Global Distribution and Characteristics of Diurnally Varying Low-Level Jets

Abstract

This study documents the global distribution and characteristics of diurnally varying low-level jets (LLJs), including their horizontal, vertical, and temporal structure, with a special emphasis on highlighting the underlying commonalities and unique qualities of the various nocturnal jets. Two tools are developed to accomplish this goal. The first is a 21-yr global reanalysis performed with the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) using a horizontal grid spacing of 40 km. A unique characteristic of the reanalysis is the availability of hourly three-dimensional output, which permits the full diurnal cycle to be analyzed. Furthermore, the horizontal grid spacing of 40 km better resolves many physiographic features that host LLJs than other widely used global reanalyses. This makes possible a detailed examination of the systematic onset and cessation of the jets, including time-height representations of the diurnal cycle. The second tool is an index of nocturnal LLJ (NLLJ) activity based upon the vertical structure of the wind's temporal variation, where the temporal variation is defined in local time. The first available objectively constructed global maps of recurring NLLJs are created from this index, where the various NLLJs can be simultaneously viewed at or near their peak time. These maps not only highlight all of the locations where NLLJs are known to recur, but they also reveal a number of new jets.The authors examine the basic mechanisms that give rise to the NLLJs identified in four disparate locations, each having a profound influence on the regional climate. The first, the extensively studied Great Plains NLLJ, is used to confirm the veracity of the global analysis and the index of NLLJ activity. It also provides context for three of the many newly identified NLLJs: 1) Tarim Pendi in northwest China; 2) Ethiopia in eastern Africa; and 3) Namibia-Angola in southwest Africa. Jets in these four regions illustrate the variety of physiographic and thermal forcing mechanisms that can produce NLLJs.

A climatology of low‐level jets in the mid‐latitudes and polar regions of the Northern Hemisphere

“…They investigate the persistent katabatic flows over the coasts Greenland and found the strongest wind speeds over eastern, northern and western coasts of Greenland at the lowest model level, which agrees well with our results in Figure 2(a) and (c) that show almost persistent, shallow katabatic LLJs over the same areas. Rife et al (2010) studied nocturnal LLJs globally and found larger values of their LLJ index around and over southern parts of Greenland during January compared to July. Their results suggest that increased LLJ occurrence in the North Atlantic and southern parts of Greenland is due to enhanced synoptic activity during the winter months.…”

Taklimakan Desert nocturnal low-level jet: climatology and dust activity

“…), while there are usually minima in wind speed 1.0 to 2.0 km above the core (Rife et al, 2010). A diurnal cycle is a common and well-documented feature of the NLLJ, with onset and cessation times generally in the early evening and midmorning, respectively.…”

“…Synoptic cold fronts are known to lead to strong surface winds, resulting in dust storms over the Taklimakan and Gobi deserts (Sun et al, 2001). Another mechanism that can lead to strong surface winds in semi-arid and desert regions is through the formation of a nocturnal low-level jet (NLLJ) (Fiedler et al, 2013;Rife et al, 2010).…”

Reanalysis-driven climate simulation over CORDEX North America domain using the Canadian Regional Climate Model, version 5: model performance evaluation

“…Numerous studies of the GPLLJ region using simulations by GCMs (Jiang et al 2007;Cook et al 2008) and RCM and high-resolution regional reanalyses (Rife et al 2010;Wang et al 2010;Cerezo-Mota et al 2011) have been performed. While the detailed study of GPLLJ is not the main objective of the present study, we will use the approaches developed in these studies for the general evaluation of the CRCM5 performance in reproducing the GPLLJ.…”