Cold Pool Intensity Research Articles

Structure and Maintenance of a Quasi-Linear Mesoscale Convective System That Caused Torrential Rain in Southern Kyushu, Japan, on 11 July 2021

Abstract A quasi-linear mesoscale convective system that remained nearly stationary (hereafter referred to as a stationary QLCS) for almost 8 h in southern Kyushu, Japan, caused torrential rainfall exceeding 350 mm on 11 July 2021. The stationary QLCS consisted of several rainbands organized by back-building convection. As the cold pool intensified, the system attained a more widespread structure, leaning on the upshear side. To elucidate the mechanism responsible for the upshear tilt, numerical simulations with 250-m horizontal grid spacing were conducted, including sensitivity experiments in which the evaporative cooling rates from rain were reduced to modify the cold pool intensity. Results show that the cold pool is critical to the organization of the QLCS, and the structure normal to it is mainly governed by the balance between the low-level shear magnitude and cold pool intensity, supporting the application of so-called Rotunno–Klemp–Weisman (RKW) theory to this event. The cause of the weak updraft that accompanies the leaning system over the strong cold pool was also investigated, analyzing the trajectories, vertical momentum equation, and pressure perturbation field using the anelastic equation. It is revealed that the updraft travels a longer distance through the tilted system experiencing more mixing with the ambient air, which results in less thermal buoyancy. In addition, the updraft is decelerated by the downward perturbation gradient force owing to the vertical buoyancy gradient around the sloping surface of the cold pool and the dynamical effect caused by the baroclinicity-associated strong horizontal vorticity around the cold pool leading edge.

The Role of Changbai Mountain in an Extreme Precipitation Event in Liaoning Province, China

Based on the half-hourly Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) data product (0.1°), the fifth-generation European Center for Medium-Range Weather Forecasting atmospheric reanalysis dataset (ERA5), sounding data, and the Weather Research and Forecasting Model (WRF-ARW), this study explored the developmental process of a typical extreme precipitation event in Liaoning Province on 2 June 2021. This study focused on the impact of Changbai Mountain on this precipitation process and its corresponding physical mechanisms. The research findings revealed that Changbai Mountain significantly affected the precipitation event in three main aspects: blocking drag, forcing uplift, and leeside convergence. The blocking drag caused by the mountain topography led to an extension in the duration of heavy rainfall. The dynamic lifting and leeside convergence associated with the mountainous terrain also substantially increased the amount of precipitation. Furthermore, the topography hindered the movement of the Bohai Bay cold pool and enhanced the intensity of the cold pool, contributing to the sustained extreme precipitation in Liaoning Province. Lastly, the terrain sensitivity experiment demonstrated that when the height of Changbai Mountain was reduced, the convergence uplift, moisture condensation, and cold pool intensity were weakened, leading to significant changes in precipitation intensity and spatial distribution. These findings further confirm the crucial role of Changbai Mountain in the occurrence and development of local precipitation in Liaoning Province.

High‐resolution simulations of tropical island thunderstorms: Does an increase in resolution improve the representation of extreme rainfall?

AbstractRecent increases in computational resources have led to the application of kilometre‐ and sub‐kilometre‐scale simulations in research, numerical weather prediction, and climate modelling alike. Despite anticipated improvements with resolution, there is still considerable work needed to evaluate how well such models improve the representation of intense convection. In this study we conduct ensemble simulations with kilometre‐ and sub‐kilometre‐scale horizontal grids to investigate intense convective events in the tropical island thunderstorm system Hector, which frequently occurs over the Tiwi Islands in North Australia. To avoid losing information through spatio‐temporal averaging we apply a tracking algorithm to simulated and observed storms. When compared with observations, the model storms exhibit a lack of propagation across the study domain. In general, simulated storms are too intense but too small and too short‐lived. This is especially true for the sub‐kilometre simulations, where storms are more intense, smaller, and more numerous than in the kilometre‐scale counterparts. We argue that size and duration errors compensate for storm number and intensity errors, which could lead to misleading interpretations when only comparing time and space averages of rainfall fields. Investigating some properties of the simulated storms suggests that storms with high rainfall intensities have stronger updrafts in the sub‐kilometre model and are accompanied by an increase in cold pool intensity. The results and their resolution sensitivities highlight that the remaining parametrisations and their many tuning parameters in high‐resolution set‐ups influence the representation of convective storms in such models.

