1. Introduction
Moist convection affects heat and water exchange, cloud, and precipitation, as well as their distributions and even the global energy budget (Arakawa 2004; Arakawa and Schubert 1974; T. Zhang et al. 2021). Previous studies have summarized the triggering mechanisms of moist convection, from large-scale circulations to mesoscale and local systems, for example, monsoons, low-level jets, sea/lake-breeze fronts, edges of cold pools, gravity waves, and cyclones (Bai et al. 2019, 2020; Chen and Tomassini 2015; Fu et al. 2021; Haghi et al. 2017; Parsons et al. 2019; Reif and Bluestein 2017). As the ascending branches of lake breezes, lake-breeze fronts (LBFs) generate convective clouds by lifting air parcels to the lifting condensation level (LCL) and above. Some studies documenting LBFs triggering moist convection have been reported from the Great Lakes region (Daggupaty 2001; King et al. 2003; Wang et al. 2019). The common situation in the Great Lakes region is that LBFs penetrate inland and merge with each other, and strong convective activity is triggered in the convergence area (Daggupaty 2001).
As the source of numerous major rivers in Asia, the Tibetan Plateau (TP) acts as the “Asian water tower,” and it affects the regional atmospheric circulation and precipitation over eastern China and influences the energy and water cycles across Asia (Fu et al. 2020; Immerzeel et al. 2010; Yao et al. 2012). Water vapor is abundant over the TP (Zhou et al. 2019). Also, shallow convective clouds are frequent (Chen et al. 2017) in summer, and convective precipitation on the TP provides water vapor for downstream areas (Fu et al. 2020). The total area of lakes on the TP accounts for 54% of the lake area in China, among which approximately 1204 lakes larger than 1 km2 were found statistically in a survey in 2010 (Wang et al. 2015; Zhang et al. 2014), and this had increased to ∼1400 lakes by 2018 (Zhang et al. 2019). Dai et al. (2018) compared meteorological station observations downwind of the Nam Co Lake with observations from other nearby stations in October 2006 and found the precipitation amount observed at Nam Co station (downwind) is more than twice that observed at the other three stations. Then, they carried out simulations with and without the lake, and the results showed that the lake caused an increase in precipitation of up to 60% over the lake’s area and downwind of the lake. Observations show that areas downstream of a lake may experience frequent extreme rainfall and snowfall events in both summer and winter (Li et al. 2009; Yao et al. 2021). Hence, it is very important to understand the triggering mechanism of shallow moist convection and the transition to deep moist convection (DMC) in the lake region of the TP more accurately.
Lake area and soil moisture (SM) are two major factors that affect the development of moist convection. Lakes on the TP vary in diameter from much less than 50 km to as large as 100 km. Satellite and remote sensing data show that the area of some lakes on the TP has exhibited an expanding trend over the past few decades (Zhang et al. 2019, 2020), and Yang et al. (2018) use a lake mass balance model to predict that such lakes will continue to expand in the coming decades. Thus, it is necessary to explore the relationship between lake area and lake-breeze circulations and how the lake area affects the development of moist convection. Soil moisture affects the partitioning of surface available energy (Koster et al. 2004). The weak sensible heat (SH) flux and shallow atmospheric boundary layer over land with large soil moisture result in increased convective available potential energy (CAPE) and moist static energy (MSE) and decreased convective inhibition (CIN). These are beneficial for the triggering of moist convection, and the process is often referred to as “positive feedback” (Eltahir 1998; Pal and Eltahir 2001; Santanello et al. 2013). Chen et al. (2020) suggested that moist convection is usually initiated over dry soil regions. Dry land leads to a large sensible heat flux, and strong surface heating makes air parcels rise and triggers moist convection (Ford et al. 2015; Hohenegger et al. 2009). Fast et al. (2019) claimed that the triggering of shallow cumulus cloud occurs earlier in dry soil regions because of the higher temperature and depth of the convective boundary layer (CBL). However, Cioni and Hohenegger (2017) have pointed out that although dry land can trigger moist convection, this is unlikely to produce widespread precipitation. In summary, our understanding of the relationship between soil moisture and moist convection triggering on the TP remains limited, especially in lake regions of the TP.
