Xianjie Cao

Turbulence regimes and the validity of similarity theory in the stable boundary layer over complex terrain of the Loess Plateau, China

To gain an insight into the characteristics of turbulence in a stable boundary layer over the complex terrain of the Loess Plateau, data from the Semi-Arid Climate and Environment Observatory of Lanzhou University are analyzed. We propose a method to identify and efficiently isolate nonstationary motions from turbulence series, and then we examine the characteristics of nonstationary motions (nonstationary motions refer to gusty events on a greater scale than local shear-generated turbulence). The occurrence frequency of nonstationary motions is found to depend on the mean flow, being more frequent in weak wind conditions and vanishing when the wind speed, U, is greater than 3.0 ms(-1). When U exceeds the threshold value of 1.0 ms(-1) for the gradient Richardson number Ri 0.3, local shear-generated turbulence on timescales of less than 4min depends systematically on U with an average rate of 0.05 U. However, for the weak wind condition, neither the mean wind speed nor the stability is an important factor for local turbulence. Then turbulence is categorized into three regimes based on the behaviors of nonstationary motions and local turbulence. Regime 1 considers stationary turbulence with a wind speed greater than 3.0 ms(-1), and the Monin-Obukhov similarity theory (MOST) can be used to calculate the turbulence momentum flux. Regime 2 examines intermittent turbulence where the MOST is competent to evaluate the local turbulence momentum flux but not nonstationary motions. Regime 3 involves wind speed that is less than the threshold value, where nonstationary motions are dominant, local turbulence is independent of the mean flow, and where the MOST may well be invalid.

Lidar measurement of planetary boundary layer height and comparison with microwave profiling radiometer observation

The paper is on the determination of the height of the planetary boundary layer (BLH) by means of lidar measurements and application of the continuous wavelet transform method. The retrieved heights are compared to results from numerical models based on the parcel method. The latter allows to determine the entrainment zone; the required information concerning the surface heat flux and the temperature profile are provided from a microwave radiometer and sonic anemometer. The authors retrieve a set of BLHs for Lanzhou and Yuzhong.

Energy balance in the semiarid area of the Loess Plateau, China

To understand the energy balance over the complex terrain of the Loess Plateau, we analyzed data from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL). From 1 June to 15 August 2008, the energy balance closure ratio ranged from 0.52 to 0.90 during the daytime when the available energy was greater than 150 Wm(-2); during nocturnal hours, it was about 0.25. We proposed a method based on water mass conservation to estimate the energy transmitted by the vertical movement of moisture in the soil and set out to explain the imbalance. During the daytime, the heat stored in the soil above the heat flux plate placed at 5 cm below the surface, the energy transmitted by the vertical movement of moisture in the soil, and the energy assimilated by plant photosynthesis contributed to 29.6%, 2.0%, and 1.8% of the energy residual, respectively. During stable conditions, the soil heat storage contributed to 46.4% of the energy residual, whereas the other energy terms contributed little. The instantaneous energy closure ratio was about 0.80 during unstable conditions; it was about 0.40 when the wind speed U was greater than 3.0 ms(-1), while depends systematically on U under condition of U<3.0 m s(-1) during stable conditions. Under the weak wind stable condition, affected by topography-induced nonstationary motions, the turbulence is anisotropic with a strong horizontal fluctuation and a weak vertical fluctuation, resulting in weakened heat mixing in the vertical direction and stronger unclosure of energy.

Enhanced Bottom-of-the-Atmosphere Cooling and Atmosphere Heating Efficiency by Mixed-Type Aerosols: A Classification Based on Aerosol Nonsphericity

AbstractThe current understanding of the climate effects of mixed-type aerosols is an open question. The optical and radiative properties of the anthropogenic, mixed-type, and dust aerosols were studied using simultaneous observations of a sun photometer and a depolarization lidar over the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL), northwestern China. The aerosol radiative effect was calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model and was in good agreement with the Aerosol Robotic Network (AERONET) product. The anthropogenic, mixed-type, and dust aerosols were identified mainly based on the lidar-measured depolarization ratio, which was supported by the airmass back trajectories. The mixed-type aerosols exhibit lower (higher) extinctions below (above) 1.5 km above the ground, indicating anthropogenic pollution from the atmospheric boundary layer and dust aerosols above. The dust aerosols exhibit the highest absolute radiative effect...

Impacts of initial soil moisture and vegetation on the diurnal temperature range in arid and semiarid regions in China

To assess the impacts of initial soil moisture (SMOIS) and the vegetation fraction (Fg) on the diurnal temperature range (DTR) in arid and semiarid regions in China, three simulations using the weather research and forecasting (WRF) model are conducted by modifying the SMOIS, surface emissivity and Fg. SMOIS affects the daily maximum temperature (Tmax) and daily minimum temperature (Tmin) by altering the distribution of available energy between sensible and latent heat fluxes during the day and by altering the surface emissivity at night. Reduced soil wetness can increase both the Tmax and Tmin, but the effect on the DTR is determined by the relative strength of the effects on Tmax and Tmin. Observational data from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) and the Shapotou Desert Research and Experimental Station (SPD) suggest that the magnitude of the SMOIS effect on the distribution of available energy during the day is larger than that on surface emissivity at night. In other words, SMOIS has a negative effect on the DTR. Changes in Fg modify the surface radiation and the energy budget. Due to the depth of the daytime convective boundary layer, the temperature in daytime is affected less than in nighttime by the radiation and energy budget. Increases in surface emissivity and decreases in soil heating resulting from increased Fg mainly decrease Tmin, thereby increasing the DTR. The effects of SMOIS and Fg on both Tmax and Tmin are the same, but the effects on DTR are the opposite.