Low-level Shear Research Articles

Low-level shear stress induces differentiation of liver cancer stem cells via the Wnt/β-catenin signalling pathway

Shear stress-activated cellular signalling is involved in cellular migration, proliferation and differentiation. However, it remains unclear about the effects of shear stress on the differentiation of liver cancer stem cells (LCSCs). In this study, using a parallel plated flow chamber system, we found that low-shear stress loading (2 dyne/cm2) for 48 h could significantly inhibit the sphere-forming ability, increase the chemotherapeutic drug sensitivity, downregulate cancer stem cell marker expression and suppress the in vivo tumorigenicity potential of LCSCs. Moreover, using an atomic force microscope (AFM), we found that shear stress increased the Young's modulus of LCSCs. These results indicate LCSC's differentiation after shear stress exposure. In addition, low-shear stress loading suppressed the β-catenin expression in LCSCs. More importantly, pharmacological activation of Wnt/β-catenin signalling restored the shear stress-suppressed β-catenin expression in LCSCs, and abolished the shear stress-altered sphere-forming ability, chemotherapeutic drug sensitivity, cancer stem cell marker expression, Young's modulus and in vivo tumorigenicity potential of LCSCs. Our results suggest that low shear stress could induce LCSC differentiation via the Wnt/β-catenin signalling pathways, providing a new insight into the role of shear stress in cancer cell biological behaviors that might contribute to the development of new therapeutic strategies for liver cancer treatment.

Dynamical Insights into Extreme Short-Term Precipitation Associated with Supercells and Mesovortices

Abstract In some prominent extreme precipitation and flash flood events, radar and rain gauge observations have suggested that the heaviest short-term rainfall accumulations (up to 177 mm h−1) were associated with supercells or mesovortices embedded within larger convective systems. In this research, we aim to identify the influence that rotation has on the storm-scale processes associated with heavy precipitation. Numerical model simulations conducted herein were inspired by a rainfall event that occurred in central Texas in October 2015 where the most extreme rainfall accumulations were collocated with meso-β-scale vortices. Five total simulations were performed to test the sensitivity of precipitation processes to rotation. A control simulation, based on a wind profile from the aforementioned event, was compared with two experiments with successively weaker low-level shear. With greater environmental low-level shear, more precipitation fell, in both a point-maximum and an area-averaged sense. Intense, rotationally induced low-level vertical accelerations associated with the dynamic nonlinear perturbation vertical pressure gradient force were found to enhance the low- to midlevel updraft strength and total vertical mass flux and allowed access to otherwise inhibited sources of moisture and CAPE in the higher-shear simulations. The dynamical accelerations, which increased with the intensity of the low-level shear, dominated over buoyant accelerations in the low levels and were responsible for inducing more intense low-level updrafts that were sustained despite a stable boundary layer.

Low-level shear stress promotes migration of liver cancer stem cells via the FAK-ERK1/2 signalling pathway

Cancer stem cells (CSCs) are a small subpopulation of tumour cells that have been proposed to be responsible for cancer initiation, chemotherapy resistance and cancer recurrence. Shear stress activated cellular signalling is involved in cellular migration, proliferation and differentiation. However, little is known about the effects of shear stress on the migration of liver cancer stem cells (LCSCs). Here, we studied the effects of shear stress that are generated from a parallel plated flow chamber system, on LCSC migration and the activation of focal adhesion kinase (FAK) and extracellular signal regulated kinase1/2 (ERK1/2), using transwell assay and western blot, respectively. We found that 2 dyne/cm2 shear stress loading for 6 h promotes LCSC migration and activation of the FAK and ERK1/2 signalling pathways, whereas treatment with the FAK phosphorylation inhibitor PF573228 or the ERK1/2 phosphorylation inhibitor PD98059 suppressed the shear stress-promoted migration, indicating the involvement of FAK and ERK1/2 activation in shear stress-induced LCSC migration. Additionally, atomic force microscopy (AFM) analysis showed that shear stress lowers LCSC stiffness via the FAK and ERK1/2 pathways, suggesting that the mechanism by which shear stress promotes LCSC migration might partially be responsible for the decrease in cell stiffness. Further experiments focused on the role of the actin cytoskeleton, demonstrating that the F-actin filaments in LCSCs are less well-defined after shear stress treatment, providing an explanation for the reduction in cell stiffness and the promotion of cell migration. Overall, our study demonstrates that shear stress promotes LCSC migration through the activation of the FAK-ERK1/2 signalling pathways, which further results in a reduction of organized actin and softer cell bodies.