Large-eddy simulation (LES) has been used to reproduce the mechanisms of lake–land breezes (Crosman and Horel 2012), shallow convective clouds (Huang and Margulis 2013; Kang and Ryu 2016), and transition to DMC (Lee et al. 2019; Rieck et al. 2014). In addition, LES has been used to explore the relationship between soil moisture and moist convection (Han et al. 2019). The Weather Research and Forecasting (WRF) Model is widely used in meteorological studies (Powers et al. 2017). Hence, we coupled the WRF and LES (WRF–LES) under idealized conditions after editing part of the code. Here, we use the WRF–LES and observed atmospheric vertical profiles to investigate moist convection development in summer afternoons in the lake region of TP. We mainly focus on the interaction between lake breezes and the triggering of shallow convective clouds, the effect of lake area on the transition to DMC, and the mechanism of how soil moisture changes in the lake region of TP affect moist convection development. The remainder of this paper is arranged as follows: Section 2 describes the model and analysis methods; section 3 provides a detailed analysis of the results; and section 4 summarizes our findings.
2. Numerical experiments
a. Model configuration and experimental setup
We used a new model—version 4.4 of the WRF Model coupled with LES—to generate the simulations with ideal initialization. The WRF Model is a nonhydrostatic mesoscale numerical weather prediction system and serves a wide range of meteorological applications across scales from 10 m to 1000 km. It can produce simulations based on realistic or idealized conditions. Here, a new 3D scale-adaptive turbulent kinetic energy (TKE) (SMS-3DTKE) scheme developed by Zhang et al. (2018) for subgrid turbulent mixing was used. The SMS-3DTKE scheme was first introduced into the WRF Model in version 4.2. It is an extension to the original LES subgrid model 1.5-order TKE closure scheme (Deardorff 1980) and is suitable for simulations at various resolutions across the LES scale, mesoscale, and gray zone resolutions in between (Ye et al. 2023). Table 1 shows some of the physical process parameterization schemes chosen herein.
Parameterization schemes in WRF–LES.
Instead of specifying constant surface fluxes, the Noah land surface model (LSM) was coupled into the WRF–LES model in this study. The Noah LSM provides turbulent fluxes of momentum, heat, and moisture as boundary conditions for the WRF–LES and also predicts vertical profiles of soil temperature and moisture by solving a set of 1D equations for soil hydrologic and thermodynamic variables (Chen and Dudhia 2001). To account for feedback between the lake surface and the atmosphere, we applied the default lake model in the WRF to calculate momentum, heat, and moisture fluxes over the lake surface. The default lake model originates from the Community Land Model, version 4.5 (Oleson et al. 2013), and has been further improved by Subin et al. (2012) and Gu et al. (2015). In our simulation, we used the surface layer option of the revised MM5 Monin–Obukhov scheme (Jiménez et al. 2012), WRF single-moment 6-class microphysics scheme (WSM6) (Hong and Lim 2006), Dudhia shortwave (Dudhia 1989), and Rapid Radiative Transfer Model (RRTM) longwave (Mlawer et al. 1997) radiation schemes. Fifth-order and third-order advection schemes were applied in the horizontal and vertical directions, respectively, along with the third-order Runge–Kutta time differencing scheme.
