Shallow Foehn on the Northern Leeside of Tianshan Mountains and Its Influence on the Boundary Layer over Urumqi, China—A Climatological Study

The Tianshan Mountains, with an average altitude of 4000 m, are large independent zonal mountain ranges that run through the Xinjiang Uygur Autonomous Region in northwestern China and divide Xinjiang into northern and southern regions with distinctly different climate. The capital city, Urumqi, is located near the eastern edge of Tian Shan near the exit of a deep north-south oriented valley known as the Middle Tianshan Mountains Valley (MTMV). Under certain synoptic weather conditions, southeasterly gap winds that belong to the category of shallow foehn can develop through MTMV affecting areas downwind including Urumqi. These southeasterly gap flows can be either ground-based, referred locally as ground-based southeasterly gale (GBSEG), or elevated, known locally as elevated southeasterly gale, (ESEG). GBSEG occur more frequently in spring and autumn while ESEG happens mostly in winter. Much attention has been given to GBSEG because they can cause a significant amount of damage in Urumqi. The formation of ESEGs and their potential impacts, however, have been largely neglected, despite that this kind of foehn flows can significantly change the local boundary layer structure and thereby influence local air quality.

In this climatological study, we use twice daily rawinsonde data at Urumqi during the period of 2007-2012 to examine the general behaviour of ESEGs in the region. The analysis shows that ESEGs have occurred in every month, with average occurrences of 94 days per year, or an annual frequency of 26%. There is a strong seasonal variation in the occurrence of ESEGs, with frequencies of 19 days (21%), 9 days (9.0%), 29 days (31.%) and 37 days (40%) in spring, summer, autumn and winter, respectively. The highest frequency (45%) is in December while the lowest (0.05%) is in June.

The vertical structure of ESEGs also displays marked seasonal variations. The base and top of the ESEG layer and the height at which maximum wind speed occurs are relatively low in winter, with average values of 568 m, 1744 m and 1057 m, respectively, but high in summer, with values of 908 m, 2186 m and 1612 m, respectively. The thickness of the ESEG layer, exhibits small seasonal changes, varying between 1055 in November and 1490 in October. The monthly average maximum wind speeds exceed 10 m/s for every month, with the peak wind speed of 13 m/s found in March.

ESEGs are characterized by several parameters and statistically significant relationships are found between certain pairs of parameters. Examples include those between the bottom height and top height, the thickness and top height, the maximum wind speed and bottom height, the top height and height at which the maximum wind speed occurs, and the thickness and maximum wind speed. ESEGs are also related to temperature inversions as well as mixing heights over Urumqi. The top of the inversion layer is typically several hundred meters above the level of the maximum wind speed and the inversion strength and depth gradually increase with increasing speed and height of maximum winds. Mixing layer depths also tend to increase with increasing maximum wind speed and the height where the maximum occurs.

The corresponding atmospheric circulation patterns on ESEG days are analyzed using NCEP global reanalysis and ECMWF High Resolution Model analysis datasets. The pattern is characterized by a 500-hPa ridge over Xinjiang, accompanied by weak southwesterly 700 hPa winds. At 850 hPa, a colder air mass over western Mongolia is situated opposite to a warmer air mass over western Xinjiang, which is consistent with a west–east orientation surface pressure gradient. Under such synoptic scale atmospheric pattern, large amounts of cold air re-enter both sides of the Tianshan. Without the blocking effect of Tianshan, and assisted by the gap to the east of the Tarim Basin, the cold air mass can easily flood into the Tarim Basin. The cold air entering southern Xinjiang cannot flow out of the Tarim Basin owing to the blocking effect of the surrounding mountains. It thus has to change its trajectory and enter northern Xinjiang through the MTMV and another wider breach known as the ‘50 km Windy Belt’, in the east of the Tianshan.

ESEGs differ from GBSEGs not only in the areas of influence, strength, seasonality and vertical structure but also in their formation mechanisms. Although the circulation patterns at the middle and upper levels are similar for both types, the 850 hPa trough over Xinjiang is deeper and the 850 hPa temperature over Urumqi is higher on days with GBSEGs than on days with ESEGs. In addition, an abrupt rise in surface temperature over Urumqi is a critical condition for the formation of GBSEGs. For GBSEGs, pressure-driven channelling and the thermally driven mechanisms are important while terrain-driven channelling and downward momentum transfer appear to play a bigger role in the formation of ESEGs.