Initiation Of Deep Convection Research Articles

Spatiotemporal characteristics of deep convection initiation in the Central United States

AbstractThis research examines the spatiotemporal characteristics of deep convection initiation (DCI) using a dataset of approximately 182,000 instances of DCI occurring over an 11‐year period in the Central United States. Spatial statistical analysis reveals differences in the frequency of DCI occurrence across the study area. Favoured areas of DCI are most common across areas of higher terrain and in proximity to the Gulf Coast, while DCI occurs less frequently over the northern Great Plains. Spatial differences in the degree of interannual variability of DCI are also noted, with higher (lower) interannual variability generally noted in areas of less (more) frequent DCI occurrence. Unsupervised machine learning is utilized to identify spatial variability in the diurnal cycle of DCI. The data and methods in this study are able to reliably identify more obvious differences in diurnal time series of DCI occurrence, while more subtle differences such as those resulting from localized or weaker mechanisms are identified less reliably. However, from these results it is obvious that spatial variability in the diurnal time series of DCI exists.

Understanding the factors influencing cloud-core vertical accelerations during deep convection formation in the WRF model

By introducing a linear diagnostic equation to compute perturbation pressure, the vertical momentum equation of the WRF model in the η coordinate is rederived, which constructs a direct relationship between vertical acceleration and physical factors we concern in the deep convection initiation (DCI) process. Important physical processes to the rapid growth of convective cloud are studied, aiming to fully understand the DCI process in the model. It is found that the DCI process is highly related to a cloud-core vertical acceleration. Due to this acceleration, the updraft is strengthened and the cloud is able to develop higher. Based on the diagnostic results of the vertical momentum equation, some different processes contributing to the high-level cloud-core accelerations in the model are found. A divergent pattern of the three-dimensional wind field is favorable for vertical acceleration. An inner physical process is that the horizontally convergent mass should be pumped up instantly by the vertical updraft to sustain a strong vertical acceleration. Second-order vertical inhomogeneity of perturbation geopotential also has an impact. Because of geopotential changes by vertical velocity (geopotential equation), this factor implies larger vertical motions will induce larger accelerations. The effects of the vertical gradient of perturbation potential temperature and moisture may be cancelled out since phase transitions can heat the convective air, but simultaneously decrease the water vapor content. Moisture makes a direct contribution to vertical acceleration, but is mostly cancelled out by the drag of cloud hydrometeors. Clearly understanding the direct impact of the basic prognostic variables on convection may help to identify the reason why DCI predictions within the model fail.摘要本文将扰动气压利用一个线性诊断关系代替, 重新推导了WRF模式框架地形追随坐标系下的垂直动量方程, 建立了垂直加速与对流触发 (DCI) 影响因子 (如温度, 水汽等) 的直接联系. 研究发现, DCI过程与对流核垂直加速相关, 三维副散, 扰动位势在垂直方向的二阶非均匀性, 扰动位温垂直梯度, 比湿及其垂直梯度, 水凝物拖曳, 均是能够直接影响垂直加速和对流触发的物理因子, 这些量与模式基本预报量相关, 通过解析基本预报量对对流发展的直接影响, 可能有助于理解模式对DCI过程预测失败的原因.

Observations of Boundary Layer Convergence Lines and Associated Updrafts in the U.S. Southern Great Plains