The Influence of the Diurnal Cycle in Wind Shear and Thermodynamics on Squall Lines in the West African Monsoon

Abstract The West African monsoon has a clear diurnal cycle in boundary layer properties, synoptic flow, and moist convection. A nocturnal low-level jet (LLJ) brings cool, moist air into the continent and we hypothesize that it may support storms by providing vertical wind shear and a source of moisture. We use idealized simulations to investigate how the mean diurnal cycle in temperature and humidity compared with that of the wind shear impacts on mature squall lines. Thermodynamic diurnal changes dominate those of the winds, although when isolated the LLJ wind is favorable for more intense systems. Bulk characteristics of the storms, including in-cloud upward mass flux and—if precipitation evaporation is accounted for—total surface rain rates, correlate well with the system-relative inflow of convectively unstable air and moisture into the storms. Mean updraft speeds and mean rainfall rates over the storms do not correlate as well with system-relative inflows due to variations in storm morphology such as cold pool intensity. We note that storms tend to move near the speed of the African easterly jet and so maximize the inflow of convectively unstable air. Our results explain the observed diurnal cycle in organized moist convection, with the hours from 1800 to 0000 UTC being the most favorable. Storms are more likely to die after this, despite the LLJ supporting them, with the environment becoming more favorable again by midday. Significance Statement Large organized storms dominate rainfall in the West African Sahel, but models struggle to predict them at the correct time of day and the underlying mechanisms that control their timings are not well understood. Using idealized simulations, we show that the temperature and humidity of the late evening are favorable for such storms whereas inflow from the low-level jet supports storms overnight. Storm inflows of available energy and moisture predict upward mass transport and total rainfall rates, whereas the strength of the storm’s cold pool is important for storm structure and intensity. Our results demonstrate how the environmental wind profile (which varies throughout the day) interacts with internal storm dynamics, posing a major challenge to parameterized models.

Rain, Wind, and Dust Connections in the Sahel

AbstractThe Sahel is a dust source region where dust emission could be drastically modified in the future due to climatic and land use changes. Based on observations of meteorological parameters and dust concentration for about 1,000 rain events, we investigated the processes leading to dust emission during the rainy season when Mesoscale Convective Systems (MCSs) regularly cross the Sahel. We show that the highest wind speed is strongly linked to the MCS cold pool intensity, which is characterized by a drop in surface temperature. This is observed during the premonsoon period (∼May to June) when the midtroposphere is still sufficiently dry to allow intense evaporation of raindrops. Because this coincides with the time of the year that the surface is the least protected by the vegetative residue, the premonsoon wind speed leads to the highest observed dust concentration in our record. Most of the highest wind speed occur before or just at the beginning of a rainy event allowing a large part of the dust raised to be transported ahead the rain limiting dust removal by wet scavenging. Finally, we show that the number of 5‐min dust concentration higher than 5,000 μg m−3 is almost only occurring during the rainy season. These results suggest that until the dust models fail to correctly resolve MCS, it will be difficulty to obtain reliable estimates of dust emission from the Sahel for the present or future scenarios.

Land Surface Effects on Shear Balance of Squall Lines

AbstractWe explore whether and how the land characteristics, especially surface drag, affect the squall line, in an idealized modeling framework. We find that the balance between the ambient low‐level shear and cold‐pool‐induced shear in the squall line can be modified by surface drag: it not only changes the ambient shear if near‐surface environmental wind is nonzero, but also reduces the cold‐pool‐induced shear by both weakening the low‐level wind and deepening the cold pool. The land surface impact on squall lines is larger with lower convective available potential energy (CAPE), partly because of stronger convection with higher CAPE. The cold pool intensity (vertically integrated negative buoyancy) could not explain the reduced cold pool shear induced by the land. Furthermore, land characteristics—such as roughness lengths—help maintain the shear balance and potentially make the squall line long‐lived, suggesting that they be included in convective (or cold pool) parameterizations to improve climate and Earth system models.

