MaoShan Li , YaoMing Ma , WeiQiang Ma , Ishikawa Hirohiko ,FangLin Sun , Shinya Ogino

1. Key Laboratory of Land Surface Process and Climate Change in Cold and Arid Regions, Cold and Arid Region Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

2. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China

3. Disaster Prevention Research Institute, Kyoto University, Kyoto 611-0011, Japan

4. The Graduate School of Science and Technology, Kobe University, Kobe 657, Japan

Different characteristics of the structure of atmospheric boundary layer between dry and rainy periods over
the northern Tibetan Plateau

MaoShan Li1*, YaoMing Ma1,2, WeiQiang Ma1, Ishikawa Hirohiko3,FangLin Sun1, Shinya Ogino4

1. Key Laboratory of Land Surface Process and Climate Change in Cold and Arid Regions, Cold and Arid Region Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

2. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China

3. Disaster Prevention Research Institute, Kyoto University, Kyoto 611-0011, Japan

4. The Graduate School of Science and Technology, Kobe University, Kobe 657, Japan

In this paper, based on in-situ observational data of the Coordinated Enhanced Observing Period (CEOP) Asia-Australia Monsoon Project (CAMP) on the Tibetan Plateau (CAMP-Tibet), structure of the Atmospheric Boundary Layer (ABL) was preliminarily studied during the dry and rainy seasons. The main results show: (a) Diurnal variation of the ABL is obvious over the northern Tibetan Plateau area. The height of the ABL is different with the season change, which ranges from 2,211 m to 4,430 m during the pre-monsoon season and from 1,006 m to 2,212 m during the monsoon season. The ABL height is higher during the dry period than during the rainy period. (b) The humidity is lower during the dry period than during the rainy period,and there are reverse humidity during both periods. (c) Horizontal wind direction is mostly west during the dry period, east under the height of 2,500 m and west above the height of 2,500 m during the rainy period. The wind speed is low during both the rainy and dry periods in the lower ABL layer. The wind speed is stronger within the upper ABL during the dry period than during the rainy period.

Tibetan Plateau; structure characteristics; atmospheric boundary layer; dry period; rainy period

1. Introduction

The energy of atmospheric movement comes from the ground surface. The interaction between the land surface and atmosphere is implemented in the Atmospheric Boundary Layer (ABL). Thus, we need to understand the dynamic and thermodynamic processes in the ABL. The northern Tibetan Plateau is located in Kunlun, Tanggula and Gangtisi Mountains, and is 2,400-km long and 700-km wide. The average altitude is above 4,500 m, and covers an area of 60×104km2. The geography includes grassland,alpine meadow, Gobi, lakes, mountains and valleys. In previous literature about the Tibetan Plateau, the ABL structure was studied as follows: Ye and Gao (1979) evaluated the height of the ABL from 2 km to 3 km, some scientists realized it was from 1,850 m to 2,750 m over the Tibetan Plateau (Zhouet al., 2000), from 1.4 km to 1.8 km in the Dangxiong region (Liu and Miao, 2001), and from 1.6 km to 2.3 km in Lhasa (Pan and Jiang, 1982). After the Qinghai-Xizang Plateau Meteorology Experiment(QXPMEX) in 1979 (Chenet al., 1985; Fenget al., 1985;Zhu and Fan, 1988; Yanai and Li, 1994) and the GEWEX Asia Monsoon Experiment Tibet (GAME-Tibet,1996-2000), the CAMP-Tibet was carried out over the northern Tibetan Plateau from 2001 to 2010 (Maet al.,2005). Numerous interpretations were brought forward involving the interaction between the land surface and atmosphere (Qiet al., 1996; Miaoet al., 1998; Maet al.,2000, 2002a, b; Wanget al., 2002), but what is required is increased research on the ABL structure over the Tibetan Plateau (Liuet al., 2002; Zhouet al., 2002; Zuoet al.,2004a, b, c).

