Global Climatology and Regional Modeling of Coastal Low-Level Jets

Sammanfattning: Localized coast-parallel wind-speed maxima at low altitude, known as coastal low-level jets (CLLJs) have important ramifications to the coastal climate and a number of human activities. This thesis documents the existence of the CLLJs around the globe including their mesoscale structure, dynamics and spatio-temporal variability.A CLLJ-detection algorithm is presented, which identifies their occurrence and can distinguish between CLLJs and other types of low-level wind maxima. The method is based on vertical profiles of wind speed and temperature, and is applied to the ERA-Interim reanalysis dataset to obtain a 31-year-CLLJ climatology. Coastal jets are found to exist on many continents, including the previously undocumented CLLJs along the coasts of Oman and Iberian Peninsula. The study highlights a pronounced seasonality among the CLLJ regions and links to large-scale flow. The Oman coastal jet exhibits the globally highest CLLJ frequency (~70%).The thesis also includes detailed analysis of the Oman and Iberian CLLJs using high-resolution regional modeling by dynamical downscaling. The Oman CLLJ is located close to the coast, at low altitude and is forced primarily by the coastal baroclinicity, unlike the previously known Somali-Jet, driven by the Asian summer-monsoon circulation. Although on a large-scale, the Oman CLLJ and the Somali jet appear to merge, the high-resolution simulations clearly illustrate that these are two distinctive phenomena with different forcing. The 20-year-climatology of the Iberian CLLJ reveals a strong seasonality with large inter-annual variations within spring, summer and autumn seasons while the maximum CLLJ frequency is found during the summer. Regional modeling studies were able to resolve detailed mesoscale structure of CLLJs, not visible from the coarse resolution reanalysis climatology. It is concluded that 6-km horizontal resolution can reproduce most of the small-scale features in a reasonable manner.