Abstract Boundary layer convergence lines (CLs) are highly effective at deep-convection initiation (DCI), suggesting that their associated updraft properties differ from those of more widespread turbulent updrafts in the planetary boundary layer (PBL). This study exploits observations at the Atmospheric Radiation Measurement Southern Great Plains (ARM SGP) observatory in Oklahoma from 2011 to 2016 to quantify CL properties and their relation to turbulent PBL eddies preceding CL arrival. Two independent methods for estimating CL properties are developed at two locations in the SGP region, both relying on the assumption of a 2D circulation in the CL-normal plane but using different combinations of instruments. The first (the radar method) relies mainly on scanning radar data and is applied to 61 CLs passing near a high-resolution scanning radar based in Nardin, Oklahoma, while the second (the surface method) relies mainly on surface wind data and is applied to 68 CLs crossing the SGP facility in nearby Lamont, Oklahoma. Mean daytime (1000–1900 LST) CL width (∼2 km) and convergence magnitude (∼0.003 s−1) are similar for both methods, and mean daytime CL depth is ∼0.75 km. The two methods disagree at night (0000–1000 and 1900–2400 LST), where the surface method estimates wider and weaker CLs than the radar method. This difference may stem from the radar beam overshooting the shallow, highly stable nocturnal PBL. The largest CL updrafts are slightly wider (∼20%) and stronger (∼40%) than the largest PBL updrafts in the pre-CL period, generating 50%–100% larger updraft mass fluxes over most of the PBL depth. Significance Statement Deep convection is commonly initiated by boundary layer convergence lines (CLs), which are associated with intense surface-based wind convergence and strong updrafts that may lift air to saturation. Although CLs form regularly, they are far less common than ordinary, short-lived turbulent thermals in the daytime boundary layer. To better understand why CLs are so effective at deep-convection initiation, we observationally quantify their morphologies and strengths and compare these properties to those of surrounding turbulent updrafts. Perhaps surprisingly, the CLs are found to exhibit only slightly larger scales and strengths as the turbulent updrafts. Although these marginal increases help to explain the preference for storms to initiate along CLs, they likely are not the whole story.

Summary of atmospheric characteristics of days with inland penetrating sea breezes from 2015 to 2021

AbstractSea breezes penetrate inland more than 100 km. Using 7 years of meteorological observations, we have identified 470 cases of deep inland (>100 km) penetrating sea breezes at the Savannah River Site between March and October (27% of days) of 2015–2021. We compared measurements of temperature, dewpoint temperature, incoming solar radiation, cloud fraction, and lightning on days of sea breeze initiation, the day after the sea breeze passage, and all other nonsea breeze (NSB) days for these 8 months over the 7 years. Days of sea breeze initiation were found to have lower cloud fraction, higher temperature, and greater incoming solar radiation compared with NSB days. Variations occurred by time of year as days after the sea breeze passage were found to have higher dewpoint temperature than NSB days in the spring. Lightning density measurements indicated that residual sea breeze conditions could drive earlier initiation of deep convection on days following the sea breeze than normal non sea breeze days. This data set provides a 7‐year record of sea breezes which can be leveraged for future studies.

Statistics of Isolated Deep Convection Initiation and Its Relation to Topography in the North China Area

AbstractThis study investigates the statistical characteristics of isolated deep convection initiation (DCI) and its relation to topography in the North China area during the summer season of 2017–2019. A total of 2,534 isolated DCI events are identified using the infrared brightness temperature data from Himawari‐8 satellite with a resolution of 0.05° × 0.05°. The spatial distribution of DCI shows clustering over mountains and hills, indicating that the local topography may be critical for DCI. Topography is described with elevation and relief amplitude in this study. Results show that normalized DCI counts increase with elevation and relief amplitude. For the regime characterized by fewer morning clouds (regime one), DCI strongly depends on elevation and relief amplitude, while for the regime with more morning clouds (regime two), topography only moderately impacts DCI. Linear regression analyses suggest the existence of significant linear relationships between DCI and relief amplitude when elevation is relatively high. Topography is also categorized into four types: plain/sea areas, plateaus, hilly areas, and mountains. Normalized DCI counts show that mountains and hilly areas are more favorable for initiating isolated deep convection, especially for regime one. Moreover, DCI counts in regime one show a stronger diurnal variation and a peak occurring 2 hr earlier than that of regime two. We suggest that special attention should be paid to elevation and relief amplitude, as well as morning cloud cover condition when forecasting DCI in the North China area. Further study is necessary to understand this DCI occurrence preference on specific topography.