In this study, using the observed data during the prepare enhanced observation period (PEOP) in August, 2002 and two times enhanced observation period (EOP) of the CAMP-Tibet in April and August, 2004, the ABL structure was analyzed during the dry and rainy seasons. This analysis will contribute to research on the interaction between the land surface and atmosphere, regional climate model parameterization, air pollution release, environment protection,and sustainable development.

2. Observed field, instruments and data process

The CAMP-Tibet PEOP and two times EOP were carried out over the Tibetan Plateau in August, 2002, and in April and August, 2004, respectively. The radiosonde (instruments, Vaisala RS80 GPS. The receiver is Vaisala DigCORA Π MW15), laser radar (SESI Co., MPL1002),and wind profile (RADIAN LAP-3000) were installed at the BuJiao Station (31.37°N, 91.90°E, at 4,509 m above sea level). The station is located on the wide and plain (inclement ＜2°) grassland near Nagqu City, which belongs to the sub-frigid climate zone. The annual average temperature ranges from -0.9 °C to -3.3 °C, the annual average relative humidity ranges from 48% to 51%, and the annual average sunshine duration is from 2,852.6 hours to 2,881.7 hours.During the year, there is no absolute frost-free period. The climate is arid, low temperature, very sandy from November to March of every year, and it is warm and rainy with 80%of the precipitation occurring from May to September of every year. The growing season occurs during the rainy season. Before the Asian monsoon, short grass covers the ground and the sensible heat flux is dominate, but the latent heat flux plays a key role and the grass is 5-cm high during the monsoon season.

In this paper, using the observed radiosonde data during EOP in April and August, 2004, the ABL structure was analyzed at the BuJiao Station. April is the dry period where the ground is covered with wilted grass, and August is the rainy period where the ground is covered with 5-cm tall grass.During the EOP, the collected data frequency was one time per 3 hours. The pilot spends around 1 hour every time in the air and the receiver receives one data per 10 s (vertical resolution was 50 m). The highest altitude of the balloon was 20,000 m.

The potential temperature is calculated by the Passion equation (John and Peter, 1977) and the specific humidity equations are as follow.

Potential temperature:

Virtual potential temperature:

Relative potential temperature:

Saturation relative potential temperature:

Specific humidity:

whereeis water vapor pressure,e=es×RH,esis the saturation water vapor pressure,RHis the relative humidity,ωis the mixing ratio of water vapor, andCpis the specific heat of air at constant pressure.

The height of the convective mixing layer was determined by the profile of the virtual potential temperature due to an obvious inversion temperature existing at the top of the ABL (Liu and Miao, 2001). The ABL height is determined by the bottom of the temperature inversion layer. At night,the ABL height was determined by the thick temperature inversion layer.

3. Results

3.1. Diurnal variation of the ABL structure during the dry period over the northern Tibetan Plateau

Through analyzing the observed data from April 15 to 22, 2004, the diurnal variation of the ABL structure, except for rainy days, was similar during the dry period. During the daytime, the ABL was under the convection stratification, while at night it was stable. In this paper, we selected the ABL structure on April 15 as an example. Figure 1 shows that the ABL height increases when radiation heating to the ground increases. Until 11:00 the virtual potential temperature did not change under the height of 1,957 m in the ABL, and then it became completely mixed at 14:00 and remained unchanged under the height of 2,499 m. Then, the ABL height gradually reduces to 2.3 km at 17:00, and 1.7 km at 20:00. At night, it was around 1.2 km under the stable ABL, and the specific humidity reduces with height. Before 11:00 there was inversion humidity in the lower layer due to cloudy weather. The wind direction was northwest during the daytime and southwest at night under the height of 800 m, resulting from topographically induced valley winds. This is due to the placement of NianqingTanggula Mountain, south of the BuJiao Station;and Tanggula Mountain, north of the BuJiao Station. The wind direction was westward above the height of 800 m.The wind speed was weaker near the surface layer, and then increased with height. The wind was stronger during the daytime than at night.

