Original paper
Actual versus geostrophic wind: statistics from 12‑year measurements at the 280 m high Hamburg Weather Mast
Brümmer, Burghard
Meteorologische Zeitschrift Vol. 32 No. 3 (2023), p. 245 - 258
28 references
published: Sep 11, 2023
published online: Jan 26, 2023
manuscript accepted: Nov 29, 2022
final revised version received: Nov 29, 2022
manuscript revision requested: Jul 22, 2022
manuscript received: Jun 1, 2022
Open Access (paper may be downloaded free of charge)
Abstract
The geostrophic wind, representing the horizontal pressure gradient as driving force of motion, is often taken as a first guess of the actual wind. There is, however no fixed relation between them but depends on various influencing factors. How good is this first 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 ( U g $U_{\text{g}}$ ) wind data taken from the ERA-Interim model and actual ( U $U$ ) wind data measured at five 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 ∕ U g $U/U_{\text{g}}$ and the angle difference α g - α $\alpha_{\text{g}}-\nobreak\alpha$ on various influence parameters: height z $z$ above ground, geostrophic wind speed U g $U_{\text{g}}$ , thermal wind U therm $U_{\text{therm}}$ , surface roughness z 0 $z_{0}$ , and day-night stratification differences. The actual wind has a Weibull-like frequency distribution (FD) with systematically changing parameters from lower to upper levels. In contrast, the U g $U_{\text{g}}$ ‑FD has the same constant Weibull-like shape at all levels. This does not imply that U g $U_{\text{g}}$ is constant with height, but that the various configurations of thermal wind almost balance. The U therm $U_{\text{therm}}$ ‑FD itself is Weibull-like. The all-times FDs of U ∕ U g $U/U_{\text{g}}$ ( α g - α $\alpha_{\text{g}}-\nobreak\alpha$ ) peak at 0.25 (47°) at 10 m and gradually increase (decrease) to 0.77 (17°) at 250 m. The U ∕ U g $U/U_{\text{g}}$ ratio decreases systematically with increasing U g $U_{\text{g}}$ towards height-staggered asymptotic limits for U g > 3 0 m/s $U_{\text{g}}>\nobreak 30\,\text{m/s}$ . With respect to the thermal wind influence, cold-air advection (CAA) causes on average 1–1.5 m/s larger U values accompanied with 12° less wind turning between 10 m and 250 m than warm-air advection (WAA) for the same low-level U g $U_{\text{g}}$ forcing. The z 0 $z_{0}$ values around the Hamburg Weather Mast vary between 0.3 and 1.1 m and lead to 0.1 differences in the U ∕ U g $U/U_{\text{g}}$ ratio. Vertical stratification has the largest impact on U ∕ U g $U/U_{\text{g}}$ and α g - α $\alpha_{\text{g}}-\nobreak\alpha$ . Stable stratification 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 > U g $U>\nobreak U_{\text{g}}$ ) cases increases with height. At low levels, U > U g $U>\nobreak U_{\text{g}}$ occurs only during daytime and with U g > 5 m/s $U_{\text{g}}>\nobreak 5\,\text{m/s}$ . At upper levels, U > U g $U>\nobreak U_{\text{g}}$ can occur during all times of the day, preferably during night with about 30 % of time, and with U g < 5 m/s $U_{\text{g}}<\nobreak 5\,\text{m/s}$ . For U g > 1 5 m/s $U_{\text{g}}>\nobreak 15\,\text{m/s}$ stable stratification cannot sustain.
Keywords
Actual versus geostrophic wind • 12‑year data set • frequency distributions • 280 m high Hamburg Weather Mast