Influence of the low-level jet on the intensity of the nocturnal oasis cold island effect over northwest China

摘要

The nocturnal oasis cold island (OCI) effect is an important phenomenon seen over the heterogeneous surfaces of oases and deserts in northwest China and is influenced by many unknown complex factors. Using field observations and wind profile radar data over oases and deserts in Zhangye city, China, we analyze the relationship between the OCI effect and the low-level jet (LLJ). Vertical mixing induced by the LLJ has a profound impact on the intensity of the OCI effect during the night. The mean nocturnal intensity of the OCI effect and the strength of the nocturnal LLJ are significantly correlated. We investigate two examples in detail-one with a strong OCI effect and one with a weak OCI effect-using observational data and simulations by the Weather Research and Forecasting model. During a weak LLJ, when the wind speed decreases and vertical mixing is weakened, the increase in evaporation over the oasis causes the temperature to decrease dramatically and results in a strong OCI effect. During a strong LLJ, the strong shear induced below the nose of the LLJ enhances turbulent mixing, heat is transported downward, and the temperature over the oasis decreases slightly. The near-surface wind speed over the desert increases, and the decrease in temperature is larger than that during a weak LLJ. Therefore, the intensity of the OCI effect decreases during a strong LLJ.