3.2. Diurnal variation of the ABL structure during the rainy period over the northern Tibetan Plateau

Through analyzing the observed radiosonde data at Bu-Jiao Station from August 16 to 19, 2004, the diurnal variation of the ABL was similar during the rainy period. Thus,we selected the diurnal variation on August 17 as an example to study the ABL structure during the rainy period. Figure 2 shows that the diurnal variation of the virtual potential temperature increases with height at 02:00 under the height of 512 m, and was residual layer of the mixing ABL during the daytime from 512 m to 1,500 m. At 08:00, the ABL was still under the stable stratification. The ABL was completely mixed until 14:00, and then the lower of the ABL was stable again with the decreasing temperature. At 20:00 the whole ABL was stable.

Figure 2b shows the diurnal variation of the specific humidity. In the morning it was low, and increased at noon,then decreased in the afternoon. Figure 2c shows the diurnal variation of the wind speed. It was the weakest under the height of 600 m in the early morning at 02:00, and then increased; in the afternoon it quickly increased under the height of 500 m; until 17:00 it arrived at the highest value, and then decreased. It was opposite above the height of 600 m.

Figure 1 Diurnal variation of virtual potential temperature (a), specific humidity (b), wind direction (c), and wind speed (d)in the boundary layer above the northern Tibetan Plateau on April 15.

3.3. Comparison of the ABL structure during the dry and rainy periods

Table 1 shows the diurnal variation of the ABL height.The height of the convection layer was determined by the cap inversion. The top of the cap inversion near the surface layer determined the height of the stable layer. In the early morning the convection layer was shallow. After sunrise it slowly developed and gradually increased in thickness due to the stable layer covered by the young mixed layer. Before noon the thermal bubble penetrated quickly upwards. When the bubble arrived at the top of the inversion cap layer due to disappear of the stable layer, it encountered the resistance to the lift movement, then its increasing ratio quickly reduced. Therefore,Table 1 shows that the thick ABL was stable in the afternoon;but the thick mixed layer was different from the different weather conditions and meso-scale situation. Table 1 also shows that the largest height of the mixed layer for each day was 2,499 m on April 15; 3,942 m on April 16; 2,755 m on April 17; 2,960 m on April 18; 3,089 m on April 19; 4,430 m on April 20; 2,140 m on April 21; 2,211 m on April 22; 2,616 m on April 23; and 3,919 m on April 24.

Figure 2 Diurnal variation of virtual potential temperature (a), specific humidity (b), wind speed (c), and wind direction (d)in the boundary layer above the northern Tibetan Plateau on August 17.

Table 1 The ABL height during the dry period

Table 2 lists the diurnal variation of the ABL during the rainy period over the northern Tibetan Plateau. In the early morning, the convection layer was shallow, the same as that during the dry period. At sunrise the mixed layer was developing slowly due to the residual stable layer; at night it was covered the top of the young mixed layer and the light heating to the ground. Before noon the thermal bubble penetrated rapidly upwards, and the thick mixed layer quickly increased. Because of weak heating to the ground during the rainy period, the lift speed and height of the thermal bubble was lower than those during the dry period. In the afternoon,when the thermal bubble arrived at the top of the inversion cap and encountered vertical movement resistance, the increasing ratio of the mixed layer decreased. Therefore, Table 2 shows an unchanged thick mixed layer in the afternoon.Table 2 also shows that the largest height of the ABL for each day was 1,309 m on August 14; 1,189 m on August 15;1,383 m on August 16; 2,212 m on August 17; 1,703 m on August 18; 1,006 m on August 19; 1,148 m on August 20;1,354 m on August 21; 1,245 m on August 22; 1,139 m on August 23; 1,057 m on August 25; 1,250 m on August 26;and 1,703 m at 20:00 on August 18 due to the hail.

In general, the height of the ABL was obviously higher during the dry period than during the rainy period. A possible reason was that the heat flux and the convection were strong during the dry period. On the other hand, increased water vapor content resisted the development of the ABL during the rainy period.

