M. Tjernstrom et B. Grisogono, THERMAL MESOSCALE CIRCULATIONS ON THE BALTIC COAST .1. NUMERICAL CASE-STUDY, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 101(D14), 1996, pp. 18979-18997
A three-dimensional mesoscale numerical model is utilized to investiga
te a well-documented seabreeze event taken from experimental data. The
simulations are not set up as a model validation. Instead, the model
is driven by generalized and simplified initial and boundary condition
s extracted from the field experiment data. The aim is to simulate the
main characteristics, if not all the details, of the observed case. T
his generates a comprehensive set of fully three-dimensional and time
dependent data that can be used to obtain a more complete description
of the case characteristics. Analyzing these data facilitates understa
nding of the flow mechanisms in more detail. Once such a control simul
ation is done, the case can be resimulated for slightly different forc
ing and the cause of observed features can be illuminated. Significant
features in the simulation are the significant temperature difference
between land and sea, moderate coastal terrain, and an almost 90 degr
ees change in coastline orientation. Among the conclusions from the co
ntrol Simulation presented here is that the seabreeze stage is precede
d by a coastal wind jet stage with a significant influence on the flow
by the quite moderate topography. This is due to the ratio of the dep
th of the marine boundary layer to the terrain height; the terrain is
acting on the Bow in three ways. First the stability impedes vertical
motions and the terrain thus acts as a barrier inhibiting geostrophic
balance. The flow is blocked and deflected down the synoptic-scale pre
ssure gradient. Second, it generates a perturbation in the temperature
field causing local baroclinicity and a thermal wind. Finally, a slop
e flow is also generated that acts to enhance the temperature perturba
tion by advection of cold air to the coastal zone. A seabreeze evolves
on both coastlines. The first forms on the offshore flow coast but re
mains stationary at the shoreline until the second, later, seabreeze o
n the almost perpendicular coastline, where the coastal jet dominates
the flow well into the morning, moves inland and-removes the offshore
flow balancing the first seabreeze at the shoreline. After this obstru
ction is removed, the initial seabreeze rapidly propagates inland.