Download idealized simulations of vertical velocity fields andthermal

IDEALIZED SIMULATIONS OF VERTICAL VELOCITY FIELDS ANDTHERMAL
CONVECTION OVER A COMPLEX HILLY TERRAIN
Huang Qian 1 Tian Wenshou1 Wang Wen 1 Zhang Qiang 2 Shen Xueshun 3
Zhang Lei 1 Kang Fengqin 2
1 College of Atmospheric Sciences, Lanzhou University, Lanzhou 730000
2 Key Laboratory of Arid Climate Change and Reducing Disaster of Gansu Province,
Institute of Arid Meteorology, China Meteorological Administration, Lanzhou 730020
3 Chinese Academy of Meteorological Sciences, Beijing 100081
Abstract
A series of idealised model simulations have been performed over a real hilly terrain using a high
resolution boundary layer model. The model used is the Met Office boundary layer model called
Boundary Layer Above Stationary, Inhomogeneous Uneven Surface (BLASIUS). Based on those
simulations, the characteristics of vertical velocity fields and thermal convection over the
real/complex terrain are investigated and effects of orography are diagnosed. The model results
indicate that the vertical velocity field is more predictable under higher wind speeds in a stable
atmosphere, while it is more randomly distributed in a neutral atmosphere. When Froude number
is less than 0.5, air flow tends to be blocked and uplifted over the windward slope by orography,
and under such circumstances deep convection is likely to be triggered. Due to the division of
airflow over the windward slope the vertical motion can also be induced on the lee side. The
confluence of air flow on the lee side of mountains is another important factor which may induce
deep convective systems. When the wind speed is high or Froude number is larger, gravity waves
can be triggered on the lee side of mountains, and interaction between terrain-induced gravity
waves and thermal convection can be clearly seen. When there is no energy input at the model's
surface, the vertical velocity field is dominated by the forced ascents and gravity waves, and when
the model's surface is heated gradually, thermal convection begin appear. In the lee side of the
mountain, there are convective lines tending to converge due to the confluence of the flow from
both sides of the mountain. The convergence of convective lines has an important impact on
convection triggering. The top of the convective mixed layer is the interface for the interaction
between thermal convection and gravity waves. When the upward motion of convection is
overlapped by the ascent due to gravity waves, the upwards vertical motion tends to be
strengthened, otherwise, the convection is likely to be depressed. Because terrain
induced
gravity waves are able to affect the strength of convective activity, the propagation of convective
system may also be modulated by gravity waves. The alignments of convective lines and gravity
waves seem to be related to each other, and. more model simulations are worthy to be performed
to obtain quantitative conclusions and theoretical mechanisms.
Key words:Complex terrain, Thermal convection, Orographic gravity waves, Convective rolls.