Requirements on the Duration of Observation Sessions
As is well known [1,2,4,7], during measurements of parameters of atmospheric turbulence in the isotropic boundary layer (plane-parallel flows over a homogeneous and homogeneously heated surface), a measurement session should necessarily be conducted in approximately the same meteorological situation, corresponding to a certain established pattern of turbulent motions. During field measurements, we should exclude the influence of diurnal variations, significant changes in the cloud situation, and, especially, cloud cover index variables. Then the stratification parameters controlling the near-surface turbulent meteorological situation over the territory (e.g., the Monin—Obukhov number or the Richardson number) are approximately constant, and the measured time average values are stable (depending only on the type of regional meteorological situation).
For measurements in the mountain anisotropic boundary layer, according to the results presented in Section 3.2.2, the requirement of the stable meteorological situation is not principal. In this boundary layer, all turbulence characteristics become functions of the Monin—Obukhov number, and every point in the layer is characterized by its own value of this number. However, to be certain, it is necessary to conduct a series of measurement sessions, in each of which the regional meteorological situation is approximately stable. From the comparison of these results, we can find the degree of influence of the meteorological situation.
In addition, as follows from Equations 3.7, 3.12, and 3.16, for the comparison of the experiment with the theory, it is necessary to measure different partial derivatives of meteorological fields. At the same time, the approximation of derivatives by difference relations usually leads to significant errors. These errors can be decreased considerably, if we use data obtained at the stable meteorological situation for measurement of the derivatives.
Thus, to conduct the necessary measurements, we should use the series of measurement sessions, in each of which the regional meteorological situation is approximately stable.
Since the known stratification parameters (Monin—Obukhov number or Richardson number) in the mountain boundary layer are different for every point of the layer, they cannot serve for indication of the general meteorological situation of the mountain region. The stratification parameters averaged over many points of the surface can likely be such indicators. However, in the process of measurements, these effective parameters are not known prior.
In this connection, during the measurements, the meteorological situation was controlled, first of all, by the stability of the solar radiation intensity, and by the cloud amount and cloud motion. During a measurement session, the sun should not be periodically covered by clouds or, if it was covered, should not be unclosed. This stability is usually observed in the region of the Baikal Astrophysical Observatory in October in the afternoon, starting from 13:00 to 14:00 local time. The duration of measurement intervals with the highly stable meteorological situation both for the open sun (scattered clouds located, as a rule, near the horizon) and for closed sun (overcast and dense clouds) is approximately identical and ranging from 3 to 5—7 h. However, unstable meteorological situations characterized by significant broken (or overcast, but not dense) clouds are observed more often (mostly, before noon). The duration of the open sun periods at broken clouds is usually shorter that 1—1.5 h.
In addition to the high-quality control of the cloud situation, the dynamics of variation of the regional turbulent meteorological situation for a particular time period can be estimated from measurements of the near-surface values of C% (or Ct, CV) at one point. Preliminary measurements conducted in the afternoon at the same place have shown that in the case of permanently open sun at approximately the same cloud amount, the near-surface value of СП can vary (decrease 1.5—2 times) for 2—3 h due to the diurnal motion of the sun (the result is different for different geographic locations of the measurement point). At overcast or dense clouds, the same change takes longer time, approximately 2—4 h. The change of СП values appears to be more significant in the case of unstable meteorological situation and, when the sun is periodically open or closed by clouds or the density of clouds covering the sun changes.
It is clear that the duration of observation sessions should be minimized. If turbulent characteristics at one point are recorded for 5—7 min, including the measurement time of 2 min, the time for device setting-up procedures, and the time needed for transportation from one observation point to the next one, then at the highly stable meteorological situation lasting 3 h, it is possible to record about 30 points without marked errors for the measurement session. At not so stable meteorological situation, the measurement session should not last longer than 1—1.5 h.
For faster measurements and in connection with the need to move over a vast territory, we used a cross-country car; pre-measurement and post-measurement procedures were minimized. Measures were provided to avoid the influence of foreign thermal sources on the measurement results (during the measurements, the car engine was turned off, equipment and people were at a sufficient distance on the leeward).
For the period of research mission (October 12—28, 2002), five measurement sessions were carried out. In addition, preliminary observations were conducted to monitor the stability of regional turbulent meteorological situations. The first measurement session was conducted on October 17 before noon (sunny morning, wind speed 1—2 m/s, temperature from — 1°C to 0°C, and humidity 49%—61%); the second session took place on October 17 in the afternoon (overcast conditions, white haze, wind speed 0.3—5 m/s, temperature from — 0.4°C to +1.7°C, humidity 46%—62%); the third session took place on October 18 in the afternoon (sunny day, wind speed 1—8 m/s, temperature from 0°C to +3.7°C, and humidity 35%— 63%); the fourth and fifth sessions were conducted on October 22 in the afternoon (overcast conditions, wind of 1—3 m/s, temperature — 0.9°C to +1.1°C, humidity 37%—57%). In the first three sessions, the measurements were conducted along the entire path at different long sections, whereas in the fourth and fifth sessions the measurements were conducted only on the top of the LSVT mountain with a more detailed resolution, and in the fifth session the vertical measurements in the lowest 5-m layer were carried out. The first and third measurements were carried out at the fine sunny weather in similar stable meteorological situations, while the second, fourth, and fifth measurements were carried out at the stably closed sun.