TOPOGRAPHIC FLOWS AT THE NORTH-EASTERN ADRIATIC COAST

A typical summertime anticyclonic situation over the north-eastern Adriatic coast is studied using a 3-D nonhydrostatic meso-γ-scale model. The aim is to evaluate small-scale variability in the thermally driven mesoscale circulations over very complex terrain. The simulations show a development of two diurnal mesoscale eddies inside the Kvarner Bay. The nighttime deeper ones exhibit anticlockwise rotation, while the late afternoon shallow one shows the rotation in the opposite direction. Their appearance and dynamics are connected primarily to the coastline shape and topography. The convergence zones (CZs), as a result of the merged sea breezes, develop over Istria and the island of Krk. The intensity, speed and position of these CZs are examined. The results reveal that the CZ over the island of Krk is connected with a baroclinic low-level jet (LLJ). While the governing factor of the Istrian CZ is differential heating between Istria and the surrounding sea surface, the CZ over the island of Krk (and LLJ) is basically determined by the terrain and coastal geometry. The influence of the synoptic flow on the mentioned local characteristics is relatively small

The ABL due to a Mountain Pass and Coriolis Effect

Idealized airflow over a mountain with a pass is studied using a numerical mesoscale model with a trustful higher-order turbulence parameterization scheme. A uniformly stratified inflow of 8 m/s over mountain, 100km x 20km x 1km, yields Froude number 0.6, while Rossby number along the flow ranges from 7.6 to infinity. A pass drops the mountain top to ~ 400m locally increasing Froude number and modifying the overall wave breaking and the ABL. In the presence of the Earth rotation, f ≠ 0, which already breaks the lee-side flow symmetry, the pass induces additional variations in the ABL extending far from the mountain. Both, the rotation and the pass, alter the low-level jet structure and the specific humidity field. This mesoscale process may stretch to synoptic scale within a reasonable time, say 15-25h, only when the rotation is included. Otherwise, a somewhat similar process with f=0 might take unreasonably long time

Glacier winds and parameterisation of the related surface heat fluxes

The katabatic flow over glaciers is studied with data from automatic weather stations (AWS). We analyse data from the Morteratschgletscher (Switzerland), Vatnajökull (Iceland) and West Greenland, and conclude that katabatic flow is very common over melting glacier surfaces and rarely disrupted by the large-scale flow. Over small and medium-size glaciers the height of the wind maximum is generally low (typically 10 m), and vertical temperature differences near the surface are very large (up to 15 K over 4 m). In glacier mass-balance models there is a great need for parameterisations of the surface heat flux. We develop a simple method to estimate the sensible heat flux Fh associated with the glacier wind. It is based on the classical Prandtl model for slope flows. We set the turbulent exchange coefficient proportional to the maximum wind speed (velocity scale) and the height of the wind maximum (length scale). The resulting theory shows that Fh increases quadratically with the temperature difference between the surface and the ambient atmosphere; Fh decreases with the square root of the potential temperature gradient of the ambient atmosphere; and Fh is independent of the surface slope

Justifying the WKB approximation in pure katabatic flows

Pure katabatic flow is studied with a Prandtl-type model allowing eddy diffusivity / conductivity to vary with height. Recently we obtained an asymptotic solution to the katabatic flow assuming the validity of the WKB method, which solves the fourth-order governing equation coupling the momentum and heat transfer. The WKB approximation requires that eddy diffusivity may vary only gradually compared to the calculated quantities, i.e., potential temperature and wind speed. This means that the scale height for eddy diffusivity must be higher than that for the calculated potential temperature and wind speed. The ratio between the maximum versus the mean eddy diffusivity should be less than that for the scale heights of the diffusivity versus the calculated quantities temperature and wind). Here we justify (a posteriori) the WKB method independently based on two arguments: (i) a scaling argument and (ii ) a philosophy behind a higher-order closure turbulence modeling. Both the eddy diffusivity maximum and the level of the relevant maximum turbulent kinetic energy are above the strongest part of the nearsurface inversion and the low-level jet which is required for the WKB validity. Thus, the numerical modeling perspective lends credibility to the simple WKB modeling. This justification is important before other data sets are analyzed and a parameterization scheme written

Characteristics of the near-surface turbulence during a bora event

During a bora event, the turbulence is strongly developed in the lee of the Dinaric Alps at the eastern Adriatic coast. In order to study its properties, a 3-D ultrasonic anemometer operating at 4 Hz sampling frequency was placed in the town of Senj at 13 m above ground. The strong bora case that occurred on 7 January and lasted till 11 January 2006 is analyzed here. This data set is used for evaluation of the turbulent kinetic energy, TKE, and its dissipation rate, ε. The computation of ε is performed using the inertial dissipation method. The empirical length scale parameter for this event is estimated with respect to ε and TKE. Some considerations about defining turbulent perturbations of the bora wind velocity are also pointed out