1. Introduction
The arid and semiarid regions in China often suffer from dust storms. A dust storm is characterized by a visibility of less than 1 km. The strongest dust storms are termed “black storms” in China, which have a visibility of less than 50 m and a wind speed greater than 25 m s−1. Although the frequency of dust storms is 20–30 events in a year (Parungo et al. 1994), black storms occur only once or twice in a 10-yr period.
On 5 May 1993, a black storm occurred over the arid region in northwest China. The storm traveled as far as 500 km, making cities as dark as night, and killed 49 people. Mitsuta et al. (1995b) proposed that this storm might be induced by a series of downbursts emanating from a rapidly propagating, long-lived squall line. They described the dust storm and its related meteorological changes. In the present paper, a further investigation of the structure and evolution of this severe squall line is conducted in order to show the characteristics of a squall line that occurs in an extremely dry environment.
The Sino-Japanese cooperative research program, called the Heife River Field Experiment (HEIFE), was carried out from 1989 to 1993 in the arid area of northwest China (see Fig. 1). The main purpose of this project was to study the atmosphere–land surface interaction processes (Yamamoto and Guo 1993). Gamo (1996) showed that a mixed layer develops up to 4 km above the ground level (AGL), and a near-neutral vertical gradient of potential temperature is typically found within the lower layer of the troposphere over the HEIFE area during the summer season. Itano (1997) pointed out that without the forcing of a synoptic-scale disturbance, deep cumulus convection does not develop owing to a very high level of free convection (LFC) associated with a dry atmospheric boundary layer (ABL). Itano also suggested that most of the rainfall evaporates in the dry, deep ABL during the early stage of the rain event, never reaching the desert surface. Thus, rainfall over the desert is as small as 100 mm yr−1, whereas rainfall over the higher-elevated stations located on the surrounding mountains, that is, about 3000 m above mean sea level (ASL), is around 600 mm yr−1. In this way, the dry, deep ABL is characteristic of the precipitation in this dry environment.
The evaporation of falling precipitation has a significant effect on the squall line dynamics through producing a surface cold-air pool (Sawyer 1946; Braham 1952; Krumm 1954; Kamburova and Ludlam 1966). Under dry conditions, it is expected that rainfall evaporation occurs readily beneath a convective cloud, leading to the rapid development of a surface cold pool. Some mechanism to overcome the very high LFC conditions that are unfavorable for the development of deep convection is required to sustain the deep convection at the leading edge of the cold pool.
The structure and evolution of midlatitude squall lines have been described by Newton (1950), Newton and Newton (1959), and others. Some of these squall lines over the United States cause a severe, widespread windstorm called derecho (Johns and Hirt 1987). These studies have been based on observations over the United States. Few case studies, however, have been conducted on the squall line over an extremely dry region, except for those over the semiarid, High Plains of the United States (e.g., Schmidt and Cotton 1989; Fankhauser et al. 1992).
The objective of the present study is to examine the structure and evolution of the long-lived squall line causing a severe dust storm over an arid region, using data from routine observations at stations in China and those acquired from the special surface stations during HEIFE. In section 2, a summary of the preliminary study on this squall line by Mitsuta et al. (1995b) is reviewed. In the subsequent sections, the structure of the present squall line is described. The similarities of the present dust storm with derechos are discussed in the last part of this paper. The local time in the HEIFE area is UTC plus almost 6.5 h.
2. A review of the 5 May 1993 squall line
Figure 2 shows the arid region in northwest China. The elevation of the base terrain of the region studied is 1000–1500 m ASL. The dust storms and black storms were observed over this area as indicated in Fig. 2. A picture of a dust storm can be found in Fig. 2 of Mitsuta et al. (1995b), which is similar to the gust fronts presented by Simpson (1969) and Idso et al. (1972).
