General Results of Measurement of Turbulence Characteristics in the Mountain Boundary Layer
It was found experimentally that the turbulence in the mountain boundary layer is characterized by significant anisotropy. Even at the stable regional meteorological situation, depending on the location of the measurement point, the structure characteristics Cy, Cy, and Cy can vary by more than two orders of magnitude. The strong influence is exerted by surface areas with the developed heat exchange and high heat capacity (e.g., the influence of the Lake Baikal surface). Near these areas, Cy, Cy, and СП are practically independent of the type of meteorological situation.
96 ? Optical Waves and Laser Beams in the Irregular Atmosphere
Figure 3.5 Wind map of the measurement path at the stable meteorological situation. Arrows with open triangles correspond to session 1, arrows with closed triangles are for session 3. The measurement path is shown by the solid curve (HO, OB, HOB routes).
Large inhomogeneities of terrain and human-made objects create stable rotor perturbations of air flows. Figures 3.5 and 3.6 depict the wind maps of the observation area. As can be seen from Figure 3.5, air flows on the slopes of two parallel ridges are directed to the canyon bottom (between the ridges). At close meteorological situations, the stability of these flows was observed (near point O).
At the top of the LSVT mountain, wind flows from the lake reflect from a height (26 m) of the LSVT building, and, as can be seen from Figure 3.6, stable rotor flows take place (especially, near points 8, 10, 11, 14 in Figure 3.6). Near the centers of these vortex formations, we observed low values of the velocity vector, increased pressure, and decreased humidity.
In the vicinity of the centers, the local temperature stratification can alternate from very unstable to stable. This can be seen, for example, from Figure 3.7, which shows the field of recorded values of the Monin—Obukhov number Z at the top of the LSVT-mountain at a height of 2.7 m from the surface. Thus, at the same, as in Figure 3.6, point 8 the local very unstable stratification is observed (Z = -388). However, just near, at point 11 (several tens of meters from point 8), the Monin— Obukhov number already corresponds to the local stable stratification (Z = +0.3).
As follows from our findings, the values of the Monin—Obukhov number Z measured at the same height of 2.7 m vary at the same observation point at the alternation of the type of regional meteorological situation. The main turbulence characteristics, namely, average dissipation rates of kinetic energy e and temperature N, as well as turbulent scales of temperature T, and velocity V,, also vary considerably in the mountain boundary layer.
Figure 3.6 Wind map of the top of the LSVT-mountain (field of 3D vectors of mean wind at the mountain top at a field near LSVT, whose boundary is shown by the solid curve): p is latitude, e is longitude, (pM, eM) are the latitude and longitude of the reference point M. Observation session 4, measurements at a height of 2.7 m from the underlying surface. Vertical bars show the vertical component of the mean 3D wind vector.
Figure 3.7 Field of values of the Monin-Obukhov number Z on the top of the LSVT mountain. Observation session 4, measurements at a height z = 2.7 m from the underlying surface. The values of Z widely different from the stratification boundaries |Z| = 0.05 are shown separately; for other points: -0.34 < Z < -0.05.
98 ? Optical Waves and Laser Beams in the Irregular Atmosphere
The average temperature T (°C) is also variable. From measurements of the dissipation rate ? by the equation l0 = v3/4?-1/4, where v = 1.3 • 10-5 m2/s is the kinematic viscosity of air at 0°C, we have obtained the values of the Kolmogorov inner scale of turbulence l0. The values of the scale l0 fall in a range from 0.3 to 1.2 mm (average of 0.64 mm). The Kolmogorov inner scale is in the inverse dependence with the intensity of fluctuations of the air flow velocity. The smaller the scale, the larger the velocity fluctuations, which increase near rather large obstacles.