A low-level wind maximum is often found over the oceans near many coasts around the world. These Coastal Low-Level Jets (CLLJs) play an important role in the coastal weather and have significant impacts on regional climate and ecology as well as on a number of human activities. The presence of CLLJs is related to various local circumstances such as land-sea temperature contrasts, upwelling, coastal terrain, orientation of the coast, etc., but also to the large-scale atmospheric dynamics. This makes studies of CLLJs not only interesting but also challenging.

In the present study, based on ERA-Interim reanalysis data, the global distribution, spatio-temporal structure, and the seasonal variability of CLLJs are documented. Seasonal data from 1980 to 2011 are used to identify areas where CLLJs are frequently found in the lowest 2 km, following criteria based on the vertical profiles of wind speed and temperature. The results are analyzed to highlight the fundamental aspects and distinctive features of the CLLJs across the globe, including their occurrence rate, jet height, wind-speed maximum and horizontal extent.

Global maps of CLLJs are constructed for the summer and winter seasons. The west coasts of North America, the Iberian Peninsula, north-western Africa and the south-eastern coast of the Arabian Peninsula make up the northern-hemispheric CLLJ regions, while the west coasts of South America, Australia and southern Africa comprise the south-hemispheric equivalents. The existence and characteristics of CLLJs along the southern coast of Oman and the western coast of the Iberian Peninsula regions are also discussed, not fully envisaged before in the context of CLLJs. The highest occurrence of CLLJs is found during the summer in both hemispheres, and the coast of Oman has the globally highest CLLJ frequency, with also the highest maximum wind speeds. The most commonly found CLLJ has a maximum wind speed between 9 and 15 m s^-1, and occurs at heights between 500 and 700 m a.s.l.

Atmospheric flows in coastal regions are impacted by land-sea temperature contrasts, complex terrain, shape of the coastline, among many things. Along the west coast of central North America, winds in the boundary layer are mainly from north or northwest, roughly parallel to the coastline. Frequently, the coastal low-level wind field is characterized by a sharp wind maximum along the coast in the lowest kilometer. This feature, commonly referred to as a Coastal Low-Level Jet (CLLJ), has significant impact on the climatology of the coastal region, influencing aviation, shipping, fishing and a number of other human activities along the littoral zone, and better understanding and forecasting of this is vital.

The purpose of this study is to elucidate the issue of proper resolution in a mesoscale numerical model to describe the physics of a CLLJ, and its impact on the upper ocean. The COAMPS^® model is used for a summer event to determine the realism of the model results compared to observations, from an area of supercritical flow adjustment between Pt. Sur and Pt. Conception, California. Simulations at different model horizontal resolutions, from 54 km to 2 km are performed. While the model produces realistic results with increasing details at higher resolution, the results do not converge even at a resolution of only few kilometers and an objective analysis of model errors do not show an increased skill with increasing resolution. New methods may thus have to be developed to evaluate models at very high resolution. Based on all available information, a compromise resolution appears to be at least 6 km.

Coastal low level jets are a low-tropospheric wind feature driven by the pressure gradient produced by a sharp contrast between high temperatures over land and lower temperatures over the sea. This contrast between the cold ocean and the warm land in the summer is intensified by the impact of the coast-parallel winds on the ocean generating upwelling currents, sharpening the temperature gradient close to the coast, and giving rise to strong baroclinic structures at the coast.

During summertime the Iberian Peninsula is often under the effect of the Azores High and a thermal low pressure system inland, leading to a seasonal wind, in the west coast, called the Nortada (northerly wind).  This study presents climatology of the costal low level jet off the west coast of the Iberian Peninsula, based on a high resolution (9 km) downscaling data produced using the WRF mesoscale model, forced by 20 years of ERA-Interim reanalysis (1989-2008). The simulation results show that the jet hourly frequency of occurrence in summer is above 30%, and decreases for about 10% in spring and autumn. The monthly frequencies of occurrence can reach higher values, around 40% in summer months, and reveal large inter-annual variability in all the three seasons. In summer, on a daily basis, the low level jet is present in almost 70% of the days. The jet wind direction is mostly from north-northeasterly, and occurs more persistently in three areas, where the interaction of the jet flow with local capes and headlands are more pronounced. The jet occurs at heights between 300 and 400 m, and its speed has a median around 15 m s^-1, reaching maximum speeds of 25 m s^-1.

During the boreal summer strong southwesterly, coast parallel low-level winds prevail off the coast of Oman and over the Arabian Sea. The offshore large-scale structure and variability of these coastal winds has been a topic of numerous studies because of their strong connection to the south-Asian summer monsoon. However, low-level coastal jets along the coast of Oman have not been studied in detail, especially on a mesoscale level, despite their globally high frequency of occurrence. In the current study, regional atmospheric modeling has been utilized in an attempt to resolve the mesoscale structure, spatial variability and temporal characteristics of the Oman coastal low-level jet (CLLJ). The limited area model COAMPS^® has been used at 6-km resolution for a five month period from May through September, during 2009. The model output compares favorably with the seasonal climatology for the coast of Oman.

In this study, analysis of high resolution model fields reveals the mesoscale structure of the Oman CLLJ, clearly distinguishing it from the large-scale southwesterly monsoon circulation farther offshore over the Arabian Sea. The Oman CLLJ is closer to the coast and spread northeastwards along the coast of Oman. Although the synoptic forcing that preconditions the Oman CLLJ is different from other areas of CLLJ occurrences, nevertheless, the jet is still driven primarily by the coastal baroclinicity due to sharp land-sea thermal contrasts. Within the study period, July exhibits the highest CLLJ frequency with ~80% occurrences and also manifests highest winds around 27 m s^-1.