Figure 3 shows Geostationary Meteorological Satellite (GMS-4) IR images at 0930 (Fig. 3a) and 1130 UTC (Fig. 3b). A wide, banded cloud system can be seen in Fig. 3. A surface cold front is identified as a sharp gradient in temperature in the 850-hPa contour chart, at nearly the same elevation as the ground level of the region, at 1200 UTC (Fig. 4). This cold front corresponds to the leading edge of the cloud band as shown in Fig. 3. In the IR imagery, the squall line was first identified at 0530 UTC (not shown) and began to evolve into a linearly organized cloud system ahead of this cold frontal cloud band. At 0930 UTC (Fig. 3a), a well-defined squall line is seen and evolves to a length of about 270 km and a width of about 100 km. The system moved from 304° with an average speed of about 19 m s−1, estimated from the motion of the leading edge of the squall line in IR images. The squall line can be traced in the IR images for over 6 h; after that, it changed in appearance to a large cloud cluster. Unfortunately, no radar data were available, so that the detailed internal structure of the squall line cannot be shown.
Figure 5 shows the vertical structure of the pre–squall line environment at Minqin [(MQN) see Fig. 2] on the morning (0000 UTC) of 5 May 1993. Thermodynamic variables are calculated following the formula given by Bolton (1980). An absolute stable stratification existed near the surface, which was thought to be due to nocturnal radiative cooling. This inversion layer developed up to the 831-hPa level. Above this stable layer, a convectively unstable layer (∂θe/∂z < 0) formed (Fig. 5b), which suggests the potential for induced strong convection. This morning profile shows that the convective available potential energy (CAPE), evaluated by pseudoadiabatically raising a parcel from 831 hPa (top of the surface inversion) to 285 hPa (neutral buoyancy level), is approximately 450 m2 s−2. This value is not so large as those found in severe spring squall lines over Oklahoma (Bluestein and Jain 1985). In the afternoon, however, CAPE estimated by Mitsuta et al. (1995b) is 1340 m2 s−2. This CAPE value was estimated by raising a surface air parcel having the maximum equivalent potential temperature in the daytime to its intersection with the saturated equivalent potential temperature profile in the morning. The large CAPE in the afternoon reflects the destabilization of the surface layer due to strong daytime surface heating. This destabilization also leads to the development of a mixed layer.
The presence of vertical shear of horizontal winds is important to the organization of a squall line (Rotunno et al. 1988; Weisman et al. 1988). From Fig. 5c, the vertical shear between the surface and the 400-hPa level (≈6 km AGL) at MQN was 4.7 × 10−3 s−1. At Jiuquan and Zhangye (see Fig. 2), the values were 4.9 × 10−3 s−1 and 4.9 × 10−3 s−1, respectively. These vertical shears are a little larger than those of the severe squall lines investigated by Bluestein and Jain (1985). However, shears in low levels are relatively small. For example, a low-level shear between the surface and the 700-hPa level (about 1.7 km AGL) at MQN was 1.7 × 10−3 s−1. A weak low-level shear will be mentioned again in section 3a.
Surface observations by the automatic weather stations (AWS) of the HEIFE network clearly marked the passage of the squall line, as shown in Fig. 6 for MQN. Before the arrival of the squall line gust front at MQN, the surface water vapor mixing ratio qυ was as low as 2.3 g kg−1, which corresponds to a relative humidity of less than 10%. With the arrival of the gust front at about 0840 UTC, the temperature decreased as the pressure began to rise, while qυ increased from 2.3 to 4.2 g kg−1. The increase in qυ can result from the evaporation of falling precipitation. The wind (sustained wind) component u, parallel to the squall line motion, sharply increased to be more than 20 m s−1. The strong wind speed lasted for more than 2 h. During the 2 h after the passage of the gust front, four major peaks in the u component are found. Similar wind speed changes were also observed at the Linze [(LNZ) see Fig. 2] AWS (not shown). Furthermore, at Zhangye [(ZGY) see Fig. 2], located near LNZ, the peak gust reached 37.9 m s−1.
In the present case, almost no rain was observed at the ground. Between 0600 and 1200 UTC, several stations observed the 6-h precipitation as being less than 0.1 mm. Between 1200 and 1800 UTC, five stations observed precipitation of more than 0.1 mm, but at most 3 mm. Instead, qυ increased after the passage of the gust front. These facts sharply contrast with those found in rain storms that typically develop over humid regions. The increase in qυ has also been reported for other dust storms over arid regions of the world (Farquharson 1937; Idso et al. 1972) and appears to be a common feature of dust storms in arid regions.