Table 2 The atmospheric boundary layer height during the rainy period

3.3.1 Comparison of the structure of the ABL during the dry and rainy periods

In this paper, we selected typical sunny days on April 17 and August 17 during the dry and rainy periods as examples to study the influence of different meteorology elements on the ABL. Figures 3a and 3b show that the specific humidity was lower during the dry period than during the rainy period and lower than 1.5 g/kg on April 17. It was 9.62 g/kg on August 17 due to increased precipitation and high surface temperature during the rainy period (Figures 3i and 3j). It was also correlated with high evaporation of the ground surface. The wind direction was westward during the dry period (Figure 3c), eastward during the rainy period under the height of 2,500 m (Figure 3d), and westward above the height of 2,500 m during these two periods. Figure 3e shows that the wind speed was weaker near the surface layer, but increased with height on April 17. The wind speed was weaker in the ABL on August 17 and its largest value was 15 m/s. The diurnal variation of the virtual potential temperature in the ABL was larger during the dry period than during the rainy period (Figures 3g and 3h).The ABL height was higher during the dry period than during the rainy period due to large diurnal variation of the surface temperature during the dry period (Figure 3i). Figures 3i and 3j shows that the difference of the surface temperature was 40 °C during the dry period, while it was 30 °C during the rainy period.

3.3.2 Comparison of the largest average height of the convection mixed layer

In this paper, we selected the observed radiosonde data at 14:00 every day during the observed period (Figure 4),because the ABL height was the highest and stable in the afternoon. The height of the convection ABL was determined as follows: (1) no-entrainment neutral buoyancy layer;(2) the lowest value of the profile of relative potential temperature; (3) the bottom of the inversion temperature layer of the saturation relative potential temperature (Kumaret al.,2010).

Figure 4 shows that the largest average height of the convection layer was 3,580 m during the dry period and 2,194 m during the rainy period.

Figure 3 Diurnal variation of specific humidity (a, b), wind direction (c, d), wind speed (e, f), potential temperature (g, h), and surface temperature (i, j) above the northern Tibetan Plateau on April 17 and August 17.

Figure 4 The average value of all the daytime sounding (1400BST) for virtual, equivalent and saturated equivalent potential temperature during the dry (a), and rainy (b) periods.

4. Conclusions and discussions

Through analyzing the observed radiosonde data during the PEOP and EOP of the CAMP-Tibet in April and August,2004, we obtained results of the ABL structure over the northern Tibetan Plateau as follows.

(a) The diurnal variation of the virtual potential temperature, specific humidity and other elements were large. The ABL height was from 2,211 m to 4,430 m during the dry period and from 1,006 m to 2,212 m during the rainy period,thus the ABL height was higher during the dry period than during the rainy period. The largest average ABL height was 3,580 m during the dry period and 2,194 m during the rainy period. Possible reasons are large heating to the ground,strong turbulence and the active convection during the dry period, and also increased humidity during the rainy period.

(b) The specific humidity was lower during the dry period than during the rainy period. There was inversion humidity during both periods.

(c) The horizontal wind direction was westward during the dry period in the whole ABL, while it was eastward under the height of 2,500 m and was westward above the height of 2,500 m during the rainy period. The wind speed was low near the surface layer and increased quickly with height during the dry period. It was low in the whole ABL during the rainy period.

In this study, the ABL structure was analyzed and the seasonal difference was interpreted during the dry and rainy period over the northern Tibetan Plateau, but detailed studies still need to be done in the future.

This paper was under the auspices of the Chinese National Key Programme for Developing Basic Sciences(2010CB951703), the Chinese National Key Programme for Developing Basic Sciences (2005CB422003), the National Natural Science Foundation of China (41175008,40810059006 and 40675012) and the JICA Project of "China-Japanese center of the cooperative research on meteorological disaster".

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10.3724/SP.J.1226.2011.00509

*Correspondence to: Dr. MaoShan Li, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science. No. 320, West Donggang Road, Lanzhou, Gansu 730000, China. Email: mshli@lzb.ac.cn

20 May 2011 Accepted: 19 August 2011