The absence of background wind and topography in our idealized simulations was intended to emphasize land–air interactions on the underlying surface of the lake and adjacent land and to isolate interactions between soil moisture and moist convection. The initial profiles of potential temperature and water vapor mixing ratio used in this study (Fig. 1) were obtained from radiosonde observations at 1400 Beijing standard time (BST) 18 July 2012, at the Nam Co Station for Multisphere Observation and Research, Chinese Academy of Sciences (NAMORS). It was a sunny day, and the atmosphere above was unstable and stratified. The CAPE of the initial profile was 785.18 J kg−1 and the CIN was close to zero, indicating a favorable condition for the development of moist convection. By default, there are four layers of soil in the Noah LSM, at depths of 0.1, 0.3, 0.6, and 1 m underground, respectively. Because thermal and hydrological processes within the soil were not the focus of this study, we set the soil temperature and soil moisture profiles with constant values. According to the long-term (2005–16) dataset of hourly integrated land–atmosphere interaction observations on the TP (Ma et al. 2020), we took the soil temperature at different depths (0, 0.2, 0.4, and 0.8 m) and shortwave radiation value at NAMORS at 1400 BST in July 2012 as reference. We set the land surface temperature to 300 K (26.85°C), the soil temperature to 285 K (11.85°C), and the downward shortwave radiation at the ground surface of the entire domain to 836.48 W m−2. Taking the measured 6-m water temperature below the surface of Lake Nam Co in the afternoon of July 2012 by Wang et al. (2023) as reference, which is the closest lake surface temperature in observation, we set the lake surface temperature as 285 K (11.85°C). According to the research of Li et al. (2012), sand was selected as the soil type from the default soil parameter table in Noah, and the soil moisture was taken as the reference soil moisture (unit: volumetric fraction) of sand, that is, 56% of the saturated soil moisture content. The land use of lake and land areas was set as water body and grassland, and the vegetation fraction of grassland was set to 80%. The values of the above parameters are displayed in Table 2. Unlike selecting the optimal combination of parameterization schemes to reproduce real-world conditions, our goal here was to investigate the development process of moist convection in the summer afternoon in the lake region of TP.
Model initial profiles of potential temperature (red line) and water vapor mixing ratio (blue line) over the whole domain.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
Parameters for sensitivity experiments with different lake diameters (D_) and SM. Note: When the soil type is sand, the saturated soil water content is 0.339 m3 m−3.
In this study, the horizontal domain covered an area of 12 km × 240 km with a grid spacing of 100 m in the cross-stream and streamwise directions, and thus, 120 × 2400 horizontal grid points were used. Figure 2 shows a sketch of the heterogeneous underlying surface. To avoid the influence of the lateral boundary condition on the simulated lake-breeze circulations, we placed the lake in the center of the domain. Lakes with diameters of 20 km (case D_20), 50 km (case D_50), and 70 km (case D_70) were used to investigate the effect of lake area on local circulation and moist convection. Another set of sensitivity experiments was performed with soil moisture set to 20% (case SM20), 56% (case SM56), and 85% (case SM85) of the saturated soil moisture content in the grassland area and with a 50-km diameter lake placed in the domain. The purpose of three simulations was to investigate the effects of dry, moderate, and wet soil on the development of moist convection. Detailed information on the experimental setup is shown in Table 2. The physical time step was 1.0 s. There were 160 levels and approximately 50-m grid spacings in the vertical direction, with a vertical grid having a minimum spacing of 27 m in the lowest layer. The deep convective clouds primarily have tops of approximately 6–8 km above the ground due to their dry environment on the TP (Chen et al. 2017). Considering this, we set the model top of 8 km. Rayleigh damping with a coefficient of 0.003 was applied to the 1-km layer immediately below the model top to absorb vertically propagating gravity waves. Periodic lateral boundary conditions in the x direction and open boundary conditions in the y direction were applied, with a rigid lid (zero vertical motion) at the top of the model domain. Each simulation was run for 6 h, and the model outputs were saved every 10 min. In our simulation, the lake-breeze circulation over land on both sides of the lake is symmetrical without the background wind. The range of land affected by lake-breeze circulations was limited, and the part near the lakeshore was more evident, so we only analyzed the simulated results within 45 km from the lakeshore (as shown in the red box in Fig. 2), and the “land” refers to land within 45 km of shore in the following.
Sketches of heterogeneous underlying surfaces. The white and black domains represent the lake and land, respectively. The different sizes of the lake are marked. The areas in red boxes are areas analyzed and reported in later sections of this paper.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
b. Method
1) Phase-averaged analysis
2) Procedures for composite updrafts and cloud core
We defined the mean x coordinate and y coordinate (xm, ym) of all grid points within an individual updraft at the lower layer (0.3zi), middle layer (0.5zi), and upper layer (0.8zi) of the CBL as the center of the individual updraft. Then, these individual updrafts were shifted horizontally so that their centroids coincide. Finally, the ensemble average of all the individual updrafts was used to obtain the composite updrafts near the LBF at a given output time. The above is a procedure of obtaining composite updrafts similar to that used by Finnigan et al. (2009) and Schmidt and Schumann (1989). By repeating this process, we can obtain the composite updrafts for all output times and finally obtain the time-averaged composite updrafts for a period of time.
