عنوان مقاله [English]
Nocturnal low-level jets (NLLJs) occur frequently in many parts of the world. These low level jets are important in heat, dust, moisture and even insects transport for long distances; hence their characteristics have been the subject of many studies. There are many regions in mountainous areas of Iran that experience NLLJs for which the NLLJ over the Dashte Kavir (DK) is an important one that occur in summer months. The occurrence and other detailed characteristics of NLLJs over the DK in the north desert region of Iran and south of the Alborz chain are not well known. There are not much observational wind profiles available to study the jet in the region. So, we have used the ERA-Interim Reanalysis data that provide long enough historical grid data that can be used for this kind of studies. This paper climatologically presents the occurrence of NLLJs and its characteristics over the DK by analyzing multi-year ERA-Interim reanalysis.
We have compared the reanalysis data with surface observations from 11 synoptic stations in the DK region in the long period (1979-2017) and also a 40m tall observation platform to find correspondence between the two. It is found that the ERA-Interim data set can capture the real atmospheric parameters and thus can be used to study NLLJs’ features in the region. The NLLJs occur in most of the summer nights, which are primarily easterly to northeasterly. The jet core typically appears at 850 hPa at the top of the surface inversion layer with monthly average speed of 9 to 14m/s in its core. Based on the Inertial oscillation theory or Blakadar (1957) effect, most NLLJs are located above the nocturnal inversion during the warm season nights, while during the cold season, the wind regime changes and weak westerly winds dominate in lower levels in this area. Based on the 6 hours’ resolution of the available reanalysis data, NLLJs above the inversion have strong daily oscillations and the maximum wind speed occurs at 00 and the minimum at 12 UTC and an annual cycle with a mean monthly maximum speed in June.
We quantified mean monthly NLLJ parameters using some definitions and construct magnitudes of the NLLJ in every grid point in the DK region. The magnitude of the momentum in the lower atmosphere from the top of the surface layer to the top of the mixed layer is large for NLLJs of the warm season and using the bulk Richardson number, in a few case studies, the downward momentum transfer in weak stability conditions leads to dust rising in the region. The winds below the NLLJ core to the desert surface gain strength in summer, and these summer winds are coincident with an enhancement of rising dust that reduces visibility in the cities in the desert margins. The nocturnal jet seems to flow along the southern mountain range of Alborz that extents far west even to the Tehran greater plain. Such strong flow may have implication for air pollution ventilation of the Tehran area in summer. This phenomena can be interesting subject for future study in this area (Tehran) that suffers from acute air pollution episodes.