Effects of the Low-Level Wind Profile on Outflow Position and Near-Surface Vertical Vorticity in Simulated Supercell Thunderstorms

Abstract Supercell thunderstorms are simulated using an idealized numerical model to analyze the effects of modifications to the environmental low-level wind profile on near-surface rotation. Specifically, the orientation, magnitude, and depth of the low-level vertical wind shear are modified in several suites of experiments and compared to control simulations with no vertical wind shear in the prescribed layer. The overall morphology of the simulated supercells is highly sensitive to even shallow changes in the low-level wind profile. Moreover, maximum near-surface vertical vorticity varies as the low-level wind profile is modified. The results suggest this is principally a consequence of the degree to which favorable dynamic forcing of negatively buoyant outflow is superimposed upon the near-surface circulation maximum. Simulations with easterly shear and weaker storm-relative winds over the depth of the gust front promote forward-surging outflow and smaller separation between the near-surface circulation maximum and the mesocyclone aloft compared with other hodograph shapes. This promotes near-surface vertical vorticity intensification in these simulations. Similar trends in near-surface vertical vorticity as a function of low-level shear orientation are observed for varying shear-layer depths and bulk-shear magnitudes over the shear layer. The degree to which specific hodograph shapes promote strong near-surface rotation may vary with different deep-layer wind profiles or thermodynamic environments from those simulated here; however, this study concludes that favorable positioning of the near-surface circulation maximum and mesocyclone aloft are a necessary condition for supercell tornadogenesis and this positioning may be modulated by the low-level wind profile.

Severe Convective Storm Environments in Turkey

Severe convective storms occasionally result in loss of life and property in Turkey, a country not known for its severe convective weather. However, relatively little is known about the characteristics of Turkish severe weather environments. This paper documents these characteristics using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data on tornado and severe hail days in Turkey from 1979 to 2013. Severe storm environments are characterized by larger convective available potential energy (CAPE) in Turkey compared to the rest of Europe, but the CAPE values are less than those in typical U.S. severe storm environments. Severe hail is associated with large CAPE and vertical wind shear. Nonmesocyclonic tornadoes are associated with less CAPE compared with the other forms of severe weather. Deep-layer vertical wind shear is slightly weaker in Turkish supercell environments than in U.S. supercell environments, and Turkish tornadic supercell environments are characterized by much weaker low-level shear than in the United States and Europe, at least in the ECMWF reanalysis data. Composite parameters such as the supercell composite parameter (SCP) and energy–helicity index (EHI) can discriminate between very large hail and large hail environments.

Identification of favorable environments for thunderstorms in reanalysis data

The relations between lightning occurrence over Europe from the EUCLID network (2008–2013) and parameters derived from ERA-Interim reanalysis data were studied to increase the understanding of the conditions under which thunderstorms form. The objective was to identify relevant factors beyond instability and convective inhibition, in order to better model thunderstorms using numerical weather prediction or climate model data. It was found that latent instability is only required up to a certain amount of approximately 200–400 J kg-1 CAPE. For higher values of CAPE (∼ 800–2800 J kg-1), the relative frequency of lightning is rather constant. Relative humidity in the low to mid-troposphere has a major influence on storm occurrence with low relative humidity strongly suppressing thunderstorm development. For an average 850–500 hPa relative humidity below 50%, the frequency of lightning decreases to below 15%, even when CIN is negligible and CAPE sufficient. A subtle dependency on wind shear was found in which two regimes of higher frequency of lightning were identified. For very weak and for high shear the probability was higher than for intermediate values of both deep-layer and low-level shear.

