Atmospheric boundary layer measurements at the 280 m high Hamburg weather mast 1995-2011: mean annual and diurnal cycles

Abstract

In this paper, the 280 m high Hamburg weather mast and its instrumentation are introduced. Digital data recorded since 1995 are used to calculate the mean annual and diurnal cycles of the primary climate variables (pressure, temperature, humidity, wind, short- and long-wave radiation, cloud coverage, cloud base, precipitation). The annual average of 2 m temperature is 9.8 °C indicating an increase compared to the period 1971-2000 at the Hamburg airport climate station. Absolute humidity follows the temperature cycle with a maximum in July/August. Relative humidity is highest in winter and lowest in April/May. The fraction of received to clear-sky short-wave radiation is between 61 % in May and 34 % in December. Cloud coverage classes of 0-1 octas and 7-8 octas occur most frequently, but have opposite annual cycles. Cloud base distribution is narrow in winter and peaks around 300 m and is distributed over a wide height range in summer. Average annual precipitation amounts to 716 mm and falls in 9.3 % of the time. Monthly mean wind speed is highest (lowest) in January (August). Winds from west are most frequent followed by winds from southeast. A channelling by the Elbe river valley is indicated. The diurnal temperature cycle is weak in winter but strong in summer showing the evening generation and morning rise of the inversion. While relative humidity has a single diurnal cycle, absolute humidity has a double cycle in summer, but not in winter. Short-wave radiation in summer shows a weak asymmetry between forenoon and afternoon. The diurnal cycles of cloud cover and base are small in winter. In summer, cloud bases show a continuous increase from morning to afternoon and a break afterwards simultaneously with the diurnal rain maximum. Wind speed has opposite diurnal cycles at lower and upper levels. The upper-level cycle shows a temporal asymmetry in summer, i.e.the upper-level wind minimum does not occur simultaneously with the lower-level wind maximum. The reversal height between the opposite cycles is around 130 m in summer and 80 m in winter. The wind direction difference (250 m-10 m) shows a strong diurnal variation between 15 (day) and 45 (night) in summer and a small one between 23 and 35 in winter. The annual and diurnal cycles of all primary climate variables together present an excellent basis for the validation of process, weather or climate models.