed data. Third, the z0h scheme that performed best (i.e., Y07) was then selected to replace the defaulted one (i.e., Z98); the revised scheme and the superiority of Y07 over Z98 was further demonstrated by comparing the simulated results with the observed data. Admittedly, the revised model did a relatively poor job of simulating the diurnal variations of surface soil heat flux, and nighttime soil temperature was also underestimated, calling for further improvement of the model for desert regions.

Impact factors of soil wind erosion in the center of Taklimakan Desert

The development and progress of soil wind erosion are influenced by the factors of climate, terrain, soil and vegetation, etc. This paper, taking Tazhong region, a town in the centre of the Taklimakan Desert, as an example and using comparative and quantitative methods, discussed the effects of climate, surface roughness (including vegetation cover) and surface soil properties on soil wind erosion. The results showed that the climate factor index C of annual wind erosion is 28.3, while the maximum of C is 13.9 in summer and it is only 0.7 in winter. The value of C has a very good exponential relationship with the wind speed. In Tazhong region, the surface roughness height is relatively small with a mean of 6.32 × 10 −5 m, which is in favor of soil wind erosion. The wind erosion is further enhanced by its sandy soil types, soil par- ticle size, lacking of vegetation and low soil moisture content. The present situation of soil wind erosion is the result of concurrent effects of climate, vegetation and surface soil properties.

Observation of saltation activity at Tazhong area in Taklimakan Desert, China

A two-year field observation of saltation activity was carried out at Tazhong area, the hinterland area of the Taklimakan Desert with highly frequent dust storms. From 1 September 2008 to 31 August 2010, a piezoelectric saltation sensor (Sensit) was used to continuously collect the data on saltation activity at a level sand surface. Analysis on the data suggests that saltation activity can occur at any time of the year when conditions are favorable; however, the necessary conditions are rarely satisfied in most time. In the daytime of spring or summer, saltation activity can persist even over a continuous one-hour-or-so period. It is found that, from 1 September 2008 to 31 August 2010, saltation activity accounts for more than 3% of the total yearly time, and it tends to peak in spring and summer months with strong winds. During winter months when winds are weak, however, it is often at a minimum. It seems that precipitation does not appear to be significant in reducing saltation activity in arid regions like Tazhong.

Estimation of the land surface emissivity in the hinterland of Taklimakan Desert

An accurate accounting of land surface emissivity (ɛ) is important both for the retrieval of surface temperatures and the calculation of the longwave surface energy budgets. Since ɛ is one of the important parameterizations in land surface models (LSMs), accurate accounting also improves the accuracy of surface temperatures and sensible heat fluxes simulated by LSMs. In order to obtain an accurate emissivity, this paper focuses on estimating ɛ from data collected in the hinterland of Taklimakan Desert by two different methods. In the first method, ɛ was derived from the surface broadband emissivity in the 8-14 μm thermal infrared atmospheric window, which was determined from spectral radiances observed by field measurements using a portable Fourier transform infrared spectrometer, the mean ɛ being 0.9051. The second method compared the observed and calculated heat fluxes under nearneutral atmospheric stability and estimated ɛ indirectly by minimizing the root-mean-square difference between them. The result of the second method found a mean value of 0.9042, which is consistent with the result by the first method. Although the two methods recover ɛ from different field experiments and data, the difference of mean values is 0.0009. The first method is superior to the indirect method, and is also more convenient.

Nondimensional Wind and Temperature Profiles in the Atmospheric Surface Layer over the Hinterland of the Taklimakan Desert in China

Observed turbulent fluxes, wind, and temperature profiles at Tazhong station over the hinterland of the Taklimakan Desert in China have been analyzed to evaluate empirical parameters used in the profile functions of desert surface layer. The von Karman constant derived from our observations is about 0.396 in near-neutral stratification, which is in good agreement with many other studies for different underlying surface. In our analysis, the turbulent Prandtl number is about 0.75 in near-neutral conditions. For unstable range, the nondimensional wind and temperature profile functions are best fitted by the exponents of −1/4 and −1/2, respectively. The linear relations still hold for stable stratification in this extremely arid desert. However, the parameters used in their profile functions need to be revised to be applicable for desert surfaces.

Determination of Desert Soil Apparent Thermal Diffusivity Using a Conduction‐Convection Algorithm

Surface soil temperatures impact land-atmosphere interactions in desert environments. Soil apparent thermal diffusivity (k) is a crucial physical parameter affecting soil temperature. Previous studies using the conduction-convection algorithm reported k values of desert soils for only a few days. The main objective of this study is to determine the daily and monthly variations of desert k for a range of water contents over a ten-month period. The k values were estimated with a conduction-convection algorithm using soil temperature measured at the 0.00 m and 0.20 m depths from January 1 to October 11, 2011 at the Tazhong station in the Taklimakan Desert of China. Generally, the daily values of k ranged from 1.46 × 10-7m2  s-1 to 5.88 × 10-7m2  s-1, and the ten month average k value was 2.5(±0.8) × 10-7m2  s-1 for the 0.00 m to 0.20 m soil layer. The k values varied significantly with soil water content. The apparent convection parameter (W), which is the sum of the vertical gradient of k and apparent water flux density, was also determined. Comparison of the magnitudes of W and k gradients indicated that little water movement occurred during the dry months, some water infiltrated downward during the wet months, and some water moved upwards in response to evaporation following the wet months. These findings confirmed that the conduction-convection algorithm described the general pattern of soil water movement. The presented daily and monthly values of k can be used as soil parameters when modeling land-atmosphere interactions in the Taklimakan desert.