Diurnal and semidiurnal variations in global surface wind and divergence fields

Diurnal and semidiurnal variations in surface winds and wind divergence ove r 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 repo rts 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 o f 0.6-1.1 m/s for wind speed and 0.5-0.7 m/s for the zonal and meridional w ind components. Surface wind speed peaks in the early afternoon over most o f the globe. It is suggested that increased downward turbulent mixing of mo mentum during the day could be one of the main causes for the early afterno on maximum of surface wind speed. The diurnal anomalies of surface wind div ergence tend to be out of phase in adjacent regions. In particular, land ar eas (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 evi dence 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 morni ng. Over the tropical Atlantic and Pacific Oceans (10 degrees S-10 degrees N), a zonally coherent pattern of maximum convergence (divergence) north (s outh) of the equator around 0900-1200 LST is generally similar to the latit udinal profile of the mean daily divergence, indicative of an enhancement o f the local Hadley cell around 1030 LST relative to the daily mean. Another zonally coherent north-south dipole occurs over the North Pacific, with ma ximum surface wind divergence (convergence) around 0600-0800 LST in the sub tropics (midlatitudes). This phase pattern correlates with cloud cover over the two regions. Over the United States, surface divergence peaks around 0 600 LST in the west and the east and around 2000 LST in the center.