Comparison of three microphysics parameterization schemes in the WRF model for an extreme rainfall event in the coastal metropolitan City of Guangzhou, China

Abstract An extreme rainfall event in the coastal metropolitan city of Guangzhou, China is simulated by the Weather Research and Forecasting (WRF) model using three bulk microphysics schemes to explore the capability to reproduce the observed precipitation features by these schemes and their differences. The detailed comparison among the three runs in terms of radar reflectivity, precipitation, thermodynamic fields and microphysical processes are conducted. Results show that all the simulations can reproduce the two main heavy rainfall centers in Guangzhou and the first convection initiation. The accumulated precipitation in the simulation using the WSM6 scheme performs better than the others in terms of intensity and distribution compared to observations. The weaker accumulated precipitation in the second heavy rainfall center in the simulations using the Thompson and Morrison schemes result from their more dispersed precipitation distributions dominated by the cold pool intensity and distribution. The latent heating from the water vapor condensation dominates the convection initiation and storm development. The latent cooling from the rain water evaporation dominates the cold pool intensity and distribution, which influences the storm moving and subsequent convection propagation, and finally the intensity and distribution of surface precipitation. Sensitivity experiments of the latent heat confirm the dominant roles of latent heating/cooling, especially the water vapor condensation heating and rain water evaporation cooling, in the differences of the thermodynamic fields, storm development, convection propagation and surface precipitation among the three simulations.

Dynamics of Lower-Tropospheric Vorticity in Idealized Simulations of Tropical Cyclone Formation

Abstract Idealized simulations are conducted using the Cloud Model version 1 (CM1) to explore the mechanism of tropical cyclone (TC) genesis from a preexisting midtropospheric vortex that forms in radiative–convective equilibrium. With lower-tropospheric air approaching near saturation during TC genesis, convective cells become stronger, along with the intensifying updrafts and downdrafts and the larger area coverage of updrafts relative to downdrafts. Consequently, the low-level vertical mass flux increases, inducing vorticity amplification above the boundary layer. Of interest is that while surface cold pools help organize lower-tropospheric updrafts, genesis still proceeds, only slightly delayed, if subcloud evaporation cooling and cold pool intensity are drastically reduced. More detrimental is the disruption of near saturation through the introduction of weak vertical wind shear. The lower-tropospheric dry air suppresses the strengthening of convection, leading to weaker upward mass flux and much slower near-surface vortex spinup. We also find that surface spinup is similarly inhibited by decreasing surface drag despite the existence of a nearly saturated column, whereas larger drag accelerates spinup. Increased vorticity above the boundary layer is followed by the emergence of a horizontal pressure gradient through the depth of the boundary layer. Then the corresponding convergence resulting from the gradient imbalance in the frictional boundary layer causes vorticity amplification near the surface. It is suggested that near saturation in the lower troposphere is critical for increasing the mass flux and vorticity just above the boundary layer, but it is necessary yet insufficient because the spinup is strongly governed by boundary layer dynamics.

Lightning data assimilation with comprehensively nudging water contents at cloud-resolving scale using WRF model

A new lightning data assimilation (LDA) scheme comprehensively nudging water contents in the WRF model is developed at cloud-resolving scale, which takes the dynamical and thermodynamic conditions into consideration and nudges the low-level water vapor and graupel mass within the mixed-phase region according to the detected total lightning flash rate and model environments (here named C18). The main nudging functions of the LDA scheme in this work are modified based on the work of Fierro et al. (2012, hereafter F12) and Qie et al. (2014, hereafter Q14). To evaluate the application of LDA in high-impact weather simulations and quantitative precipitation forecasts, the three LDA schemes, including F12, Q14 and C18, are tested and compared for two severe squall line cases occurred over the Beijing metropolitan region (BMR). Benefiting from assimilation of lightning data, the simulated storm structure and surface cold pool are improved and closer to the observations for all the three schemes than the control run without LDA. For the F12 scheme, although the simulated thunderstorms are characterized by wide spread stratiform clouds and relatively weaker cold pool intensity and coverage, the positive impacts on both storm evolution and precipitation endures most prominently. For the Q14 scheme, intense downdrafts lead to the strongest cold pool and earlier dissipation of storms comparing with the observations. For the C18 scheme, increased graupel mass affect the cold-cloud processes leading to an earlier onset of rainfall, and the simulated surface cold pool is in better agreement with the observations. Profiting from more moisture added into the lower layers in C18, the storm develops more intensively toward higher altitude and sustains its development over a longer period after LDA, resulting in the best quantitative precipitation forecasts consequently.