Near-Cloud Atmospheric Ingredients for Deep Convection Initiation

Abstract A lack of routine environmental observations located near deepening cumulus congestus clouds limits verification of important theorized and simulated updraft–environment interaction processes occurring during deep convection initiation (CI). We analyze radiosonde profiles collected during several hundred CI events near a mountain range in central Argentina during the CACTI field campaign. Statistical analyses illustrate environmental conditions supporting radar-observed CI outcomes that span a spectrum of convective cell depths, widths, and durations, as well as events lacking precipitating convection. Tested environmental factors include a large variety of sounding-derived measurements of CAPE, CIN, moisture, terrain-relative winds, vertical shear, and lifted parcel properties, with supplemental model reanalysis of background larger-scale vertical motion. CAPE and CIN metrics do not consistently differentiate CI success from failure. Only a few environmental factors contain consistent monotonic relationships among the spectrum of cloud depths achieved during CI: (i) the depth and strength of background ascent, and (ii) the component of low-level flow oriented parallel to the ridgeline. These metrics suggest that the ability of the surrounding flow to lift parcels to their LFC and terrain-modified flow are consistently relevant processes for CI. Low- to midlevel relative humidity strongly discriminated between CI and non-CI events, likely reflecting entrainment-driven dilution processes. However, we could not confidently conclude that relative humidity similarly discriminated robust from marginal CI events. Circumstantial evidence was found linking cell width, an important cloud property governing the probability of CI, to LCL height, boundary layer depth, depth and magnitude of the CIN layer, and ambient wind shear.

Convection Initiation Associated with a Boundary Layer Convergence Line over a Real-World Sharp Vegetation-Contrast Area

Abstract Convection initiation (CI) has remained a major challenge in weather forecasting worldwide. The Hetao area in North China, the location of Asia’s largest irrigation area, contains highly heterogeneous vegetation where near-surface convergence lines (boundaries) parallel to the oasis–desert border often emerge over the desert and initiate convection. This study investigated the evolution of such a boundary and its influence on the CI process where a series of cells were successively initiated along the boundary on 4 June 2013. Our results indicated that uneven surface heating across the oasis–desert border produced mesoscale thermal circulation. The westerly oasis breeze in the thermal circulation converged with the southerly background wind and formed a boundary over the desert along the high-temperature contrast line. A middle-hemisphere westerly trough further enhanced uplift and facilitated CI. Our simulation revealed that the first 30-dBZ parcels in each cell originated from either the desert side at a low level or the oasis side at a middle level, rather than from the oasis at a low level, as indicated by previous idealized studies. Southerly low-level parcels veered above the boundary and experienced a longer lifting time over the desert, while western parcels originating from the oasis experienced a shorter lifting time and smaller vertical displacement, resulting in middle-level parcels instead of low-level parcels that reached their level of free convection. Even though CI occurred over a surface boundary without a near-surface stable layer, the inflow may have originated from middle levels rather than in contact with the surface. Significance Statement The purpose of this study is to understand the evolution of a real-world near-surface convergence line and the associated deep convection initiation (CI) along the border of Asia’s largest irrigation area and the desert in northern China. Notably, previous works have mainly focused on shallow convection over uneven vegetation distributions based on idealized simulations, which may be quite different from real-world interactions between thermal circulation and background flow. Our results highlight different parcel sources in different convections initiated by the same convergence line, which is different from the idealized situation where parcels mainly originate from the low-level oasis side.

Results of Field Experiments for the Creation of Artificial Updrafts and Clouds

This study documents results from a series of field experiments on the creation of artificial updrafts and convective clouds at a test site in the United Arab Emirates (UAE). The proposed method incorporates a vertically directed jet from an aircraft turbojet engine saturated with active hygroscopic aerosols for the purpose of energetically feeding the jet with water vapor condensation heat below cloud base level. This paper presents the description and main characteristics of the experimental equipment, methodology of experiments and atmospheric conditions, analysis of the obtained results, and prospects for further development of the proposed method. On the whole, the experiments showed that under the conditions of low air humidity, typical for the UAE, and the slowness of the condensation process, the replenishment of the jet energy by the heat of condensation is too small, and the power of the used jet engine in the experiments is insufficient to overcome surface temperature inversions, horizontal winds, and initiation of deep convection. Nevertheless, the results of field experiments and numerical simulation made it possible to outline promising directions for further research on improving the considered method for creating artificial clouds and precipitation.

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

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.