3. Squall line structure
a. Vertical cross section
In order to clarify the vertical structure of the squall line, vertical cross sections along the line A–B shown in Fig. 2 were constructed using twice-daily aerological soundings at six stations.
The vertical cross section of the prestorm environment at 0000 UTC is shown in Fig. 7. A region of low θe (equivalent potential temperature) is seen near the ground, demonstrating that the inversion layer extended horizontally over the region. Above this layer, a neutral-to-weakly unstable layer is seen around MQN.
The vertical cross section of the squall line at 1200 UTC is shown in Fig. 8. There is a significant horizontal θe gradient to the west of Yinchuan (YCN), recognized as the gust front of the squall line. A significant vertical θe gradient is also found at around 3.5 km ASL (2 km AGL) around MQN. Ahead of the gust front, a convectively unstable layer is found at low levels (CAPE ≈ 530 m2 s−2, LFC ≈ 4.4 km AGL at YCN). The prestorm YCN sounding shows that the vertical gradient of virtual potential temperature (θυ) in lower levels (850–500 hPa) is about 4 K (4.3 km)−1. This small θυ gradient value shows the presence of a deep mixed layer. The depth of a mixed layer during the daytime and its role in the squall line evolution will be described in the subsequent sections. Behind the gust front, a well-defined cold pool is seen, which at its coldest values is colder than the environment. The propagation speed of the cold surface outflow can be estimated from the pressure change at MQN, using the formula discussed in Seitter and Muench (1985) or Mahoney (1988), to be about 20 m s−1. This value is almost the same as that estimated from the satellite images. In Fig. 8, a weak horizontal θe gradient is seen at around 800 km behind the gust front. This θe gradient is a synoptic-scale cold front as shown in the surface analysis (Fig. 4).
The vertical cross section of the present squall line generally looks similar to those found over the United States (Newton 1950; Newton and Newton 1959; Ogura and Liou 1980). It should be noted that the relative humidity in the cold pool was less than the high relative humidity region of the cloud area (Fig. 8). This fact, as well as the near-zero observed rainfall, indicates that the lower atmospheric layer over the desert region was so dry that it could not reach saturation, even if all the precipitation evaporated. After the subcloud layer becomes moistened throughout its depth due to the evaporation, rain will reach the ground. At ZGY far behind the gust front at 1200 UTC, the relative humidity is relatively higher than that within the cold pool over the other stations (Fig. 8). In this high relative humidity condition (more than 50%), ZGY observed a little rainfall of less than 0.1 mm.
The vertical wind profile at YCN in Fig. 8 shows that at low levels (surface to 3 km ASL) the horizontal winds are very weak and have no substantial vertical shear in the direction of the squall line motion. These low-level winds are a strong inflow to the gust front in the system-relative sense. In the mid- to upper troposphere, however, strong shear exists. The shear magnitude between 700 and 300 hPa was 5.6 × 10−3 s−1. A strong midlevel and weak low-level shear was also found in the MQN morning wind profile as mentioned in section 2. This indicates that the present squall line developed in an environment of the strong midlevel and weak low-level shear. Rotunno et al. (1988), however, stressed that low-level ambient shear interacting with a surface cold pool is important for the maintenance of strong, long-lived squall lines. Thus, the present case suggests another maintenance mechanism of squall lines in addition to Rotunno et al.’s theory.
b. Surface convergence
The divergence fields of the surface winds at 0600, 0900, and 1200 UTC on 5 May are shown in Fig. 9. At 0600 UTC, no significant convergence related to the squall line is seen. However, at 0900 UTC, a large magnitude of convergence (greater than 10−4 s−1) is found around 39°N, 104°E. This region extends a few hundred kilometers, from the southwest to northeast, along the direction parallel to the squall line. Even at 1200 UTC, a significant convergence zone translated southeastward from 0900 UTC is found. The wide areas of large convergence are related to the large horizontal extent of the squall line.
4. Prestorm dry boundary layer
In section 3a, the presence of a deep mixed layer (ML) in the prestorm environment is suggested based on the 1200 UTC sounding at YCN. In this section, the evolution of the ambient ABL ahead of the squall line during the daytime is described.
