Intensive Radiosonde Measurements of Summertime Convection over the Inner Mongolia Grassland in 2014: Difference between Shallow Cumulus and Other Conditions

Cumulus clouds play an important role in the global redistribution of water and energy and in the transport of surface heat, moisture and momentum to the free troposphere. Shallow cumulus (SCu) clouds or fair-weather cumuli are characterized by small size, relatively weak convection, and no precipitation, which are dramatically different from those of cumulus congestus and deep convection clouds (Stull and Eloranta, 1984; Brown et al., 2002; Chandra et al., 2013). SCu convective transport is a key process in the lower atmosphere because it regulates the partitioning of the surface heat and moisture fluxes and the temporal evolution of chemical reactants (Lohou and Patton, 2014). The presence of SCu clouds can cause a substantial change in both the turbulence intensity and the internal structure of the convective boundary layer (Wilde et al., 1985).

Maritime SCu clouds (trade wind cumulus) have been investigated based on large-scale budget results from field experiments, such as the Barbados Oceanographic and Meteorological Experiment and the Atlantic Trade Wind Experiment (Siebesma and Cuijpers, 1995; Brown, 1999; Stevens et al., 2001). It was shown that the sea is characterized by statistically quasi-steady convection, an undisturbed trade wind regime, and a lower convective boundary layer.

Figure 1.  (a) SCu clouds over the IMG as shown by Landsat Thematic Mapper imagery at 1100 LST on 17 July 2014. (b) A picture of SCu clouds overhead at BQ at 1430 LST on 26 July 2014.

The climatology of the macroscale properties of continental SCu clouds, e.g., cloud base height (CBH), cloud top height (CTH), cloud fraction, and cloud-chord lengths (CCLs), was obtained from active remote sensing measurements in the South Great Plain (SGP) of the U.S. The results showed that continental SCu clouds exhibit significant diurnal evolution, and that the CCLs closely fit an exponential distribution (Berg and Kassianov, 2008; Chandra et al., 2013). The large vertical extent of clouds is favored by greater boundary layer inhomogeneity, higher moisture content, and atmospheric instability [convective available potential energy (CAPE)] in summer (Zhang and Klein, 2010).

Simulations of continental SCu using a large-eddy simulation model suggested that the lifting condensation level (LCL) shows significant diurnal variation consistent with that of the CBH (Endo, 2009; Endo et al., 2015). The growth of the boundary layer height (BLH) over land was found to be strongly forced by the surface heat flux (Vogelmann et al., 2015). A smaller surface Bowen ratio (the ratio of sensible to latent heat flux) corresponds to a greater probability of SCu occurrence (Zhu and Albrecht, 2002). The wind shear at the lower troposphere (0-5 km) leads to a quasi-linear structure of clouds that is perpendicular to the shear, which impacts the atmospheric instability and the vertical advection of temperature and moisture (Chen et al., 2015).

SCu clusters can often be observed in summer over the Inner Mongolia Grassland (IMG)——the largest grassland in China. A typical example of SCu is shown in Fig. 1 from Landsat Thematic Mapper imagery on 17 July 2014, and a sky imagery on 26 July 2014, over the IMG. Since SCu often occurs in summer over the IMG, and studies on SCu in this region are very limited, an intensive radiosonde experiment was performed in the summer of 2014. One of the most important objectives of this campaign was to improve our understanding of the macroscopic aspects of SCu clusters over the IMG. Furthermore, environmental parameters associated with different cloud types were compared, to discuss which conditions are favorable for the occurrence of SCu clouds. This is, to the best of our knowledge, the first report of SCu observation over this region.

The paper is organized as follows. Section 2 describes the measurements, data and methodology. Details on the macroscopic properties of the SCu, its thermodynamic structure, and the environmental parameters associated with it, are discussed in section 3. A summary and conclusions are presented in the final section.

The launching of the radiosonde was performed at Baiqi (BQ) (42.23^°N, 114.99^°E, 1362 m ASL), where the vegetation is dominated by grasses. The station is located approximately 300 km northwest of Beijing.

An intensive radiosonde campaign was conducted during 26-30 July 2014. Radiosondes were launched from sunrise to sunset. A total of 48 profiles were obtained during the campaign. The radiosonde was produced by the Changfeng Company, which participated in the Eighth World Meteorological Organization International Radiosonde Comparison at Yangjiang in 2010 and showed good performance (Li and Li, 2011). The radiosonde measured air temperature (T), relative humidity (RH), pressure, wind speed, and wind direction, during the ascending and descending modes.