How are heat waves over Yangtze River valley associated with atmospheric quasi-biweekly oscillation?

Over Yangtze River valley (YRV) where heat wave (HW) events most frequently occur in China during 1979–2014, 30 out of 57 HW events (nearly 55%) in July and August is found to be related with the dry phases of atmospheric quasi-biweekly oscillation (QBWO). When a significant low-level anticyclonic anomaly (LAA) associated with QBWO appears over YRV, temperature rises sharply according to the adiabatic heating caused by subsidence and the enhanced downward solar radiation due to decreased clouds. The LAA with subsidence over YRV is primarily generated by quasi-biweekly atmospheric waves, which are classified to three types through case-by-case categorization, named as “mid-latitude wavetrain”, “WNP (western North Pacific) wavetrain” and “double wavetrains”, respectively. The mid-latitude wavetrain QBWO causes the LAA through subsidence induced by upper-level cyclonic vorticity which is associated with an eastward/southeastward migrating wave train from Eastern Europe to WNP in the upper troposphere. The WNP wavetrain QBWO forms LAA through a northwestward migrating lower-tropospheric wave train emanating from tropical WNP to southeastern China. The double wavetrains QBWO triggers LAA through both the low-level shear anticyclonic vorticity provided by a low-level northwestward/westward propagating wave train from tropical WNP to South China Sea and the upper-level positive vorticity associated with an eastward/southeastward migrating wave train from Eastern Europe to southeastern China in the upper troposphere. In all cases, South Asian High extends eastward and WNP subtropical high extends westward during HW events. Tracing these distinct precursory circulation anomalies may facilitate better understanding and short-medium range forecast of HW.

Multi-Source Data Analyses of Processes of a Squall Line and the Gale Weather with Heavy Hails before the Squall Line

Through multi-source data analyses of regional automatic station data, wind profiling radar, lightning information, new-generation weather-radar echo data and conventional observations in Fujian Province, and others, this paper finds out aspects to focus on for potential forecasts and the nowcasting of wide-range gale weathers with thunderstorms and hail weathers in west Fujian Province on April 26th, 2016. Thus providing a basis for future forecasting of such catastrophic meteorological activities. Results of analyses showed that being affected by the eastward moving of upper troughs and the eastward and southward moving of low-level vortex shears, cold air and warm air strongly intersected in west Fujian Province; noticeable cold and warm advection formed the temperature frontal-zone; and under the triggering of the ground convergence line, impetuses formed by the strong convergent uplift before the south troughs forcibly produced wide-range squall lines, hails and other strong convective weathers. The squall line was caused by baroclinic frontogenesis. Hails with a diameter of up to 3 cm happened in the prefrontal warm zone. Supercells were generated and developed in the 80 km hot low-pressure convergence zone before the squall line moved along the direction at about 25° to the right of the mean wind field of the environment, belonged to right-shifting hailstorms, were of characteristics representing the hook echo and were of characteristics that the strength of the echo was high. On the afternoon of 26th, on the ground, temperature and humidity strongly increased. Fujian was located in the warm zone in the south side of the inverted trough. The specific humidity at 850 hPa was higher than 12 g/kg. The positive temperature change lasted for 24 hours. In the inland, the ridge was warm, while the trough was cold. Strong vertical wind shears that reached 20 m/s at 925 - 500 hPa and others were beneficial environmental conditions for forming the process of strong convective weather of baroclinic frontogenesis for this time.