As mentioned in section 1, a deep ML is typically found over the desert region of China in the summer season. The growth of such a deep ML can be accomplished by a strong surface sensible heat flux during warmer seasons, particularly in May and June (Mitsuta et al. 1995a). A strong sensible heat flux resulted from high ground temperature Tg. In the present case, Tg increased to about 50°C at MQN, while the surface air temperature increased to about 27°C in the afternoon, leading to a strong sensible heat flux. Thus, a deep ML was expected to develop during the daytime. In this section, the maximum mixed-layer depth (MMD) and LFC in the prestorm environment are estimated to show the role of the ML in the squall line evolution. The estimates are based on a simple assumption that the growth of the ML arises from surface heating only, and the potential temperature within the ML is constant with height, which is called encroachment growth (Garratt 1992).
The MMD can be estimated by lifting the maximum surface virtual potential temperature θυ to its intersection with the morning θυ profile. This method is equivalent to that discussed in Holtzworth (1964). The LFC is estimated by lifting the maximum surface equivalent potential temperature θe to its intersection with the morning saturated θe profile. This procedure is schematically illustrated in Fig. 10 for the MQN sounding. The estimations are made from profiles at ZGY and MQN. Since there were no continuous observations at ZGY, data from the LNZ AWS, located near ZGY, are used to estimate the MMD and LFC at ZGY. The maximum values of θυ and θe are defined as the means over a certain period during the daytime. This averaging may remove extreme values in surface superadiabatic layer. The averaging period is 0300–0500 UTC for the LNZ AWS, and 0500–0800 UTC for the MQN AWS.
Table 1 lists the estimated values of MMD and LFC. The MMD at ZGY is lower than that at MQN, probably because the passage of the squall line over ZGY occurred early enough in the afternoon to prevent the ML from obtaining a sufficient amount of sensible heat. The MMD at MQN is estimated as greater than 4 km. Even though the estimated LFC for a surface air parcel at MQN is very high, the growth of a deep ML contributes to decreasing the difference in height between the MMD and LFC.
It should be noted that the vertical saturated θe gradient at MQN (Fig. 10) is nearly zero between 4 and 9 km ASL. Thus, the LFC is sensitive to the choice of the surface maximum θe. If the vertical mixing of θe in the ML is taken into consideration, a “mixed-layer mean” θe should be used rather than the surface value. The mixed-layer mean θe can be defined as the density-weighted average of θe within the ML. In computing the mean θe, the vertical profile of qυ should be specified. Since there was no observation of qυ in the ML during the daytime, it is assumed that qυ decreases linearly with height from the surface value averaged over 0500 and 0800 UTC (2.55 g kg−1) to the morning value at about 4 km AGL (1.72 g kg−1). This assumption is plausible since a decrease of moisture with height is frequently observed over the semiarid regions (Mahrt 1976) and also over the desert regions in China. With this assumption, the values of mixed-layer mean θe = 322.0 K, and LFC = 6590 m AGL are then obtained. This value of LFC is 2 km higher than the estimated MMD. It should be noted that the moisture in the upper part of the ML during the daytime might be higher than the morning value due to vertical mixing. Thus, the mixed-layer mean θe could be higher than the above value of 322 K and, hence, the LFC may not reach as high as 6.5 km AGL. Although this computed LFC is sensitive to the choice of θe, a deep ML decreases the difference in height between the MMD and LFC. The role of the deep ML in the squall line evolution will be discussed in the next section.
The presence of a deep ML can also be confirmed by aerological sounding data. A mean vertical profile of θυ was constructed using the data of the evening soundings (1200 UTC) at ZGY on fair weather days in May 1991. These days were selected when the following surface observation conditions were met: 1) the maximum air temperature >25°C, 2) the maximum insolation >700 W m−2, and 3) the maximum ground temperature >40°C. Twelve days were found to meet the criteria (the MQN surface data on 5 May 1993 also satisfied these conditions). Figure 11 shows the mean vertical profile of θυ that has been averaged over the selected days. The mean ML depth was determined as 3206 m with a standard deviation of 749 m. Since these soundings were taken in the early evening, the maximum ML depths are expected to be much higher than those shown in Fig. 11.