ERA-Interim data were used to analyze the synoptic system during the campaign. ERA-Interim is the latest global atmospheric reanalysis dataset available from the European Centre for Medium-Range Weather Forecasts (Berrisford et al., 2011). Gridded data products include a large variety of three-hourly surface parameters and six-hourly upper-air parameters covering the troposphere and stratosphere. Surface sensible heat flux (SH), surface latent heat flux (LH), specific humidity (q), pressure, the zonal (u) and meridional (v) components of wind, and CAPE data, over the IMG, were used.

The vertical profile of cloud occurrence was derived from radiosonde-based RH measurements following the algorithm of Zhang et al. (2010, 2016), which was modified from the method of (Wang and Rossow, 1995). The rationale of this algorithm is that the occurrence of cloud requires that the RH exceeds a specified threshold. The CBH and CTH of a single cloud layer were studied here. The simple parcel method initialized by Holzworth (1964, 1967) was used to determine the BLH, which is based on the gradient of virtual potential temperature (θ) and the RH difference from the sounding measurements. The Bowen ratio (β) is calculated from the θ and q as follows: \begin{equation} \beta=\dfrac{c_p\Delta\bar{\theta}}{L_{\rm v}\Delta\bar{q}} , (1)\end{equation} where c_p is the specific heat at constant pressure (units: J kg-1 K-1); L _v is the latent heat of vaporization (units: J kg-1); \(\Delta \bar{\theta}\) is the difference of potential temperature at different heights (units: K); and \(\Delta\bar{q}\) is the difference of q at different heights (units: g kg^-1).

During the campaign, BQ experienced variable weather conditions and multiple cloud types occurred, which provided an opportunity to discuss the macroscopic properties of SCu clouds and study the differences in environmental parameters associated with each cloud type.

Case 1: SCu on 26 July 2014. BQ was under stable weather conditions, with weak winds at the 500 hPa level (Fig. 2a). The occurrence and maintenance of SCu clouds spanned from about 1400 to 1800 local standard time (LST; UTC+8). Thirteen radiosonde profiles were obtained. The first and last soundings were launched at 1000 LST and 1700 LST, respectively, which almost covered the lifetime of the SCu.

Figure 2.  Synoptic system analysis of ERA-Interim data at 500 hPa at 1400 LST for (a) Case 1 to (d) Case 4 [Arrows represent wind direction; maximum value of winds: 34 m s^-1; black lines are geopotential height (units: gpm); the color shading represent temperature (units: ^°C); red diamond is the location of BQ].

Case 2: Cumulus congestus and cumulonimbus (Cb) clouds on 28 July 2014. BQ was located at the eastern edge of low pressure at the 500 hPa level (Fig. 2b). The cumulus congestus clouds occurred at 1340 LST and transformed into Cb clouds at 1700 LST. Nine profiles were launched from 0900 LST to 1700 LST.

Case 3: Cb clouds and rain on 29 July 2014. The synoptic system was similar to that in Case 2, but the trough was stronger than that in Case 2 (Fig. 2c). It rained throughout the daytime until 1930 LST. Thirteen radiosondes were launched. The first and last soundings were launched at 1100 LST and 2100 LST.

Case 4: A clear day on 30 July 2014. BQ was located east of a high-pressure system with strong winds, which led to a sunny day after the low-pressure system moved systematically eastwards (Fig. 2d). Thirteen radiosondes were launched in total. The first and last soundings were launched at 0700 LST and 2200 LST.

We showed the average values and standard deviations of CBH, CTH, depth, temperature at cloud base (T _base), top (T _top) and cloud layer (T), and the 0^°C isotherm height (H₀) on the SCu day. The depth was 1.5 0.7 km. The CTH and CBH values were 5.0 0.1 km and 3.4 0.7 km, respectively. The CTH and CBH values were much larger than those over the sea (CBH of 0.5 km and CTH of 1.5 km) (Siebesma and Cuijpers, 1995), but were relatively close to that of the SGP (CBH of 2 km and CTH of 3 km) (Chandra et al., 2013). The T _base and T _top values were -1.6^°C 5.4^°C and -12.4^°C 0.6^°C, respectively. The temperature of the SCu layer of these examples was below freezing (-6.6^°C 4.1^°C). The H₀ value was 4.5 0.1 km. It was difficult to distinguish the particle phase here because, as Del Genio et al. (1996, 2005) pointed out, the liquid or ice phase is permitted to exist at temperatures below freezing. Excessive ice phase growth rates might exists at different altitudes, where the temperature varies in the range -15^°C <T<-10^°C because of maximum differences between saturated water pressure water and ice (Sednev et al., 2009).