3. Results and discussion
a. Characteristics of lake-breeze circulations
To illustrate the new model that the ideal case of WRF coupled with LES in this paper can produce the lake-breeze circulations, the lake-breeze circulation initiates near the lakeshore and expands laterally at 3 h in case D_50 as shown in Fig. 3. The region of low-level onshore flow with horizontal wind speeds > 2 m s−1 was confined to within 15 km of the lakeshore, as indicated by the Y–Z cross sections of the υ wind field (Fig. 3a). Figure 3b shows that the horizontal temperature gradient between the lakeshore and the leading edge of the LBF was approximately 3 K, and the lake-breeze low-level onshore flow remained at a relatively constant depth (≈600 m) behind the LBF. The lake breezes extend inland and carry the cooler, moister air over the lake (Figs. 3b,c). Hence, our model successfully simulated the lake-breeze circulations.
The averaged Y–Z cross sections in the x direction of the (a) υ wind field, (b) potential temperature, and (c) water vapor mixing ratio at 3 h in case D_50.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
According to Crosman and Horel (2012), we defined the furthest distance that the lake breezes reach is the inland extent l of the lake breezes, the maximum height of the lake breezes developing in the vertical direction is the breeze depth h, the ratio between the inland extent and breeze depth is the breeze aspect ratio, and the near-surface cross-lakeshore horizontal wind speed υ. The inland extent of lake breezes gradually increased with time, reaching a maximum of approximately 18 km, and then decreased (Fig. 4a). The vertical extent of lake breezes was highest in case D_50, reaching a height of 1.2 km (Fig. 4b). In Fig. 4c, the breeze aspect ratios increased gradually after 30 min, indicating the strong inland advances of lake breezes. The near-surface cross-lakeshore horizontal wind speed υ increased rapidly during the first simulation hour before the increasing trend slowed down (Fig. 4d). This indicates that onshore lake breezes initially accelerate near shore and then slow down. We found that the lake breezes had larger depth in the D_50 and D_70 cases than that in the D_20 case. In contrast, the inland extents and near-surface cross-lakeshore horizontal wind speeds were almost identical in all three cases. Similar results were also obtained by Crosman and Horel (2012) in their idealized LES study on lake-breeze sensitivity to lake diameter.
Temporal evolution of lake-breeze characteristics for different lake diameters (D_20, D_50, and D_70). (a) Inland extent l, (b) lake-breeze depth h, (c) breeze aspect ratio l/h, and (d) cross-lakeshore wind speed υ at the lakeshore (∼27 m above ground).
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
b. Sensitivity experiments of lake area
1) Moist convection over land
The liquid water path (LWP) and the ice water path (IWP) are referred to as the cloud water path (CWP). The CWPs first increased and then decreased with time over land for each of the three cases with varying lake diameters (Fig. 5a). This tendency indicates that the moist convection gradually became stronger and then decayed. Larger lakes produced higher CWPs, leading to a greater potential to develop moist convection. These results are also critical for understanding radiation effects; this will be discussed later in the paper. After 3 h of simulation, precipitation occurred and gradually increased with time over land (Fig. 5b). Notably, when the lake diameter increased from 20 km to 50 and 70 km, the maximum precipitation increased by 71.4% and 1.29 times, respectively. We also examined the temporal evolution of domain-averaged profiles of total cloud hydrometeors over land (Figs. 5c–e) and found that clouds became deeper and stronger with time. We defined the occurrence of DMC as total cloud hydrometeors exceeding 0.01 g kg−1 at a level of 5.5 km (Rieck et al. 2014). Thus, the transition times to DMC were identified; these were 4 h 10 min, 3 h 50 min, and 3 h 50 min for cases D_20, D_50, and D_70, respectively. In other words, when the lake diameter increased from 20 km to more than 50 km, the DMC occurred 20 min earlier.