The Influence of Shear on Deep Convection Initiation. Part II: Simulations

Abstract This study evaluates a hypothesis for the role of vertical wind shear in deep convection initiation (DCI) that was introduced in Part I by examining behavior of a series of numerical simulations. The hypothesis states, “Initial moist updrafts that exceed a width and shear threshold will ‘root’ within a progressively deeper steering current with time, increase their low-level cloud-relative flow and inflow, widen, and subsequently reduce their susceptibility to entrainment-driven dilution, evolving toward a quasi-steady self-sustaining state.” A theoretical model that embodied key elements of the hypothesis was developed in Part I, and the behavior of this model was explored within a multidimensional environmental parameter space. Remarkably similar behavior is evident in the simulations studied here to that of the theoretical model, both in terms of the temporal evolution of DCI and in the sensitivity of DCI to environmental parameters. Notably, both the simulations and theoretical model experience a bifurcation in outcomes, whereby nascent clouds that are narrower than a given initial radius R0 threshold quickly decay and those above the R0 threshold undergo DCI. An important assumption in the theoretical model, which states that the cloud-relative flow of the background environment VCR determines cloud radius R, is scrutinized in the simulations. It is shown that storm-induced inflow is small relative to VCR beyond a few kilometers from the updraft edge, and VCR therefore plays a predominant role in transporting conditionally unstable air to the updraft. Thus, the critical role of VCR in determining R is validated.

The Influence of Shear on Deep Convection Initiation. Part I: Theory

Abstract This article introduces a novel hypothesis for the role of vertical wind shear (“shear”) in deep convection initiation (DCI). In this hypothesis, initial moist updrafts that exceed a width and shear threshold will “root” within a progressively deeper steering current with time, increase their low-level cloud-relative flow and inflow, widen, and subsequently reduce their susceptibility to entrainment-driven dilution, evolving toward a quasi-steady self-sustaining state. In contrast, initial updrafts that do not exceed the aforementioned thresholds experience suppressed growth by shear-induced downward pressure gradient accelerations, will not root in a deep-enough steering current to increase their inflow, will narrow with time, and will succumb to entrainment-driven dilution. In the latter case, an externally driven lifting mechanism is required to sustain deep convection, and deep convection will not persist in the absence of such lifting mechanism. A theoretical model is developed from the equations of motion to further explore this hypothesis. The model indicates that shear generally suppresses DCI, raising the initial subcloud updraft width that is necessary for it to occur. However, there is a pronounced bifurcation in updraft growth in the model after the onset of convection. Sufficiently wide initial updrafts grow and eventually achieve a steady state. In contrast, insufficiently wide initial updrafts shrink with time and eventually decay completely without external support. A sharp initial updraft radius threshold discriminates between these two outcomes. Thus, consistent with our hypothesis and observations, shear inhibits DCI in some situations, but facilitates it in others.

Deep Convection Initiation, Growth, and Environments in the Complex Terrain of Central Argentina during CACTI

Abstract This study characterizes the wide range of deep convective cloud life cycles and their relationships with ambient environments observed during the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign near the Sierras de Córdoba (SDC) range in central Argentina. We develop a novel convective cell tracking database for the entire field campaign using C-band polarimetric radar observations. The cell tracking database includes timing, location, area, depth, merge/split information, microphysical properties, collocated satellite-retrieved cloud properties, and sounding-derived environmental conditions. Results show that the SDC exerts a strong control on convection initiation (CI) and growth. CI preferentially occurs east of the SDC ridge during the afternoon, and cells often undergo upscale growth through the evening as they travel eastward toward the plains. Larger and more intense cells tend to occur in more unstable and humid low-level environments, and surface-based cells are stronger than elevated cells. Midtropospheric relative humidity and vertical wind shear also jointly affect the size and depth of the cells. Rapid cell area growth rates exhibit dependence on both large environmental wind shear and low-level moisture. Evolution of convective cell macro- and microphysical properties are strongly influenced by convective available potential energy and low-level humidity, as well as the presence of other cells in their vicinity. This cell tracking database demonstrates a framework that ties measurements from various platforms centering around convective life cycles to facilitate process understanding of factors that control convective evolution. Significance Statement The purpose of this study is to develop a framework that ties coordinated radar, satellite, and radiosonde measurements around tracking convective storm life cycles to facilitate process understanding of atmospheric environments that control storm evolution. The processes coupling storm life cycles and local environments remain inadequately understood and are poorly represented in weather and climate models. Our results demonstrate the importance of atmospheric instability, low- and midtropospheric moisture, changes of wind with height, and interactions among nearby storms in affecting the formation and growth of convective storms. The storm database developed in this work enables future studies for comprehensive exploration of processes that lead to improved mechanistic understanding of storm evolution and their representations in models.