Figure 3.  Radiosonde data analysis from 1000 LST to 1700 LST on 26 July 2014: the temporal variation of RH [filled in (a)], T [filled in (b)], wind shear, LCL (red circles) and BLH (red crosses), respectively. Black lines show the base and top boundary of SCu cloud layers.

Figure 3 shows the temporal variation of intensive radiosonde observations during 1000-1700 LST on 26 July 2014. The SCu cloud layers were observed to range from a fuzzy lower boundary to an unambiguous upper boundary of 5 km after 1400 LST. The cloud layers appeared to increase as the BLHs exceeded the LCLs and were always located above the top of the boundary layer. Wind shear (average value of 2.4 m s^-1) was observed at the interface between the free air atmosphere and the upper boundary of the cloud layer.

Figures 4 and 5 show the changes of q, θ and T from 1000 LST (absence of SCu) to 1400 LST (presence of SCu). The q and θ increased by 1 g kg^-1 and 3.9^°C from the surface to the layer below the CTH (5 km) after the SCu formation, respectively. A temperature inversion (∼ 2.9^°C) occurred at approximately 5 km at 1400 LST, likely because of evaporative cooling as a result of the entrainment of cloudy and clear air. The thermodynamic structures above 5 km were similar, with approximate constants [∼0.23^°C (100 m)^-1 and ∼0.28^°C (100 m)^-1], which was the mean potential temperature lapse rate above 5 km. The profiles of both q and θ above 5 km did not show substantial changes from 1000 LST to 1400 LST. The q below 5 km increased by 0.25 g kg^-1 h^-1 from 1000 LST to 1400 LST. The horizontal advection of q estimated from ERA-Interim q and wind fields within the layer from the surface to 5 km, was about -0.08 g kg^-1 h^-1 (Fig. 5). The results indicate that the horizontal advection of q was substantially smaller than the vertical transport of q from the surface, which implied that the latter was dominant.

Figure 4.  Profiles of (a) q (units: g kg^-1), (b) θ (units: ^°C), and (c) T (units: ^°C), for Case 1 at 1000 LST (blue lines) and at 1400 LST (red lines). The black dashed lines represent the CBH and CTH at 1400 LST.

Figure 5.  Advection rate of the q (units: g kg^-1 h^-1) averaged from the surface to 5 km at 1400 LST over BQ from the ERA-interim data. The red diamond is the location of BQ.

Considering that SCu clouds often occur in the late morning and afternoon due to solar radiation heating, the environmental parameters (surface β, SH, LH, CAPE, BLH, LCL and wind shear) at 1400 LST for the four cases are discussed.

There were large differences in β values among the four cases (Table 1). The β in Case 1 was 1.173, derived from the radiosonde data. The β in Case 2 (1.392) was relatively close to that in Case 1. However, the values in Case 3 and Case 4 were substantially lower, at 0.288 and 0.635, respectively, which was likely due to the precipitation on 29 July (Case 3). Precipitation led to an increase in surface moisture (likely larger LH) and thereby decreased β. Despite it having been a clear day in Case 4, β was still low as a result of surface wetness and high LH (495.8 W m^-2), since precipitation occurred only one day before.

Figure 6.  The diurnal variation of the BLH and LCL for (a) Case 1 to (d) Case 4. The blue and red lines are the BLH and the LCL from the sounding data, respectively.

The development of the boundary layer in Case 1 and Case 2 was more active than that in Case 3 and Case 4 (Table 1). The BLH exceeded 3 km for the former; however, it was lower than 2 km for the latter. A higher β value indicates stronger surface forcing and the feedback of more intensive turbulence and higher BLHs. High surface moisture content depressed the vertical development of the boundary layer and reduced the possibility of SCu formation. In other words, the occurrence of SCu clouds would not have increased during periods when there was a low β value and high soil moisture near the surface.

The CAPE values from ERA-interim data were approximately 142.5, 546.4, 1022 and 3.8 J kg^-1 from Case 1 to Case 4 (Table 1). The CAPE was similar to the results given by (Zhang et al., 2010), who reported a CAPE value of 700 J kg^-1 on deep convection days, but a value of 200 J kg^-1 on SCu days, at midday. A high CAPE (>600 J kg^-1) implied a stronger vertical development of clouds; whereas, a weak atmospheric instability characterized by low CAPE (150-200 J kg^-1) likely prevented shallow clouds from developing vertically, and thereby favored the maintenance of SCu.