Temporal evolution of (a) domain-averaged CWPs over land, (b) accumulated surface domain-averaged precipitation, and (c)–(e) domain-averaged profiles of cloud condensate (includes liquid water qc, ice qi, snow qs, graupel qg, and rain qr) in cases D_20, D_50, and D_70. Colored contour shading represents qc + qi + qg + qs, while the blue contour lines represent qr. Vertical dashed lines refer to the transition timing.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
Figure 6a shows the temporal evolution of the domain-averaged surface available heat flux [sum of sensible and latent heat (LH) fluxes] over land. Before 1 h 20 min, the surface available heat flux maintained a slightly increasing trend; then, the rapid decrease in surface available heat flux (Fig. 6a) responded to the increase in convective clouds (Fig. 5a). Their opposing relationship reflects the cloud shading effect. When convective clouds increase, they stop the solar radiation from reaching the surface, so the sum of sensible and latent heat fluxes starts to decrease (at 1 h 20 min). Therefore, in all three cases with different lake diameters, the period before 1 h 20 min was termed the dry convection stage (0–1 h 20 min); the period from 1 h 20 min to 3 h 40 min (before the earliest transition time of DMC in cases D_20, D_50, and D_70) was termed the shallow cumulus convection stage, which was confirmed by little surface precipitation (Fig. 5b). Figure 6b shows that the surface energy over land in summer afternoons was dominated by latent heat flux, consistent with the study of Y. Zhang et al. (2021). In addition, a larger lake equated to a cooler near-surface air temperature over land (Fig. 6c) and a cooler surface skin temperature (TSK) (Fig. 6c). The temperature gradient between the ground surface and air increased with increasing lake diameter (Fig. 6c), resulting in a larger sensible heat flux.
Temporal evolution of (a) surface available heat flux; (b) SH and LH fluxes; and (c) TSK, near-surface air temperature Ta at the lowest model layer, and temperature difference (ΔT = TSK − Ta).
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
We also plotted the temporal evolution of air temperature, water vapor mixing ratio, and cross-lakeshore horizontal wind speed υ at the lowest model level (27 m); this was done for three different distances (0, 5, and 15 km) away from the lakeshore (Fig. S1 in the online supplemental material). We found that lake breezes had an accelerating, cooling, and moistening effect on the air over land. A larger lake equated to a stronger cross-lakeshore horizontal wind speed and a larger water vapor mixing ratio, together with a lower near-surface air temperature; this explains the conclusion obtained above whereby a larger lake equated to cooler near-surface air over land. Next, we attempted to discover why there were larger precipitation and earlier transitions to DMC over land in larger-lake simulations.
2) Composite updrafts near the LBF
Observations and simulations have identified that the LBFs are important for the growth of convective clouds (Sills et al. 2011; Wang et al. 2019). We calculated the intensity and size of the composite updrafts near LBFs by taking 0.3zi, 0.5zi, and 0.8zi as the reference heights during the dry convection (Fig. 7) and shallow cumulus convection (Fig. 8) stages, respectively. During the dry convection stage, composite updrafts near LBFs were stronger and wider in the middle (0.5zi) and upper (0.8zi) levels than in the lower (0.3zi) level of the CBL. In addition, there were evident differences in the horizontal extensions of the composite updrafts near the LBF in cases with different lake diameters. In the lower level of the CBL (Figs. 7a–c), the horizontal extension and intensity of composite updrafts near the LBF decreased with increasing lake diameter. The composite updrafts were roughly of the same size in the middle and upper levels of the CBL (Figs. 7d–i). However, the intensities of composite updrafts became considerably stronger with increasing lake diameter. This indicates that during the dry convection stage, the LBFs of larger lakes generated over land grow upward rapidly to the upper level of the CBL, which is conducive to the formation of convective clouds and the further development of moist convection. During the shallow cumulus convection stage, the intensity and horizontal extension of composite updrafts near the LBF over land increased with increasing lake diameter (Fig. 8) at all levels within the CBL. The broader and stronger composite updrafts in the upper levels of the CBL in larger-lake cases were more conducive to precipitation.