Large-Eddy Simulations of Convection Initiation over Heterogeneous, Low Terrain

Abstract Large-eddy simulations are conducted to investigate and physically interpret the impacts of heterogeneous, low terrain on deep-convection initiation (CI). The simulations are based on a case of shallow-to-deep convective transition over the Amazon River basin, and use idealized terrains with varying levels of ruggedness. The terrain is designed by specifying its power-spectral shape in wavenumber space, inverting to physical space assuming random phases for all wave modes, and scaling the terrain to have a peak height of 200 m. For the case in question, these modest terrain fields expedite CI by up to 2–3 h, largely due to the impacts of the terrain on the size of, and subcloud support for, incipient cumuli. Terrain-induced circulations enhance subcloud kinetic energy on the mesoscale, which is realized as wider and longer-lived subcloud circulations. When the updraft branches of these circulations breach the level of free convection, they initiate wider and more persistent cumuli that subsequently undergo less entrainment-induced cloud dilution and detrainment-induced mass loss. As a result, the clouds become more vigorous and penetrate deeper into the troposphere. Larger-scale terrains are more effective than smaller-scale terrains in promoting CI because they induce larger enhancements in both the width and the persistence of subcloud updrafts.

Low-Level Mesoscale and Cloud-Scale Interactions Promoting Deep Convection Initiation

AbstractData 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.

Utilizing a Storm-Generating Hotspot to Study Convective Cloud Transitions: The CACTI Experiment

AbstractThe Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign was designed to improve understanding of orographic cloud life cycles in relation to surrounding atmospheric thermodynamic, flow, and aerosol conditions. The deployment to the Sierras de Córdoba range in north-central Argentina was chosen because of very frequent cumulus congestus, deep convection initiation, and mesoscale convective organization uniquely observable from a fixed site. The C-band Scanning Atmospheric Radiation Measurement (ARM) Precipitation Radar was deployed for the first time with over 50 ARM Mobile Facility atmospheric state, surface, aerosol, radiation, cloud, and precipitation instruments between October 2018 and April 2019. An intensive observing period (IOP) coincident with the RELAMPAGO field campaign was held between 1 November and 15 December during which 22 flights were performed by the ARM Gulfstream-1 aircraft. A multitude of atmospheric processes and cloud conditions were observed over the 7-month campaign, including numerous orographic cumulus and stratocumulus events; new particle formation and growth producing high aerosol concentrations; drizzle formation in fog and shallow liquid clouds; very low aerosol conditions following wet deposition in heavy rainfall; initiation of ice in congestus clouds across a range of temperatures; extreme deep convection reaching 21-km altitudes; and organization of intense, hail-containing supercells and mesoscale convective systems. These comprehensive datasets include many of the first ever collected in this region and provide new opportunities to study orographic cloud evolution and interactions with meteorological conditions, aerosols, surface conditions, and radiation in mountainous terrain.

Initiation of deep convection through deepening of a well‐mixed boundary layer

AbstractThe present study examines mechanisms by which deep moist convection may be initiated through deepening of a well‐mixed convective boundary layer under large‐scale low‐level convergence. The process is examined using a standard formulation for the well‐mixed boundary layer, in which the depth increases exponentially with time under a constant convergence with height. Consideration is also given to a case with a more realistic mean vertical velocity profile which has a maximum in the middle troposphere and attenuates to zero at the tropopause. In the latter case, the unstable well‐mixed layer under convergence grows to a troposphere‐deep mixed layer. Importantly, under these scenarios, the so‐called convective inhibition (CIN) does not inhibit deepening of a well‐mixed convective boundary layer, nor does deep moist convection need to overcome CIN to initiate.

A large-eddy simulation study of deep-convection initiation through the collision of two sea-breeze fronts

Abstract. Deep convection plays important roles in producing severe weather and regulating the large-scale circulation. However, deep-convection initiation (DCI), which determines when and where deep convection develops, has not yet been fully understood. Here, large-eddy simulations are performed to investigate the detailed processes of DCI, which occurs through the collision of two sea-breeze fronts developing over a peninsula. In the simulation with a maximum total heat flux over land of 700 or 500 W m−2, DCI is accomplished through the development of three generations of convection. The first generation of convection is randomly produced along the colliding sea-breeze fronts. The second generation of convection only develops in regions where no strong downdrafts are produced by the first generation of convection and is also mainly produced through the collision of the sea-breeze fronts. The third generation of convection mainly develops from the intersection points of the cold pools produced by the second generation of convection and is produced through the collision between the gust fronts and the sea-breeze fronts. Decreasing the maximum total heat flux from 700 to 500 W m−2 weakens each generation of convection. Further decreasing the maximum total heat flux to 300 W m−2 leads to only one generation of shallow convection.

