Zhaoguo Li

Turbulent transfer coefficient and roughness length in a high-altitude lake, Tibetan Plateau

A persistent unstable atmospheric boundary layer was observed over Lake Ngoring, caused by higher temperature on the water surface compared with the overlying air. Against this background, the eddy covariance flux data collected from Lake Ngoring were used to analyse the variation of transfer coefficients and roughness lengths for momentum, heat and moisture. Results are discussed and compared with parameterization schemes in a lake model. The drag coefficient and momentum roughness length rapidly decreased with increasing wind velocity, reached a minimum value in the moderate wind velocity and then increased slowly as wind velocity increased further. Under weak windxa0conditions, the surface tension or small scale capillary wave becomes more important and increases the surface roughness. The scalar roughness length ratio was much larger than unity under weak windxa0conditions, and it decreased to values near unity as wind velocity exceeded 4.0xa0mxa0s−1. The lake model could not reproduce well the variation of drag coefficient, or momentum roughness length, versus wind velocity in Lake Ngoring, but it did simulate well the sensible heat and latent heat fluxes, as a result of complementary opposite errors.

Biophysical regulation of carbon fluxes over an alpine meadow ecosystem in the eastern Tibetan Plateau

The eddy covariance method was used to measure net ecosystem CO2 exchange (NEE) between atmosphere and an alpine meadow ecosystem in the eastern Tibetan Plateau of China in 2010. Our results show that photosynthesis was reduced under low air temperature (Ta), high vapor pressure deficit (VPD), and medium soil water content (SWC) conditions, when compared to that under other Ta (i.e., medium and high), VPD (i.e., low and medium), and SWC (i.e., low and high) conditions. The apparent temperature sensitivity of ecosystem respiration (Q10) declined with progressing phenology during the growing season and decreased with an increase of soil temperature (Ts) during the non-growing season. Increased ecosystem respiration (Reco) was measured during spring soil thawing. By the path analysis, Ta, Ts, and VPD were the main control factors of CO2 exchange at 30-min scale in this alpine meadow. Integrated NEE, gross primary production (GPP), and Reco over the measured year were −156.4, 1164.3, and 1007.9xa0gxa0Cxa0m−2, respectively. Zoige alpine meadow was a medium carbon sink based on published data for grassland ecosystems.

An Investigation of Ice Surface Albedo and Its Influence on the High-Altitude Lakes of the Tibetan Plateau

Most high-altitude lakes are more sensitive to global warming than the regional atmosphere. However, most existing climate models produce unrealistic surface temperatures on the Tibetan Plateau (TP) lakes, and few studies have focused on the influence of ice surface albedo on high-altitude lakes. Based on field albedo measurements, moderate resolution imaging spectrometer (MODIS) albedo products and numerical simulation, this study evaluates the ice albedo parameterization schemes in existing lake models and investigates the characteristics of the ice surface albedo in six typical TP lakes, as well as the influence of ice albedo error in the FLake model. Compared with observations, several ice albedo schemes all clearly overestimate the lake ice albedo by 0.26 to 0.66, while the average bias of MODIS albedo products is only 0.07. The MODIS-observed albedo of a snow-covered lake varies with the snow proportion, and the lake surface albedo in a snow-free state is approximately 0.15 during the frozen period. The MODIS-observed ice surface (snow-free) albedos are concentrated within the ranges of 0.14–0.16, 0.08–0.10 and 0.10–0.12 in Aksai Chin Lake, Nam Co Lake and Ngoring Lake, respectively. The simulated lake surface temperature is sensitive to variations in lake ice albedo especially in the spring and winter.

Relation between the Atmospheric Boundary Layer and Impact Factors under Severe Surface Thermal Conditions

This paper reported a comprehensive analysis on the diurnal variation of the Atmospheric Boundary Layer (ABL) in summer of Badain Jaran Desert and discussed deeply the effect of surface thermal to ABL, including the Difference in Surface-Air Temperature (DSAT), net radiation, and sensible heat, based on limited GPS radiosonde and surface observation data during two intense observation periods of experiments. The results showed that affected by topography of the Tibetan Plateau, the climate provided favorable external conditions for the development of Convective Boundary Layer (CBL), deep CBL showed a diurnal variation of three- to five-layer structure in clear days and five-layer ABL structure often occurred about sunset or sunrise, the diurnal variation of DSAT influenced thickness of ABL through changes of turbulent heat flux, integral value of sensible heat which rapidly converted by surface net radiation had a significant influence on the growth of CBL throughout daytime. The cumulative effect of thick RML dominated the role after CBL got through SBL in the development stage, especially in late summer, and the development of CBL was promoted and accelerated by the variation of wind field and distribution of warm advection in high and low altitude.

