4 Meteorology of the Maidanak Observatory

   
4.1 Clear time

The number of clear night time hours is of course a major parameter defining the quality of a stellar observatory. The first serious work comparing different regions of Central Asia was carried out by Novikova NOV70. It was shown that Mt. Maidanak could provide about 2000 clear night time hours per year. This was supplemented by Gladyshev & Shirokova GLA87 with an analysis based on clear time records in intervals from mid-1978 to mid-85, and also including the 1 m telescope measurements in 1981-1983. The statistics compiled by Gladyshev & Shirokova (1987) are compared in Table 4 to similar work made at the ESO observatories of La Silla and Paranal (Sarazin 1997b). The definitions of photometric time slightly differ in the two studies. To keep this in mind, we use the term "clear time'' (the cloudness is not more than 25%) for Maidanak and reserve "photometric time'' (no clouds above 18 degree from horizon during at least 6 hours in a row) for ESO observatories.

As shown in Fig. 11, seasonal trends are stronger at Maidanak, with excellent summer months as was noted by previous authors. We note that with 58% of photometric nights, Maidanak is similar to the ESO observatory of La Silla in Chile.

   

Table 4: Monthly average fraction of clear nights at Maidanak in [1979-1985] Gladyshev & Shirokova1987 and of photometric nights at La Silla and Paranal during the period [1983-1997] Sarazin1997b

Month	Maidanak	La Silla	Paranal
January	31	73	77
February	54	79	84
March	42	76	83
April	47	63	79
May	44	40	70
June	79	34	70
July	92	42	75
August	81	48	73
September	84	54	85
October	55	57	83
November	42	67	84
December	46	69	77
Total	58	59	78

Figure 11: Monthly average fraction of clear nights at Maidanak in [1979-1985] (full line) compared to the statistics of photometric nights at La Silla (dashed line) and Paranal (dotted line) during the period [1983-1997]. The yearly percentages for each site are respectively 58, 59 and 78% of available dark time. Data are from Table 4

4.2 Temperature

In summer the average night temperature at Maidanak is +13 C. In winter it can sometimes fall down to -15 C. The night time temperature amplitude is about 5 C. Winter storms are possible, during which the wind velocity remains moderate (15 m/s). For this study the wind velocity and air temperature have been measured every hour manually. The measurements were taken at 5 m above ground on the DIMM tower and are representative of the operating conditions of Maidanak astronomical facilities. The results for the whole campaign are presented in Table 5, stacked into one single year.

4.3 Ground level wind

The wind speed was measured at the DIMM tower using a Fuss wind speedometer. The statistics presented in Fig. 12 corresponds to more than 3500 individual measurements. An overall average wind speed of 2.1 m/s was measured with a prevailing South-East wind direction (Fig. 13). This is a remarkably low value for a mountain site. All these data are in agreement with previous observations Gladyshev & Shirokova1987.

 

 

Table 5: Seasonal statistics of wind velocity (m/s) and temperature (Celsius) measured once per hour at 5 m above ground during DIMM seeing measurements at Maidanak for the period August 1996 to October 1999 reorganized on a 12 months basis

		Wind, m/s			Temperature, Celsius
Month	NData	Median	Max	Mean	Median	Min	Max	Mean
01	202	2.85	9.70	3.22	-7.20	-14.20	1.40	-6.19
02	239	1.00	8.00	1.96	-6.80	-15.40	-1.00	-7.01
03	58	2.00	9.30	2.44	-4.40	-9.40	-2.00	-4.69
04	99	3.75	8.50	3.55	2.70	-4.00	6.20	2.00
05	102	2.30	4.90	2.41	8.00	5.00	13.40	8.61
06	206	3.20	9.00	3.33	11.60	5.80	17.20	11.74
07	311	2.00	8.50	2.27	14.20	7.20	24.50	14.32
08	409	2.00	10.70	2.26	14.20	4.40	19.50	13.56
09	629	2.20	8.60	2.50	11.20	1.20	17.60	11.62
10	691	2.00	10.00	2.27	5.20	-4.80	12.50	4.99
11	356	1.50	9.00	1.87	0.80	-7.80	9.40	0.61
12	142	0.50	6.80	1.16	-6.65	-13.60	2.00	-6.43

Figure 12: Statistical distribution of the wind speed at Mt. Maidanak measured at 5 m above ground

Figure 13: Maidanak windrose measured at 5 m above ground

4.4 Relation of seeing with ground wind speed

The seeing shows some weak correlation with wind speed at the DIMM location (13 m below the mountain ridge). A fitting relation is proposed in Fig. 14 for the available data set, showing that the best seeing conditions occur for wind speed lower than 5 m/s, i.e. in 90% of the cases.

Figure 14: Dependence of the seeing on ground-level wind speed at Mt. Maidanak

Figure 15: Vertical profile of the average wind velocity obtained by balloon observations as given in Gur92. Our data are indicated by the triangle

4.5 Wind profile in the boundary layer

In Fig. 15 the average vertical profile of the wind speed obtained by balloon observations at Mt. Maidanak in 1990 is shown, as measured by E.I. Sofiev et al. and given in Gur92. The wind speed at ground level is in good agreement with the median value of 2.1 m/s obtained from our data. The wavefront speed range 4-10 m/s, determined from GSM data, does not contradict this profile if we assume that the predominant turbulent layers are located in the wind shear zones.

Figure 16: Wind velocity at 200 mB: monthly average velocity at Maidanak in [1991] (full line) and at Paranal (squares on dotted line) with the minimum and maximum monthly averages (dotted lines) during the period [1987-1999]

4.6 High-altitude wind speed

In order to estimate the wind speed at higher level we used the High Altitude Data provided by the Uzbek Meteorological Center. The tropopause level (200 mB pressure, or 12 km above sea) wind data were obtained only during the year 1991 at three meteorological stations in Uzbekistan: Kokand (400 km East of Mt. Maidanak), Tashkent (350 km North-East) and Termez (160 km South). Unfortunately, there are no high altitude measurements in the region of Mt. Maidanak, but the wind statistics of the three stations is very similar with prevailing western wind. We interpolated wind velocity and wind direction above Mt. Maidanak using weights proportional to the inverse of the distance of each station to Maidanak. We have calculated a median value of tropopause wind velocity of 26.8 m/s and with predominantly western direction. Same direction with a lower speed was measured by GSM, as described in Sect. 3. It is thus the same planetary wind which extends down to 2.5 km above the site, with the bulk of the turbulence at this level.

For comparison, we give in Fig. 16 the seasonal variation of the average 200 mB wind speed at Paranal Sarazin2000. Although additional years of data would be necessary to definitely conclude, one can expect that the seasonal variations are less apparent at Maidanak than at Paranal. The average 200 mB wind velocity above Paranal over a time span of 12 years is 31.3 m/s, only 12% more than the 1991 average of Maidanak, but the maxima reached at Paranal during southern winter months are 50% larger than any of the Maidanak monthly records.

Figure 17: The evolution of the surface layer turbulence on three nights. The integral of $C_{\rm n}^2$ from 1.9 to 3.4 m, $J_{\rm SL}$, is plotted. On July 16 (top) very strong surface layer turbulence was present in the second half of the night. On the nights of July 19 and 22 with good seeing (bottom, full and dashed lines respectively), little turbulence in the surface layer was detected