The Characteristics and Formation Mechanisms of Low Level Jets in China and their Relationship with Wind Energy

Low-level jets (LLJs) are defined as horizontal wind speed maxima in the lowest few kilometers of the troposphere. LLJs are observed in many parts of the world. Due to their significant effect on weather, aviation safety, regional climate, air pollution and wind energy, they has been attracted considerable attention since its discovery. However, owning to the limitation of observations, the previous studies mainly focus on their relation with precipitation through case studies, but the overall horizontal and vertical distributions of LLJs over China are not well documented. In this paper, the characteristics and formation mechasnims of LLJs in China were studied through observations, numerical simulations and theriotical study.

Half-hourly data from a wind profiler radar (WPR) at the Qingpu site during the Meiyu periods of 2008 and 2009 and their one month before and after (non-Meiyu periods) were used to develop a climatology of the LLJs over Shanghai, China. Two peaks of the LLJ incidence were revealed at 500–800 m and 2100–2200 m altitudes, corresponding to boundary-layer jets (BLJs, below 1 km) and synoptic-weather-system-related LLJs (SLLJs, within 1-3 km). The BLJs displayed a more evident diurnal cycle than the SLLJs, with maximum incidence occurring at night or in the early morning. The SLLJs occurred more frequently during Meiyu periods than non-Meiyu periods, whereas the occurrence frequency of BLJs increased throughout the warm seasons.

Using 2006-2011 hourly model data from the Weather Research and Forecasting Nonhydrostatic Mesoscale Model (WRF) with a 9-km horizontal resolution, the detailed spatial distributions and diurnal variations of low-level jets (LLJs) during early summer (May, June, and July) in China are documented. We found that LLJs frequently occur in the following regions of China: Tarim Basin; northeastern China; Tibetan Plateau (TP); coast of southeastern China and southern China. The LLJs in the Tarim basin, southeast coast and the TP are mainly BLJs, whereas over southern China and northeastern China both BLJs and SLLJs exsit. Their SLLJs are associated with the mei-yu front and Northeast Cold Vortex (NECV), respectively. From the momentum analysis, the diurnal variation of BLJs with maximum occurrences at night or in the early morning is mainly caused by inertial oscillation. East to Tibetan Plateau, the propagation of diurnal SLLJs exists, which is generally consistent with diurnal variations of precipitation. Furthermore, with sensitivity experiments, a strong coastal boundary layer jet (CBLJ) off the southeastern coast of China (around 28°N, 122°E) is studied in details. The CBLJ is mainly contributed by large-scale enhancement by diurnal forcing and orographic effects by the coastal terrain along the southeastern China coast and the terrain of Taiwan.

After developing a simple analytical model including both diurnal thermal forcing over sloping terrain (the ‘Holton' mechanism) and diurnally varying boundary-layer friction (the ‘Blackadar' mechanism) to account for the observed amplitude and phase of the LLJ over the Great Plains of the U.S. and to understand better the role of each mechanism. The present model indicates that, for the pure Holton mechanism (time-independent friction coefficient), the maximum southerly wind speed vmax occurs earlier than the observed. For the pure Blackadar mechanism (time-independent thermal forcing), vmax generally occurs later (closer to sunrise) than observed. For both mechanisms combined, the present model indicates that vmax occurs near to the observed time, which lies between the time obtained in the pure Holton mechanism and the time obtained in the pure Blackadar mechanism; furthermore, vmax is larger (and closer to that observed) than in each one considered individually. The low-level diurnal winds for different locations of eastern China are explained with the simple model. At a similar latitude, the maximum velocity parallel to the coastline over the ocean occurs earlier than the maximum velocity parallel to the inland chain of coastline-parallel mountains over land. This difference can be identified with the well-known Blackadar effect over the land. Furthermore, the thermally driven diurnally periodic wind propagation signals off the east coast of China, which are different between the south and north, are explored with 2-D linear land-sea-breeze theory and an idealized WRF model: offshore eastward propagation of the vertical motion in the boundary layer off the southeast coast of China exists while the diurnal signal in vertical motion is more phased-locked off the northeast coast of China.

IBM's “Hybrid Renewable Energy Forecasting” (HyRef) System was also used to investigate the wind power associated with LLJ in China. The HyRef system combines weather prediction and analytics to accurately forecast the availability of wind power. It is found that the low level jet is a significant contributor to wind power production in China.