The influence of irregular terrain on the evolution of the daytime
planetary boundary layer (PBL) and meso-β scale dry circulations is
studied using two three-dimensional hydrostatic σ-coordinate models
with different approaches for the PBL parameterizations; the 4-layer
model uses the mixed-layer (bulk-layer) approach, while the 7-layer
model adopts the eddy-diffusivity (multi-layer) approach. Numerical
experiments are carried out under the conditions of a dry, sunny summer
day with moderate prevailing westerly winds blowing over gently sloping
idealized hills in a domain of 150 km on a side. The results from the
two models are compared and their performances are evaluated.
The behaviors of the mean PBL depth and inversion strength are
analytically described using a simple one-point mixed-layer model.
Counterclockwise rotation of the mean PBL winds with time observed in
both model results can be explained only when the non-zero momentum
flux at the PBL top is taken into account. However, stresses
associated with entrainment at the PBL top are not sufficient to pull
the cold air out of the valleys so as to result in breakup of the
early morning stable layer, as is suggested in a previous study.
The regions of weak winds that persist in the morning PBL are
attributed largely to the baroclinic effect of horizontal variations of
potential temperature θ in the PBL, while the effect of surface drag is
quite small in these areas. Significant differences in the flow
patterns near the surface in two results suggest the importance of the
local pressure gradient force associated with terrain irregularities.
The effect of horizontal θ advection is also significant in helping
reduce the PBL θ anomalies and promote breakup of the stable layer.
The well-mixed assumption generally applies quite well to the
development of the θ profiles, while for momentum it seems valid only
during the peak of convective mixing and the eddy-diffusivity approach
is probably preferable for a better description of the low-level flows.
The fields of the PBL top height obtained using different procedures
in the two models are found to correspond fairly well to each other.
Mass-flux convergence associated with terrain irregularities and
resulting changes in the wind fields are shown to play a key role in
the midday PBL height patterns. The development of the PBL structure
as revealed by the θ cross sections obtained from either model corresponds
favorably to that indicated by idealized cross sections previously
constructed from observed data. The formation of a region of
mass-flux convergence and accompanying updrafts near the surface on the
leeward side of a mountain, processes which are likely to be important
in terrain-induced cloud initiations, seem to be simulated. / Graduation date: 1985
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29068 |
| Date | 01 March 1985 |
| Creators | Ueyoshi, Kyozo |
| Contributors | Deardorff, James W. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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