Aiguo Dai and Clara Deser
J. Geophys. Res., in press, 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.