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1. Impacts of Coastal Terrain on Warm-Sector Heavy-Rain-Producing MCSs in Southern China

   https://doi.org/10.1175/MWR-D-21-0190.1  

   Zhang, Murong; Rasmussen, Kristen L.; Meng, Zhiyong; Huang, Yipeng (March 2022, Monthly Weather Review)

   Abstract Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within a weakly forced synoptic environment under the influence of monsoonal airflows. It is usually located near the southern coast and is characterized by poor predictability and a close relationship with coastal terrain. This study investigates the impacts of coastal terrain on the initiation, organization, and heavy rainfall potential of MCSs in warm-sector heavy rainfall over southern China using quasi-idealized WRF simulations and terrain-modification experiments. Typical warm-sector heavy rainfall events were selected to produce composite environments that forced the simulations. MCSs in these events all initiated in the early morning and developed into quasi-linear convective systems along the coast with a prominent back-building process. When the small coastal terrain is removed, the maximum 12-h rainfall accumulation decreases by ∼46%. The convection initiation is advanced ∼2 h with the help of orographic lifting associated with flow interaction with the coastal hills in the control experiment. Moreover, the coastal terrain weakens near-surface winds and thus decreases the deep-layer vertical wind shear component perpendicular to the coast and increases the component parallel to the coast; the coastal terrain also concentrates the moisture and instability over the coastal region by weakening the boundary layer jet. These modifications lead to faster upscale growth of convection and eventually a well-organized MCS. The coastal terrain is beneficial for back-building convection and thus persistent rainfall by providing orographic lifting for new cells on the western end of the MCS, and by facilitating a stronger and more stagnant cold pool, which stimulates new cells near its rear edge. 

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2. Moist heatwaves intensified by entrainment of dry air that limits deep convection

   https://doi.org/10.1038/s41561-024-01498-y  

   Duan, Suqin Q; Ahmed, Fiaz; Neelin, J David (July 2024, Nature Geoscience)

   Moist heatwaves in the tropics and subtropics pose substantial risks to society, yet the dynamics governing their intensity are not fully understood. The onset of deep convection arising from hot, moist near-surface air has been thought to limit the magnitude of moist heatwaves. Here we use reanalysis data, output from the Coupled Model Intercomparison Project Phase 6 and model entrainment perturbation experiments to show that entrainment of unsaturated air in the lower-free troposphere (roughly 1–3 km above the surface) limits deep convection, thereby allowing much higher near-surface moist heat. Regions with large-scale subsidence and a dry lower-free troposphere, such as coastal areas adjacent to hot and arid land, are thus particularly susceptible to moist heatwaves. Even in convective regions such as the northern Indian Plain, Southeast Asia and interior South America, the lower-free tropospheric dryness strongly afects the maximum surface wet-bulb temperature. As the climate warms, the dryness (relative to saturation) of the lower-free tropospheric air increases and this allows for a larger increase of extreme moist heat, further elevating the likelihood of moist heatwaves. 

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3. Low-level Mesoscale and Cloud-scale Interactions Promoting Deep Convection Initiation

   https://doi.org/10.1175/MWR-D-20-0391.1  

   Marquis, James N.; Varble, Adam C.; Robinson, Paul; Nelson, T. Connor.; Friedrich, Katja (May 2021, Monthly Weather Review)

   null (Ed.)

   Abstract Data from scanning radars, radiosondes, and vertical profilers deployed during three field campaigns are analyzed to study interactions between cloud-scale updrafts associated with initiating deep moist convection and the surrounding environment. Three cases are analyzed in which the radar networks permitted dual-Doppler wind retrievals in clear air preceding and during the onset of surface precipitation. These observations capture the evolution of: i) the mesoscale and boundary layer flow, and ii) low-level updrafts associated with deep moist convection initiation (CI) events yielding sustained or short-lived precipitating storms. The elimination of convective inhibition did not distinguish between sustained and unsustained CI events, though the vertical distribution of convective available potential energy may have played a role. The clearest signal differentiating the initiation of sustained versus unsustained precipitating deep convection was the depth of the low-level horizontal wind convergence associated with the mesoscale flow feature triggering CI, a sharp surface wind shift boundary or orographic upslope flow. The depth of the boundary layer relative to the height of the LFC failed to be a consistent indicator of CI potential. Widths of the earliest detectable low-level updrafts associated with sustained precipitating deep convection were ~3-5 km, larger than updrafts associated with surrounding boundary layer turbulence (~1-3-km wide). It is hypothesized that updrafts of this larger size are important for initiating cells to survive the destructive effects of buoyancy dilution via entrainment. 