Composite Environments of Severe and Nonsevere High-Shear, Low-CAPE Convective Events

Abstract Severe convection occurring in environments characterized by large amounts of vertical wind shear and limited instability (high-shear, low-CAPE, or “HSLC,” environments) represents a considerable forecasting and nowcasting challenge. Of particular concern, NWS products associated with HSLC convection have low probability of detection and high false alarm rates. Past studies of HSLC convection have examined features associated with single cases; the present work, through composites of numerous cases, illustrates the attributes of “typical” HSLC severe and nonsevere events and identifies features that discriminate between the two. HSLC severe events across the eastern United States typically occur in moist boundary layers within the warm sector or along the cold front of a strong surface cyclone, while those in the western United States have drier boundary layers and more typically occur in the vicinity of a surface triple point or in an upslope regime. The mean HSLC severe event is shown to exhibit stronger forcing for ascent at all levels than its nonsevere counterpart. The majority of EF1 or greater HSLC tornadoes are shown to occur in the southeastern United States, so this region is subjected to the most detailed statistical analysis. Beyond the documented forecasting skill of environmental lapse rates and low-level shear vector magnitude, it is shown that a proxy for the release of potential instability further enhances skill when attempting to identify potentially severe HSLC events. This enhancement is likely associated with the local, in situ CAPE generation provided by this mechanism. Modified forecast parameters including this proxy show considerably improved spatial focusing of the forecast severe threat when compared to existing metrics.

Explicit Forecasts of Low-Level Rotation from Convection-Allowing Models for Next-Day Tornado Prediction

Abstract Three diagnostic fields were examined to assess their ability to act as surrogates for tornadoes in a convection-allowing ensemble system run during the spring of 2015. The diagnostics included midlevel (2–5 km AGL) updraft helicity (UH25), low-level (0–3 km AGL) updraft helicity (UH03), and low-level (1 km AGL) vertical relative vorticity (RVORT1). RVORT1 was used as a direct measure of low-level rotation strength. Each storm’s RVORT1 magnitude and near-storm environment properties were extracted from each hour’s forecasts using an object-based approach. The near-storm environments of storm objects with large magnitudes of RVORT1 were very similar to the environments identified as conducive for the development of tornadic supercells in previous proximity sounding-based studies (e.g., low lifted condensation levels and strong low-level shear). This motivated the use of RVORT1 as a direct surrogate for tornadoes, without the need to filter forecasts with environmental information. The relationship between UH25 and UH03 was also explored among the simulated storms; UH03 only exceeded UH25 in storms occurring within low-CAPE/high-shear environments, while UH03 rarely exceeded UH25 in traditional supercell environments. Next-day ensemble surrogate severe probability forecasts (E-SSPFs) for tornadoes were generated using these diagnostics for 92 forecasts, with thresholds based on the number of observed tornado reports. E-SSPFs for tornadoes using RVORT1 and UH03 were more skillful than E-SSPFs using UH25. The UH25 E-SSPFs possessed little skill, regardless of threshold or smoothing length scale. All E-SSPFs suffered from poor sharpness at skillful scales, with few forecast probabilities greater than 40%.

Modifications to the Near-Storm Environment Induced by Simulated Supercell Thunderstorms

Abstract This study investigates the changes that simulated supercell thunderstorms impart on their surroundings. Supercells are simulated in a strongly sheared convective boundary layer comprising horizontal roll vortices. In sensitivity tests, the effects of cloud shading on the near-storm environment are explored through the removal of cloud ice, water, and hydrometeor effects on parameterized radiation. All of the simulated supercells increase the low-level shear in their proximal environment; however, this effect is more pronounced when cloud shading is included. Shading stabilizes the boundary layer beneath the cirrus anvil, diminishes boundary layer rolls and their attendant thermodynamic perturbations, and reduces the intensity of resolved turbulent mixing in the convective boundary layer. Anvil shading also acts to reduce the buoyancy of inflow air and the horizontal buoyancy gradient along the forward-flank outflow boundary.