The composite updrafts near the LBF during the dry convection stage (0–1 h 20 min) of cases (a),(d),(g) D_20, (b),(e),(h) D_50, and (c),(f),(i) D_70 near the (a)–(c) low, (d)–(f) middle, and (g)–(i) upper layers of CBL. The y′ represents the relative distance of the boundary of updrafts from the center.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
The composite updrafts near the LBF during the shallow cumulus convection stage (1 h 20 min–3 h 40 min) of cases (a),(d),(g) D_20, (b),(e),(h) D_50, and (c),(f),(i) D_70 near the (a)–(c) low, (d)–(f) middle, and (g)–(i) upper layers of CBL. The y′ represents the relative distance of the boundary of updrafts from the center.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
3) Moisture conditions before DMC
One of the most significant factors contributing to the triggering of DMC is thought to be a moist atmosphere (Schiro and Neelin 2019). Zhang and Klein (2010) pointed out that the moisture condition at a height of 2–4 km is positively correlated with the onset of DMC. Therefore, we plotted the temporal evolution of the domain-averaged relative humidity (RH) and water vapor mixing ratio qυ over land at the 2–4-km level (Figs. 9a,b) and below 2 km (Figs. 9c,d) before DMC. There was no evident difference in moisture between the three lake-diameter cases during the dry convection stage, while distinct differences in RH and qυ were evident during the shallow cumulus convection stage. The RH and qυ below 2 km were greater than those at the 2–4-km level, and more water vapor in the lower layer is transported to the upper layer through turbulent mixing and updrafts (Fig. S2). The profile of water vapor flux
Temporal evolution of the domain-averaged (a),(c) RH and (b),(d) water vapor mixing ratio qυ over land at a height of (a),(b) 2–4 km and (c),(d) 0–2 km before DMC. The black lines represent the boundary between the dry convection and shallow cumulus convection stages.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
We calculated the time- and domain-averaged profiles of horizontal moisture flux induced by lake breezes during the dry convection and shallow cumulus convection stages, respectively (Figs. 10a,b). The horizontal moisture fluxes decreased gradually with height. The moisture transported in the horizontal direction of our simulation cases was mainly contained in the lower level of the CBL (below 250 m). Above 2 km, horizontal moisture fluxes were negligible during the dry convection stage. Moreover, horizontal moisture fluxes were almost identical in the three cases with different lake areas (Fig. 10a). During the shallow cumulus convection stage, the horizontal moisture fluxes increased markedly compared with the dry convection stage, and they increased with increasing lake area (Fig. 10b). In Fig. 10c, the accumulated horizontal moisture fluxes below 2 km in all three cases increased rapidly with time during the shallow cumulus convection stage and were considerably larger in cases D_50 and D_70 compared with case D_20. Hence, a larger lake equated to higher RH and qυ values below 2 km (Figs. 9c,d).
Time- and domain-averaged profiles of horizontal moisture flux Q induced by lake breezes during the (a) dry convection and (b) shallow cumulus convection stages. (c) Domain- and vertical-averaged Q induced by lake breezes below 2 km and (d) domain-averaged total PW below 4 km over land before DMC. The black lines represent the boundary between the dry convection and shallow cumulus convection stages.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
To investigate the total mass of water vapor in the atmospheric column from the surface to a height of 4 km over land in different cases, the domain-averaged total precipitable water (PW; mm) below 4 km is shown in Fig. 10d. During the dry convection stage, the PW over land below 4 km increased with time and was almost identical in the three cases with different lake diameters. During the shallow cumulus convection stage, the PW below 4 km over land increased with time and increased with increasing lake diameter. This result partially explains the higher precipitation over land around larger lakes depicted in Fig. 5b.
4) Cloud size
Some studies reported that large and broad clouds entrain relatively less than small clouds, which enables them to maintain less diluted cores, so they can penetrate deeper into the troposphere (Khairoutdinov and Randall 2006; Rousseau-Rizzi et al. 2017). Applying the compositing method described above, we obtained the time-averaged intensity and size of composite cloud cores during the shallow cumulus convection (Fig. 11). The reference height of the composite cloud cores in Fig. 11 is 2 km. The intensity of the composite cloud cores varied slightly between different lake-diameter cases during the shallow cumulus convection stage. The upper parts of convective clouds over land in larger-lake cases were broader, indicating that these clouds were more resistant to entrainment dilution and more conducive to extension upward.