Comparative analysis of the soil thermal regimes of typical underlying surfaces of oasis systems in an Arid Region

Using supplementary observational data obtained from the “Oasis System Energy and Water Cycle Field Experiment” conducted in the summer of 2005 and 2008, we performed a comparative analysis of the characteristics of soil thermal regimes of an Oasis and the Gobi in terms of the surface albedo, soil temperature gradient, soil heat flux and soil thermal conductivity. The results showed that on sunny summer days, the daily average albedos of the Gobi and Oasis were 0.215, 0.159 in 2005 and 0.207, 0.150 in 2008, respectively. The soil temperature gradient and the net radiation showed an approximate linear relationship; the soil temperature gradient decreased with the increase of net radiation and it declined faster when the net radiation was negative. The soil heat flux of the 0.05xa0m soil layer varied in phase with the soil heat flux of the 0.20xa0m layer of soil. The thermal conductivity values of the 0.05xa0m layer of the Gobi and Oasis soil were 0.193 and 0.374xa0Wxa0m−1xa0K−1. The soil heat flux can be well estimated using soil temperature gradient and soil thermal conductivity from observation in the Gobi.

Effect of Roughness Lengths on Surface Energy and the Planetary Boundary Layer Height over High-altitude Ngoring Lake

The special climate environment creates a distinctive air-lake interaction characteristic in the Tibetan Plateau (TP) lakes, where the variations of surface roughness lengths also differ somewhat from those of other regions. However, how different categories of roughness lengths affect the lake surface energy exchange and the planetary boundary layer height (PBLH) remains unclear in the TP lakes. In this study, we used a tuned Weather Research and Forecasting (WRF) model version 3.6.1 to investigate the responses of the freeze-up date, turbulent fluxes, meteorological variables, and PBLH to surface roughness length variations in Ngoring Lake. Of all meteorological variables, the lake surface temperature responded to roughness length variations most sensitively; increasing roughness lengths can put the lake freeze-up date forward. The effect of momentum roughness length on wind speed was significantly affected by the fetch length. The increase in the roughness length for heat can induce the increment of the nightly PBLH in most months, especially for the central lake area in autumn. The primary factors that contribute to sensible heat flux (H) and latent heat flux (LE) were the roughness lengths for heat and momentum during the ice-free period, respectively. Increasing roughness length for heat can increase the nightly PBLH, and decreasing roughness length for moisture can also promote growth of the PBLH, but there was no obvious correlation between the momentum roughness length and the PBLH.

Internal evaporation and condensation characteristics in the shallow soil layer of an oasis

The surface energy balance was analyzed using observations from the Jinta oasis experiment in the summer of 2005. A negative imbalance energy flux was found during daytime that could not be attributed to the soil heat storage process. Rather, the imbalance was related to the evaporation within the soil. The soil heat storage rate and the soil moisture variability always showed similar variations at a depth of 0.05xa0m between 0800 and 1000 (local standard time), while the observed imbalanced energy flux was very small, which implied that water vapor condensation occurred within the soil. Therefore, the distillation in shallow soil can be derived using reliable surface energy flux observations. In order to show that the importance of internal evaporation and condensation in the shallow soil layer, the soil temperatures at the depths of 0.05, 0.10, and 0.20xa0m were reproduced using a one-dimensional thermal diffusion equation, with the observed soil temperature at the surface and at 0.40xa0m as the boundary conditions. It was found that the simulated soil temperature improves substantially in the shallow layer when the water distillation is added as a sink/source term, even after the soil effective thermal conductivity has been optimized. This result demonstrates that the process of water distillation may be a dominant cause of both the temperature and moisture variability in the shallow soil layer.