A Statistical Analysis of the Effects of Vertical Wind Shear on Tropical Cyclone Intensity Change over the Western North Pacific

Abstract The effect of vertical wind shear (VWS) between different pressure levels on TC intensity change is statistically analyzed based on the best track data of tropical cyclones (TCs) in the western North Pacific (WNP) from the Joint Typhoon Warning Center (JTWC) and the ECMWF interim reanalysis (ERA-Interim) data during 1981–2013. Results show that the commonly used VWS measure between 200 and 850 hPa is less representative of the attenuating deep-layer shear effect than that between 300 and 1000 hPa. Moreover, the authors find that the low-level shear between 850 (or 700) and 1000 hPa is more negatively correlated with TC intensity change than any deep-layer shear during the active typhoon season, whereas deep-layer shear turns out to be more influential than low-level shear during the remaining less active seasons. Further analysis covering all seasons exhibits that a TC has a better chance to intensify than to decay when the deep-layer shear is lower than 7–9 m s−1 and the low-level shear is below 2.5 m s−1. The probability for TCs to intensify and undergo rapid intensification (RI) increases with decreasing VWS and increasing sea surface temperature (SST). TCs moving at slow translational speeds (less than 3 m s−1) intensify under relatively weaker VWS than TCs moving at intermediate translational speeds (3–8 m s−1). The probability of RI becomes lower than that of rapid decaying (RD) when the translational speed is larger than 8 m s−1. Most TCs tend to decay when the translational speed is larger than 12 m s−1 regardless of the shear condition.

Thermodynamic Constraints on the Morphology of Simulated Midlatitude Squall Lines

Abstract This study examines how environmental thermodynamics constrain the morphology of simulated idealized midlatitude squall lines (SLs). The thermodynamic soundings used for simulating various SLs are specified primarily by prescribed vertical profiles of the convective available potential energy (CAPE) and the level of free convection. This framework, which contemplates the latent instability properties of both low- and midtropospheric air, is considered to be convenient for investigating layer-lifting convective phenomena. Results show that frequently used CAPE indices are unsuitable for diagnosing SL characteristics, while integrated CAPE (ICAPE) discriminates the amplitude of the storm-induced heating for a given value of environmental shear. The skill of ICAPE follows from its relation to the buoyancy attained by low- and midtropospheric parcels as they ascend over the cold pool under layer-lifting convection. Environmental kinematics also affect the storm-induced heating, with stronger low-level shear leading to a greater proportion of inflowing latent unstable air among total storm-relative inflow, thus producing higher temperatures aloft. The precipitable water accounts for much of the precipitation-rate variation for a given value of shear. The precipitation efficiency is lower in environments with weaker shear and dryer midtropospheric conditions. Cold pool temperatures are slightly affected by environmental variations beneath the layer of minimum moist static energy, with drier midtropospheric conditions and weaker shear leading to warmer cold pools. SLs with a small vertical gradient of cold pool buoyancy propagate less rapidly and produce small surface wind speeds. Cold pool properties could be affected by a descending branch of the front-to-rear flow, which crosses over with the rear inflow jet.

Closure to “Experimental Investigation of Pullout Behavior of Fiber-Reinforced Polymer Reinforcements in Sand” by Cheng-Cheng Zhang, Hong-Hu Zhu, Bin Shi, Fang-Dong Wu, and Jian-Hua Yin

Closure to “Experimental Investigation of Pullout Behavior of Fiber-Reinforced Polymer Reinforcements in Sand” by Cheng-Cheng Zhang, Hong-Hu Zhu, Bin Shi, Fang-Dong Wu, and Jian-Hua Yin

Sensitivity of 24-h Forecast Dryline Position and Structure to Boundary Layer Parameterizations in Convection-Allowing WRF Model Simulations