Simulating the influence of topography on cut‐off lows over Southern Africa

AbstractCut‐off lows (COLs) over Southern Africa are known for their extreme rainfall, but the reliable prediction of COL rainfall remains a challenge because of complex interactions with topography. As there is a dearth of information on these interactions over this region, this study examines the influence of topography on the characteristics of COLs over Southern Africa. Multi‐ensemble simulations from the Weather Research Forecasting (WRF) model were run to simulate three COLs over Southern Africa, using the real topography [CONTROL] and three idealized terrains (i.e., “no topography” [NoTOPO], “only west topography” [WTOPO] and “only east topography” [ETOPO]). A COL tracking algorithm was used to track COLs in the simulation dataset. The characteristics of the examined COLs include their formation, track, rainfall and vertical structure. The results show that topography influences the characteristics of COLs, although such influence varies with individual COLs. The influence of topography on COL formation is more pronounced on COLs that form in the vicinity of the sub‐continent than the ones that form far from the sub‐continent. Topography promotes uplift of warm moist air and initiation of deep convection, which may weaken the zonal strength of the westerly jet and allow for the formation of the COL. As the COLs approach topography, their tracks become more southerly. Topography decelerates the westerly wave systems, hence the system moves faster in NoTOPO experiments than in CONTROL experiments. While topography seems not to influence the timing of the peaks in rainfall during the COL lifecycle, it does influence the spatial distribution of rainfall. The presence of western topography enhances the rainfall over the western part of Southern Africa, whereas the presence of eastern topography reduces the rainfall over the western parts of Southern Africa. These results have applications in improving the forecasting of COLs over the complex terrain of Southern Africa.

Sounding Characteristics that Yield Significant Convective Inhibition Errors due to Ascent Rate and Sensor Response of In Situ Profiling Systems

AbstractAccurate measurements of the convective inhibition (CIN) associated with capping inversions are critical to forecasts of deep convection initiation. The goal of this work is to determine the sounding characteristics most vulnerable to CIN errors arising from hysteresis associated with sensor response and ascent rate of profiling systems. This examination uses 5058 steady-state analytic soundings prescribed using three free parameters that control inversion depth, static stability, and moisture content. A theoretical well-aspirated first-order sensor mounted on a platform that does not disturb its environment is “flown” in these soundings. Sounding characteristics that result in the largest relative CIN errors are also the characteristics that result in the smallest CIN. Because they are more likely to support deep convection initiation, it is particularly critical that environments with small CIN are represented accurately. The relationship between relative CIN error and CIN exists because sounding characteristics that contribute to large CIN do not proportionally increase the CIN error. Analysis also considers CIN intervals with (operationally important) CIN on the threshold between environments that will and will not support deep convection initiation. For these soundings, CIN error is found to be largest for deep, dry inversions characterized by small static stability.

The Forced Secondary Circulation of the Mei-yu Front

Using National Centers for Environmental Prediction reanalysis data for the period 28 June to 12 July during 2001 to 2013, the secondary circulation (SC) associated with the mei-yu front was quantitatively diagnosed by numerically solving a primitive version of the Sawyer-Eliassen equation. Results demonstrate that a direct SC exists near the mei-yu front zone during mid-summer and the synoptic-scale geostrophic deformations are the main factors determining SC structures. About 94% of the sinking strength and 61% of the ascending strength in the SC are induced by the geostrophic deformations. Other terms, such as diabatic heating, ageostrophic dynamical forcing, and frictional forcing, mainly influence the fine flow pattern of the SC. The forced SC produces a frontogenesis area tilting to the north with altitude. Further diagnosis clarifies the positive feedback involving the geostrophic shear forcing and vorticity frontogenesis in the upper-level mei-yu front zone. Furthermore, statistical results indicate that all 34 deep convection cases that occurred in the warm region of the meiyu front over the period 2004–2013 experienced high-level frontogenesis associated with along-jet cold advection. The cyclonic shear forcing “moved” the monsoon SC’s subsidence branch to the warm side of the mei-yu front and caused the subsidence branch to extend downwards to the lower troposphere, conducive to the initiation of deep convection in the warm region of the mei-yu front.