Wind speed and shear associated with low-level jets over Northern Germany

Nearly two years of SODAR measurements in Hannover, Northern Germany have been analysed for the frequency of occurrence of low-level jets and their properties. Characteristic properties such as the height of the jet core above ground, maximum wind speed, and wind shear underneath the jet core have been derived from the data set. The occurrence of these jets, which appear in a bit more than 20% of all nights, is correlated with the occurrence of typical large-scale weather patterns ("Grosswetterlagen"). Maximum wind speed and height of the jet are positively correlated to each other and both increase with increasing geostrophic forcing. The evaluations further show that low-level jet wind speeds tend to develop until the bulk shear underneath the jet core reaches a critical threshold of about 0.08 s(-1). Further increase in speed of the jet is then assumed to be linked to a growing height of the jet core above ground keeping the shear at its critical value.