Abstract Recent NOAA Hazardous Weather Testbed Spring Forecasting Experiments have emphasized the sensitivity of forecast sensible weather fields to how boundary layer processes are represented in the Weather Research and Forecasting (WRF) Model. Thus, since 2010, the Center for Analysis and Prediction of Storms has configured at least three members of their WRF-based Storm-Scale Ensemble Forecast (SSEF) system specifically for examination of sensitivities to parameterizations of turbulent mixing, including the Mellor–Yamada–Janjić (MYJ); quasi-normal scale elimination (QNSE); Asymmetrical Convective Model, version 2 (ACM2); Yonsei University (YSU); and Mellor–Yamada–Nakanishi–Niino (MYNN) schemes (hereafter PBL members). In postexperiment analyses, significant differences in forecast boundary layer structure and evolution have been observed, and for preconvective environments MYNN was found to have a superior depiction of temperature and moisture profiles. This study evaluates the 24-h forecast dryline positions in the SSEF system PBL members during the period April–June 2010–12 and documents sensitivities of the vertical distribution of thermodynamic and kinematic variables in near-dryline environments. Main results include the following. Despite having superior temperature and moisture profiles, as indicated by a previous study, MYNN was one of the worst-performing PBL members, exhibiting large eastward errors in forecast dryline position. During April–June 2010–11, a dry bias in the North American Mesoscale Forecast System (NAM) initial conditions largely contributed to eastward dryline errors in all PBL members. An upgrade to the NAM and assimilation system in October 2011 apparently fixed the dry bias, reducing eastward errors. Large sensitivities of CAPE and low-level shear to the PBL schemes were found, which were largest between 1.0° and 3.0° to the east of drylines. Finally, modifications to YSU to decrease vertical mixing and mitigate its warm and dry bias greatly reduced eastward dryline errors.

Impacts of Increasing Low-Level Shear on Supercells during the Early Evening Transition*

Abstract The dynamical response of simulated supercells to temporally increasing lower-tropospheric vertical wind shear is investigated using idealized simulations. These simulations are based upon observed soundings from two cases that underwent an early evening transition during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Mature supercells were simulated in observed afternoon environments with moderate vertical wind shear and then compared to simulated supercells experiencing observed evening increases in lower-tropospheric shear. The primary effect of the increase in low-level shear is to establish larger values of vertical vorticity at lower altitudes in the storm’s updraft. In turn, this leads to a nonlinear increase in the updraft strength due to the enhanced dynamic pressure minimum associated with larger vorticity in the storm’s mesocyclone. This is particularly important at low levels, where it increases the storm's ability to lift cool surface air (including outflow). Trajectories launched in developing vortices show that, despite comparable buoyant accelerations, parcels experience greater vertical velocity and stretching of vertical vorticity due to increased dynamic accelerations when the low-level shear is increased. Thus, even as low-level stability gradually increases in the early evening, the supercells’ low-level updraft intensity and surface vorticity production can increase. These results are consistent with climatological observations of a supercell’s likelihood of tornadogenesis during the early evening hours.

Study on the Relationship Between Mesoscale Wind Field Changes and Rainstorm Using Windprofiler Data

By using the windprofiler data,this paper studies the relationship betw een mesoscale wind field changes and rainstorm. Firstly,it is indicated that the high spatial and temporal resolution data can clearly reveal the effect of mesoscale wind filed changes such as low-level jet and low-level wind shear before the heavy precipitation happens. Before the heavy precipitation happens,the strong wind field and the momentum continuously go dow nstream,making the low-level positive vorticity increase,with the warm and humid air,trigger the precipitation. Secondly,an index I is designed to represent the intensity and height of the low-level jet,the index will increase rapidly before the heavy precipitation. Thirdly,the low-level vertical wind shear increases significantly,and it plays a significant role in triggering the generation of heavy precipitation.

