Diurnal and semidiurnal variations in global surface wind and divergence fields

Diurnal and semidiurnal variations in surface winds and wind divergence over the globe (50 degrees S-70 degrees N) are documented using 3-hourly wind observations from similar to 10,000 land stations and available marine reports during 1976-1997. A strong diurnal cycle in surface winds is found over land areas (strongest over high terrain and in summer) with an amplitude of 0.6-1.1 m/s for wind speed and 0.5-0.7 m/s for the zonal and meridional wind components. Surface wind speed peaks in the early afternoon over most of the globe. It is suggested that increased downward turbulent mixing of momentum during the day could be one of the main causes for the early afternoon maximum of surface wind speed. The diurnal anomalies of surface wind divergence tend to be out of phase in adjacent regions. In particular, land areas (except for extreme inland locations) exhibit maximum divergence around dawn (0600-0800 local solar time(LST)), while nearby oceanic regions have their maximum divergence in the evening (1700-1900 LST). Thus, there is evidence for a large-scale diurnal circulation in which surface air rises and converges over the continents and sinks and diverges over nearby oceans in the afternoon and early evening, and the opposite occurs in the early morning. Over the tropical Atlantic and Pacific Oceans (10 degrees S-10 degrees N), a zonally coherent pattern of maximum convergence (divergence) north (south) of the equator around 0900-1200 LST is generally similar to the latitudinal profile of the mean daily divergence, indicative of an enhancement of the local Hadley cell around 1030 LST relative to the daily mean. Another zonally coherent north-south dipole occurs over the North Pacific, with maximum surface wind divergence (convergence) around 0600-0800 LST in the subtropics (midlatitudes). This phase pattern correlates with cloud cover over the two regions. Over the United States, surface divergence peaks around 0600 LST in the west and the east and around 2000 LST in the center.