The composite cloud cores during the shallow cumulus convection in cases (a) D_20, (b) D_50, and (c) D_70. The y′ represents the relative distance of the boundary of cloud cores from the center.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
c. Sensitivity experiments of soil moisture
To understand how variations in soil moisture affect lake breezes and the triggering of moist convection over land, we investigated the triggering of shallow moist convection and transition to DMC under dry (SM20), medium (SM56), and wet (SM85) soil moisture conditions of land. Note that case SM56 is the same as case D_50.
1) Moist convection over land
Figure 12a shows that the dry case (SM20) generated a larger CWP than did medium (SM56) and wet (SM85) cases before 3 h 30 min. After 3 h 30 min, the CWPs in cases SM56 and SM85 were substantially higher than that in SM20, with the CWP in SM56 being the highest. Moreover, the CWPs of cases SM20, SM56, and SM85 reached peak values at 3 h 20 min, 4 h, and 4 h 20 min, respectively, and then, the CWPs of all three cases decreased. Moreover, the CWP of SM20 reached its peak 1 h earlier than that of SM56 and SM85. The temporal evolutions of accumulated surface precipitation are shown in Fig. 12b. Before 4 h 20 min, the accumulated surface precipitation in SM20 was larger than that in SM56 and SM85, but this trend was reversed after 4 h 20 min. DMC did not occur in case SM20 because the total cloud hydrometeors did not reach 0.01 g kg−1 at 5.5 km during the simulation period (Fig. 12c). In the SM56 (Fig. 12d) and SM85 (Fig. 12e) cases, the transition to DMC occurred at 3 h 50 min and 3 h 40 min, respectively. This indicates that wetter soil equates to an earlier transition to DMC.
Temporal evolution of (a) domain-averaged CWPs over land, (b) accumulated surface precipitation, and (c)–(e) domain-averaged profiles of cloud condensate (includes liquid water qc, ice qi, snow qs, graupel qg, and rain qr) of cases SM20, SM56, and SM85. Colored contour shading represents qc + qi + qg + qs, while the blue contour lines represent qr. Vertical dashed lines refer to the transition timing.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
2) Lifting mechanism
Figure 13 shows the time-averaged composite updrafts near the LBF before DMC in cases SM20, SM56, and SM85 before 3 h 30 min, with 0.3zi, 0.5zi, and 0.8zi as reference heights. The composite updrafts near the LBF over the driest soil were strongest and widest at all test levels of the CBL (Figs. 13a–c). The intensity and width of composite updrafts near the LBF decreased with increasing land soil moisture. This result implies that there is a favorable factor lifting air parcels in the driest soil case (SM20).
Composite updrafts near the LBF of cases (a),(d),(g) SM20, (b),(e),(h) SM56, and (c),(f),(i) SM85 near the (a)–(c) low, (d)–(f) middle, and (g)–(i) upper levels of the CBL before DMC (0–3 h 30 min). The y′ values represent the relative distance of the boundary of updrafts from the center.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
3) Moisture conditions
To analyze the water vapor transported horizontally by lake breezes in cases SM20, SM56, and SM85, we calculated the horizontal moisture flux below 2 km using Eq. (6) (Fig. 14c). The driest soil case (SM20) showed the largest horizontal moisture flux in the layer below 2 km. The water vapor transported horizontally by lake breezes decreased with increasing soil moisture in land. The reason for this is that dry soil induces strong lake breezes, which leads to more moisture evaporating from the lake surface and being advected to land (Fig. S4). We also calculated the PW below 4 km, which is shown in Fig. 14d. The relative humidity and water vapor mixing ratio below 2 km over land are shown in Figs. 14a and 14b. We found that drier land equated to less moisture in the air. This may be related to lower evapotranspiration over drier land (Fig. S5).
Temporal evolution of the domain-averaged (a) RH, (b) water vapor mixing ratio qυ, (c) horizontal moisture flux Q induced by lake breezes below 2 km, and (d) total PW below 4 km over land before DMC.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
4) Cloud size
We obtained the intensity and size of time-averaged composite cloud cores from 0 to 3 h 30 min for cases with different soil moisture values. As shown in Fig. 15, the composite cloud cores were wetter in the medium (SM56) and wet (SM85) cases than in the dry (SM20) case. Most notably, the size of composite cloud cores over land increased with increasing soil moisture. These results indicate that composite cloud cores over land are wetter, broader, and less easily diluted in wetter soil cases and are more conducive to the transition to DMC.
