Diurnal and Semidiurnal Variations in Global Surface Wind and Divergence Fields
Aiguo Dai and Clara Deser
J. Geophys. Res., Vol. 104, 31109-31125, 1999.
Diurnal and semidiurnal variations in surface winds and
wind divergence over the globe (50S-70N) are documented using 3-hourly wind
observations from ~10,000 land stations and COADS 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 may be a primary cause 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
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 (10S-10N), 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 Cells 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 and is consistent with the radiative cooling hypothesis put
forth by Gray and colleagues. Over the United States, surface divergence peaks
around 0600 LST in the west and the east and around 2000 LST in the center.
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Hongjun Zhang:
zhangho@ucar.edu