5. Discussion
In sections 3a and 4, the presence of the deep ML was described. The role of this deep ML in the squall line evolution is discussed here. The growth of the ML height decreases the difference in height between the ML top and LFC, minimizing the energy required to lift the ML air above the LFC. In addition to the growth of the deep ML, a mechanical forcing is needed to lift the ML air over the LFC to develop deep convection. In the present case, this lifting may be induced by the cold surface outflow. With this forcing, the entire ML can be lifted by the depth of the cold outflow. Therefore, part of the ML reaches the LFC, leading to the development of deep convection. The wide area of the surface convergence as calculated in section 3b indicates that this lifting occurs in a large area. In this way, the role of a deep ML is considered to be a characteristic of squall line development in an arid environment.
A deep ML is also found over the central United States. Wakimoto (1985) investigated the prestorm environment on days of active dry downbursts in Colorado and found that a deep dry-adiabatic layer develops up to near the 500-hPa level (about 3.5 km AGL). Within this dry-adiabatic layer, the evaporation of precipitation falling below a cloud base contributes to the downdraft intensification. In the present squall line, the dry, deep ML can also be favorable for the evaporation of falling precipitation, which in turn contributes to strong downdrafts and the production of a very cool cold-air pool.
From Fig. 6, several major peaks in the u component at the MQN AWS are seen during about 2 h after the passage of the gust front. Furthermore, there were also several reports of gusty winds over 30 m s−1 (section 2; Mitsuta et al. 1995b). In addition to such strong winds, the wind damage area extends with a major axis length of more than 500 km (Fig. 2). Taking into account the gusty wind reports and the wide area of the damaged region, the present storm has some similarities with a derecho, a widespread convectively induced windstorm that occurs over the continental United States during the late spring and summer. This windstorm emanates from a mesoscale convective system (MCS), causing multiple wind damages (Johns and Hirt 1987).
The multiple structure in severe surface winds associated with an MCS has already been reported by Fujita and Wakimoto (1981). They defined a family of downburst clusters having a horizontal dimension greater than 400 km. Each downburst cluster, with scales of about 100 km, consisted of two or more downbursts with scales of about 10 km. Derecho is a family of downburst clusters produced by the MCS (Johns and Hirt 1987). Through the above studies, such derecho-spawning MCSs can include multiple downbursts. It seems that the present storm in China exhibits the same features as an MCS producing multiple strong downdrafts.
6. Conclusions
The long-lived squall line that occurred over the arid region in northwest China on 5 May 1993, which resulted in an exceptionally severe dust storm, called a black storm in China, has been investigated. Extremely dry conditions characterize the structure and evolution of the squall line.
A deep mixed layer develops to a depth of more than 3 km, at times up to 4 km, over the arid region during May, as well as in the whole summer season. In the present case, such a deep, dry atmospheric boundary layer (ABL) plays an important role in the squall line evolution. As the ABL deepens, the difference in height between the ABL and the level of free convection (LFC) becomes small, thus decreasing the amount of lift necessary for deep convection to occur. Cold-air outflow can provide this lifting at its leading edge through the surface convergence between the prestorm air. Due to the small amount of moisture and no supply of sufficient moisture, the destabilization of prestorm low-level stratification by the increase in moisture cannot be expected. Thus, the deepening ABL can be a major cause of sustaining deep convection at the leading edge of the cold pool. The dry mixed layer is also favorable for the evaporation of falling precipitation associated with strong convection, thus enhancing the surface cold pool. The dry conditions make the present convective system so severe, leading to the widespread black storm over the desert region in northwest China.
Although only routine observational data together with the HEIFE data were presently used, the basic structure of the squall line causing the historic dust storm over the arid region in China was described. This type of squall line in a dry environment had not previously been documented. The present storm has some similarities with a derecho, a convectively induced severe windstorm over the continental United States.
