Actual versus geostrophic wind: statistics from 12-year measurements at the 280 m high Hamburg Weather Mast

The geostrophic wind, representing the horizontal pressure gradient as driving force of motion, is often taken as a f rst guess of the actual wind. There is, however no f xed relation between them but depends on various inf uencing factors. How good is this f rst guess i.e. to which extent do these factors change the actual versus geostrophic wind relation? A 12-year set of six-hourly (00, 06, 12, 18 UT) geostrophic (Ug) wind data taken from the ERA-Interim model and actual (U) wind data measured at f ve levels (10, 50, 110, 175, 250 m) at the Hamburg Weather Mast, Germany, is used to statistically study the dependence of the speed ratio U/Ug and the angle difference alpha g - alpha on various inf uence parameters: height z above ground, geostrophic wind speed Ug, thermal wind Utherm, surface roughness z0, and day-night stratif cation differences. The actual wind has a Weibull-like frequency distribution (FD) with systematically changing parameters from lower to upper levels. In contrast, the Ug-FD has the same constant Weibull-like shape at all levels. This does not imply that Ug is constant with height, but that the various conf gurations of thermal wind almost balance. The Utherm-FD itself is Weibull-like. The all-times FDs of U/Ug (alpha g - alpha) peak at 0.25 (47 degrees) at 10 m and gradually increase (decrease) to 0.77 (17 degrees) at 250 m. The U/Ug ratio decreases systematically with increasing Ug towards height-staggered asymptotic limits for Ug > 30 m/s. With respect to the thermal wind infl ence, cold air advection (CAA) causes on average 1-1.5 m/s larger U values accompanied with 12 degrees less wind turning between 10 m and 250 m than warm-air advection (WAA) for the same low-level Ug forcing. The z0 values around the Hamburg Weather Mast vary between 0.3 and 1.1 m and lead to 0.1 differences in the U/Ug ratio. Vertical stratificati n has the largest impact on U/Ug and alpha g - alpha. Stable stratificati n developing during night with decreased or ceased turbulence decouples upper and lower levels with opposite effects: wind decreases below and increases above. The day-night crossover height is on average around 100 m. The percentage of supergeostrophic (U > Ug) cases increases with height. At low levels, U > Ug occurs only during daytime and with Ug > 5 m/s. At upper levels, U > Ug can occur during all times of the day, preferably during night with about 30 % of time, and with Ug > 15 m/s. For Ug > 15 m/s stable stratif cation cannot sustain.