Analysis of Predictability on a High-Altitude Hail/Graupel Disaster Weather in Nanjing Airspace

Synoptic analysis,radar observation and numerical simulation are used to analyze a convection weather process occurred in Nanjing airspace on 10 August 2011 w hich causing aero damage. All analysis support a deduction that this damage caused by hail strike,but there w as no hail observed on the ground. Since hail generates on high level of about thousands of meters,it may melt in the falling process and rain on the ground. Nanjing w as northw estw ard to the subtropical high pressure zone and in the southw est flow before the trough of Hetao low pressure area at that day. It w as near a low level shear line and had sufficient convective potential condition.Dry cold air distributed on upper level and w arm w et air distributed on low er level. This structure is benefit to the generation of hail. Analysis of radar observation show s that this convective system developed very high. Reflectivity in the core w as very strong. VIL and hail index all had high value. And there w ere meso-cyclone and middle altitude radial convergence in this system. The dynamic and flow structure of this system w ere corresponding to typical hail storm. This paper indicates that this high-altitude hail process w as obviously predicted.

A Study of the Role of Daytime Land–Atmosphere Interactions on Nocturnal Convective Activity in the Southern Great Plains during CLASIC

Abstract This study examines whether and how land–atmosphere interactions can have an impact on nocturnal convection over the southern Great Plains (SGP) through numerical simulations of an intense nocturnal mesoscale convective system (MCS) on 19–20 June 2007 with the Weather Research and Forecasting (WRF) Model. High-resolution nested simulations were conducted using realistic and idealized land surfaces and two planetary boundary layer (PBL) parameterizations (PBLp): Yonsei University (YSU) and Mellor–Yamada–Janjić (MYJ). Differences in timing and amount of MCS precipitation among observations and model results were examined in the light of daytime land–atmosphere interactions, nocturnal prestorm environment, and cold pool strength. At the meso-γ scale, land cover and soil type have as much of an effect on the simulated prestorm environment as the choice of PBLp: MYJ simulations exhibit strong sensitivity to changes in the land surface in contrast to negligible impact in the case of YSU. At the end of the afternoon, as the boundary layer collapses, a more homogeneous and deeper PBL (and stronger low-level shear) is evident for YSU as compared to MYJ when initial conditions and land surface properties are the same. At the meso-β scale, propagation speed is faster and organization (bow echo morphology) and cold pool strength are enhanced when nocturnal PBL heights are higher, and there is stronger low-level shear in the prestorm environment independent of the boundary layer parameterization for different land surface conditions. A comparison of one- and two-way nested MYJ results demonstrates how daytime land–atmosphere interactions modify the prestorm environment remotely through advection of low-level thermodynamic features. This remote feedback strongly impacts the MCS development phase as well as its spatial organization and propagation velocity and, consequently, nocturnal rainfall. These results indicate that synoptic- and meso-α-scale dynamics can play an important role in determining the spatial and temporal scales over which precipitation feedbacks of land–atmosphere interactions emerge regionally. Finally, this study demonstrates the high degree of uncertainty in defining the spatial and temporal scales of land–atmosphere interactions where and when organized convection is dominant.

Increased tornado hazard in large metropolitan areas

The tornado climate was compared between large metropolitan areas and neighbouring non-metro cities using modern tornado reports in the Storm Prediction Center (SPC) archives. Twenty large metro areas in the higher-risk region of the U.S. were used to boost sample sizes hence robustness of results. Observational biases were minimised by using the most densely populated zips. The analysis found 50% greater tornado frequency and a thicker-tailed severity distribution in metro areas compared to the non-metro cities. These differences are significant at the 1% level.Regarding tornado frequency, the primary question is whether the raised occurrence rates in metro areas are due to observation biases or real differences in tornado climate. Past studies found no relative biases at the population densities used here, whereas there are two potential urban drivers of tornadogenesis. First, the urban heat island raises the storm severity above and downwind of main urban areas, as recorded in precipitation and lightning datasets. Second, the increased surface roughness over metro areas raises low-level shear which in turn has been found to be favourable for tornadogenesis. Modification of convective storms over large metro areas is the more plausible explanation of raised tornado frequency.The drivers of a thicker-tailed tornado severity distribution in metro areas are less certain. Potential causes include: increased debris-loading in metro tornadoes; modification of storms' lower boundary layer by increased surface roughness in metro areas; the reduced density of damage indicators in non-metro cities.