Acknowledgments
The author would like to thank Prof. Y. Mitsuta for his valuable comments and continuous encouragement. The comments and suggestions by Drs. Y. Sasaki and Y. Ogura are greatly acknowledged. Three anonymous reviewers are also acknowledged for improving the original manuscript. The author is indebted to the Lanzhou Institute of Plateau Atmospheric Physics and the Gansu Provincial Meteorological Bureau for providing observational data used in this work. The elevation data were provided by the Data Support Section, Scientific Computing Division at the National Center for Atmospheric Research. This work was supported by a research fellowship from the Japan Society for the Promotion of Science for Young Scientists.
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Map of China and the HEIFE area. The hatched region denotes ground elevations of more than 3000 m above mean sea level.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Map of the arid region in northwest China. The stippled region represents elevations between 2000 and 3000 m, and the hatched region those higher than 3000 m. Routine aerological observation stations are denoted by crosses. The stations that observed dust storms are indicated by thin open circles, and the stations that observed black storms by bold open circles. The HEIFE AWS are located in Linze (1392 m ASL) and Minqin (1368 m ASL). The dashed lines labeled with times (UTC) represent the three-hourly position of the leading edge of the squall line. The rectangular box denotes the HEIFE area. The line A–B represents the orientation of the cross sections shown in Figs. 7 and 8.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
GMS-4 IR images over China at (a) 0930 and (b) 1130 UTC. The squall line and the banded cloud system are indicated by arrows.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
The 850-hPa level weather chart at 1200 UTC, analyzed by the Japan Meteorological Agency. The geopotential heights are contoured by solid lines (every 60 m) and the temperatures by dashed lines (every 6°C). The barbed line indicates the surface cold front.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Thermodynamic diagrams and wind hodograph at Minqin at 0000 UTC 5 May 1993. (a) Temperature and dewpoint profiles are represented by the bold solid line. The parcel path used to estimate CAPE is shown by a bold dotted line. The dry (θ = 310 K) and moist (θe = 343.5 K) adiabats, and constant mixing ratio (qυ = 5 g kg−1) are indicated by the thin labeled dotted lines. (b) The vertical profile of equivalent potential temperature θe. (c) Wind hodograph. The pressure levels (hPa) are labeled along the line.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Time series of surface variables observed at Minqin AWS during the squall line passage. Wind components u and υ indicate parallel and normal to the squall line motion (from 304°), respectively. The arrival of the gust front is at about 0840 UTC.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Vertical cross section of the prestorm environment at 0000 UTC, oriented along the line A–B shown in Fig. 2. Heights are indicated in km ASL. The equivalent potential temperature contour interval is 5 K up to 350 K, while the dotted lines indicate an auxiliary contour every 2.5 K. The stippled regions and the hatched shadings denote relative humidity of 50%–60% and of greater than 60%, respectively. The cross hatching along the bottom represents the ground. Vectors represent winds in the vertical plane parallel to the squall line motion. The vector lengths are scaled by speed; the single vector at the upper right of the figure represents 30 m s−1. The abbreviations of the observational stations are DHG (Dunhuang), JQN (Jiuquan), ZGY (Zhangye), MQN (Minqin), YCN (Yinchuan), and YAN (Yanan).
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Same as in Fig. 7 except for 1200 UTC, and the equivalent potential temperature is contoured up to 360 K. The horizontal scale displayed below the figure indicates the position of the gust front.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
The surface convergence/divergence field at 0600, 0900, and 1200 UTC. Values are contoured at 0.5 × 10−4 s−1 with negative (convergence) values indicated by dashed lines and positive (divergence) by solid lines. The hatched region denotes convergence >10−4 s−1. The surface wind is represented by a vector whose scale is given at the upper-left corner of each panel. The wind-observing stations are located at the roots of the vectors. Stippled regions denote elevations higher than 2000 m ASL.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Diagram illustrating the method used to determine the height of the mixed layer and level of free convection (m ASL). The solid line indicates the virtual potential temperature and dashed line the saturated equivalent potential temperature at 0000 UTC from the MQN sounding. Surface parcels are indicated by the small dots.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Mean vertical profile of the virtual potential temperature at ZGY at 1200 UTC on fine days during May 1991.
Citation: Monthly Weather Review 127, 6; 10.1175/1520-0493(1999)127<1301:SAEOAS>2.0.CO;2
Mean surface maximum of θυ and θe, estimated maximum mixed-layer depth, and level of free convection at Zhangye and Minqin.