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4. Environmental Controls on Simulated Deep Moist Convection Initiation Occurring during RELAMPAGO-CACTI

   https://doi.org/10.1175/JAS-D-21-0226.1  

   Nelson, T. Connor; Marquis, James; Peters, John M.; Friedrich, Katja (July 2022, Journal of the Atmospheric Sciences)

   Abstract This study synthesizes the results of 13 high-resolution simulations of deep convective updrafts forming over idealized terrain using environments observed during the RELAMPAGO and CACTI field projects. Using composite soundings from multiple observed cases, and variations upon them, we explore the sensitivity of updraft properties (e.g., size, buoyancy, and vertical pressure gradient forces) to influences of environmental relative humidity, wind shear, and mesoscale orographic forcing that support or suppress deep convection initiation (CI). Emphasis is placed on differentiating physical processes affecting the development of updrafts (e.g., entrainment-driven dilution of updrafts) in environments typifying observed successful and null (i.e., no CI despite affirmative operational forecasts) CI events. Thermally induced mesoscale orographic lift favors the production of deep updrafts originating from ∼1- to 2-km-wide boundary layer thermals. Simulations without terrain forcing required much larger (∼5-km-wide) thermals to yield precipitating convection. CI outcome was quite sensitive to environmental relative humidity; updrafts with increased buoyancy, depth, and intensity thrived in otherwise inhospitable environments by simply increasing the free-tropospheric relative humidity. This implicates the entrainment of free-tropospheric air into updrafts as a prominent governor of CI, consistent with previous studies. Sensitivity of CI to the environmental wind is manifested by 1) low-level flow affecting the strength and depth of mesoscale convergence along the terrain, and 2) clouds encountering updraft-suppressing pressure gradient forces while interacting with vertical wind shear in the free troposphere. Among the ensemble of thermals occurring in each simulation, the widest deep updrafts in each simulation were the most sensitive to environmental influences. 

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5. Self-Organization and Maintenance of Simulated Nocturnal Convective Systems from PECAN

   https://doi.org/10.1175/MWR-D-20-0263.1  

   Parker, Matthew D. (April 2021, Monthly Weather Review)

   null (Ed.)

   Abstract The Plains Elevated Convection at Night (PECAN) field project was designed to explain the evolution and structures of nocturnal mesoscale convective systems (MCSs) and relate them to specific mechanisms and environmental ingredients. The present work examines four of the strongest and best-organized PECAN cases, each numerically simulated at two different levels of complexity. The suite of simulations enables a longitudinal look at how nocturnal MCSs resemble (or differ from) more commonly studied diurnal MCSs. All of the simulations produce at least some surface outflow (“cold pools”), with stronger outflows occurring in environments with more CAPE and weaker near-ground stability. As these surface outflows emerge, the lifting of near-ground air occurs, causing each simulated nocturnal MCS to ultimately become “surface-based.” The end result in each simulation is a quasi-linear convective system (QLCS) that is most intense toward the downshear flank of its cold pool, with the classical appearance of many afternoon squall lines. This pathway of evolution occurs both in fully heterogeneous real-world-like simulations and horizontally homogeneous idealized simulations. One of the studied cases also exhibits a back-building “rearward off-boundary development” stage, and this more complex behavior is also well simulated in both model configurations. As a group, the simulations imply that a wide range of nocturnal MCS behaviors may be self-organized (i.e., not reliant on larger-scale features external to the convection). 

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