The temporal variation of the BLHs and LCLs from Case 1 to Case 4, based on radiosonde measurements, is shown in Fig. 6. In Case 1, the BLH was lower than the LCL in the absence of SCu; however, it exceeded the LCL after 1400 LST, in the presence of SCu. (Wilde et al., 1985) reported that cumulus clouds form when the top of the entrainment zone reaches the LCL. In Case 2, the BLH also exceeded the LCL after 1400 LST, when the Cb clouds occurred. However, the BLH was below the LCL most of the time in Case 4 (clear sky). In general, the SCu clouds formed under the condition that the top of the BLH reached the LCL. If perturbations of the thermals and plumes could not reach the LCL, they would have dissipated by turbulent motion in the boundary layer (Stull and Eloranta, 1984)

The time series of wind shear for the four cases, from the radiosonde measurements, are shown in Fig. 7. The average wind speed of the entire layer (from the surface to 7 km) was ∼4.8, ∼8.2, ∼12.8 and ∼8.9 m s^-1, from Case 1 to Case 4. In Case 1, easterly wind was observed above 5 km, but westerly wind occurred below, indicating a weak wind shear since wind speed was relatively lower. In Case 2, the wind direction above 4 km changed to westerly, as an upstream low pressure system was approaching. Meanwhile, the vertical development of the cloud body in Case 2 was more pronounced compared with the SCu clouds in Case 1. In general, strong wind shear gave rise to strong advection of momentum, moisture and temperature, which resulted in greater atmospheric instability (Holton, 1992). Precipitation occurred in Case 3 under the control of a low pressure trough accompanied by strong updraft motion. In Case 4, there was an apparent subsidence of air flow from the high pressure system that resulted in low RH above 3 km. Although the wind speed in Case 4 was similar to that in Case 2, it was unfavorable for cloud formation due to the strong downdraft motion from the high pressure system.

Figure 7.  Time series of wind shear for (a) Case 1 to (d) Case 4, with RH (filled areas, units: %) from the sounding data (SCu day, Cb day, rain day and clear-sky day). Maximum wind speed is approximately 25 m s^-1. The time series are not uniform.

An intensive radiosonde experiment over the IMG was conducted from 26 July to 30 July 2014. During the experiment, a total of 48 profiles were obtained. This study focused on the macroscopic properties and thermodynamic structures of SCu clouds, and discussed potential differences in the environmental parameters associated with different cloud types.

The SCu CBH of 3.4 km and CTH of 5 km over the IMG were far in excess of the CBHs and CTHs over the sea, but were relatively close to those at the SGP site. The result showed that the main body of the SCu was below freezing.

There was an evident change in the temperature and humidity profiles before and after the SCu formation. The q and θ increased by ∼1 g kg^-1 and ∼3.9^°C below 5 km from 1000 LST to 1400 LST, respectively. Above 5 km, the thermodynamic structure remained stable. The q above 5 km was ∼10^-1 g kg^-1 and the mean θ lapse rate was an approximate constant [∼0.23^°C-0.28^°C (100 m)^-1]. In addition, there was a temperature inversion and an obvious wind shear at approximately 5 km after the formation of SCu clouds. The local change rate of q mainly accounted for the vertical transports of q from the surface with weak horizontal advection.

Compared with the cumulus congestus, rain and clear-sky days, the results showed that the formation of SCu was characterized by surface forcing, with a high β and low soil moisture content (low LH) near the surface. SCu appeared if the top of the BLH exceeded the LCL. The maintenance of SCu clouds was in favor of a lower CAPE, which prevented the dramatic vertical development of shallow clouds. Note that substantial CAPE and strong wind shear resulted in Cb clouds and rain. Additionally, the weak wind shear and subsidence of the synoptic system was favorable to the maintenance of SCu clouds and inhibited the vertical development from SCu to Cb clouds.

We should emphasize again that this was the first attempt to observe SCu over the IMG. Although some interesting features associated with SCu have been revealed by the analysis of the intensive radiosonde measurements, we should keep it in mind that this is a preliminary study, since the results were mainly based on one case of SCu. Further studies of SCu in this key region are still required. First, many more radiosonde measurements of SCu are needed. Second, novel methods are urgently required to study SCu based on satellite data. Third, a combination of radiosonde and satellite measurements, as well as model simulations, would shed new light on the formation and maintenance of SCu. And forth, considerable attention should also be paid to the effects of SCu on the energy budget.