Composite cloud cores in cases (a) SM20, (b) SM56, and (c) SM85 before DMC (1–3 h 30 min). The y′ values represent the relative distance of the boundary of cloud cores from the center.
Citation: Journal of the Atmospheric Sciences 81, 6; 10.1175/JAS-D-23-0147.1
4. Summary and conclusions
In this study, the impacts of lake-breeze circulations on the triggering of shallow moist convection and transition to deep moist convection (DMC) over land were studied through the application of LES. Two sets of idealized simulations with varying lake area and soil moisture were performed using a new model that is the ideal case of WRF coupled with LES. Diameters of 20, 50, and 70 km were chosen to represent small, medium, and large lakes on the TP, respectively. The mechanisms by which lake area affects moist convection intensity over land and the transition time of DMC were investigated. We set the soil moisture to 20%, 56%, and 85% of the saturated soil moisture to represent dry, medium, and wet soil moisture conditions, respectively, in the 50-km lake-diameter case. The resultant lake breezes, triggering of shallow cumulus convection, and transition to DMC under different soil moisture conditions of land were then studied. The main conclusions are as follows.
The lake-diameter sensitivity experiments indicated that a larger lake gives rise to stronger horizontal lake breezes and more pronounced cooling and moistening effects on the atmosphere over land. The inverse relationship between the surface heat fluxes and the cloud water paths over land indirectly reflects cloud shading effects and intuitively reflects the importance of land–air interactions in the triggering of shallow cumulus convection and the transition to DMC.
We found that when the lake diameter increased from 20 km to 50 and 70 km, the maximum precipitation increased by 71.4% and 1.29 times, respectively. When the lake was larger, updrafts near the LBF were stronger and wider close to the top of the CBL, providing a powerful forcing mechanism for the upward growth of clouds. In addition, in cases with larger lake, land received more moisture advected by the lake breezes and then transported upward, moistening the atmosphere in the 2–4-km layer. This process ensures sufficient moisture for the upward growth of shallow convective clouds. When the lake diameter increased from 20 km to more than 50 km, the DMC occurred 20 min earlier. We showed that wetter and broader shallow convective clouds were generated over land adjacent to larger lakes, which were unlikely to be diluted by entrainment and more easily transitioned into DMC. The vertical velocity of cloud-initiating parcels was larger in large-lake simulations.
On the basis of results from cases with different land soil moisture values, we can conclude that the stronger and broader updrafts near the LBFs and the advection of moisture induced by the lake breezes caused the shallow moist convection to reach its peak 1 h earlier over the driest land, but the smaller evapotranspiration could not provide sufficient moisture for the occurrence of DMC. Larger clouds were generated in higher soil moisture cases and were more likely to develop into DMC.
Our study simulated lake breezes, the triggering of shallow moist convection, the transition to DMC, and their interactions over the heterogeneous surface of a lake region on the TP. It mainly focused on the influences of lake area and land surface soil moisture. Horizontal and vertical moisture transport by lake-breeze circulations was critical in moistening and broadening shallow convective clouds. Hence, lakes play a decisive role in forming and maintaining moist convection over their surrounding areas. The strength and direction of background winds have been shown to influence the structure and evolution of thermal circulations (sea-breeze circulations), as well as the interaction between circulations and atmospheric boundary layer turbulence (Azorin-Molina and Chen 2009; Allouche et al. 2023a,b; Bauer 2020). Chen et al. (2020) pointed out that background wind plays an important role in the location of clouds by moving them away from where they formed. In a future study, background wind fields will be added to the simulations to explore the influence of background wind on thermal circulations and determine how background wind affects the interaction between thermal circulations and moist convection.
Acknowledgments.
This research has been supported by the National Natural Science Foundation of China (42230610), the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (2019QZKK0103), the Innovation Program for Young Scholars of TPESER (QNCX2022ZD-01), and the National Key Scientific and Technological Infrastructure project “Earth System Numerical Simulation Facility” (EarthLab).
Data availability statement.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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