Cloud‐resolving model intercomparison of an MC3E squall line case: Part I—Convective updrafts

AbstractAn intercomparison study of a midlatitude mesoscale squall line is performed using the Weather Research and Forecasting (WRF) model at 1 km horizontal grid spacing with eight different cloud microphysics schemes to investigate processes that contribute to the large variability in simulated cloud and precipitation properties. All simulations tend to produce a wider area of high radar reflectivity (Ze > 45 dBZ) than observed but a much narrower stratiform area. The magnitude of the virtual potential temperature drop associated with the gust front passage is similar in simulations and observations, while the pressure rise and peak wind speed are smaller than observed, possibly suggesting that simulated cold pools are shallower than observed. Most of the microphysics schemes overestimate vertical velocity and Ze in convective updrafts as compared with observational retrievals. Simulated precipitation rates and updraft velocities have significant variability across the eight schemes, even in this strongly dynamically driven system. Differences in simulated updraft velocity correlate well with differences in simulated buoyancy and low‐level vertical perturbation pressure gradient, which appears related to cold pool intensity that is controlled by the evaporation rate. Simulations with stronger updrafts have a more optimal convective state, with stronger cold pools, ambient low‐level vertical wind shear, and rear‐inflow jets. Updraft velocity variability between schemes is mainly controlled by differences in simulated ice‐related processes, which impact the overall latent heating rate, whereas surface rainfall variability increases in no‐ice simulations mainly because of scheme differences in collision‐coalescence parameterizations.

Cold pool dissipation

AbstractThe mechanisms by which sensible heat fluxes (SHFs) alter cold pool characteristics and dissipation rates are investigated in this study using idealized two‐dimensional numerical simulations and an environment representative of daytime, dry, continental conditions. Simulations are performed with no SHFs, SHFs calculated using a bulk formula, and constant SHFs for model resolutions with horizontal (vertical) grid spacings ranging from 50 m (25 m) to 400 m (200 m). In the highest resolution simulations, turbulent entrainment of environmental air into the cold pool is an important mechanism for dissipation in the absence of SHFs. Including SHFs enhances cold pool dissipation rates, but the processes responsible for the enhanced dissipation differ depending on the SHF formulation. The bulk SHFs increase the near‐surface cold pool temperatures, but their effects on the overall cold pool characteristics are small, while the constant SHFs influence the near‐surface environmental stability and the turbulent entrainment rates into the cold pool. The changes to the entrainment rates are found to be the most significant of the SHF effects on cold pool dissipation. SHFs may also influence the timing of cold pool‐induced convective initiation by altering the environmental stability and the cold pool intensity. As the model resolution is coarsened, cold pool dissipation is found to be less sensitive to SHFs. Furthermore, the coarser resolution simulations not only poorly but sometimes wrongly represent the SHF impacts on the cold pools. Recommendations are made regarding simulating the interaction of cold pools with convection and the land surface in cloud‐resolving models.

Diagnosing the Intercept Parameters of the Exponential Drop Size Distributions in a Single-Moment Microphysics Scheme and Impact on Supercell Storm Simulations

AbstractIn this study, power-law relations are developed between the intercept parameter N0 of the exponential particle size distribution and the water content for the rain, hail, graupel, and snow hydrometeor categories within the Milbrandt and Yau microphysics scheme. Simulations of the 3 May 1999 Oklahoma tornadic supercell are performed using the diagnostic relations for rain only and alternately for all four precipitating species, and results are compared with those from the original fixed-N0 single- and double-moment versions of the scheme. Diagnosing N0 for rain is found to improve the results of the simulation in terms of reproducing the key features of the double-moment simulation while still retaining the computational efficiency of a single-moment scheme. Results more consistent with the double-moment scheme are seen in the general storm structure, the cold-pool structure and intensity, and the number concentration fields. Diagnosing the intercept parameters for all four species, including those for the ice species, within the single-moment scheme yields even closer agreement with the double-moment simulation results. The decreased cold-pool intensity is very similar to that produced by the double-moment simulation, as is the areal extent of the simulated storm. The diagnostic relations are also tested on a simulated squall line, with similar promising results. This study suggests that, when compared with traditional fixed intercept parameters used in typical single-moment microphysics schemes, results closer to a double-moment scheme can be obtained through the use of diagnostic relations for the parameters of the particle size distribution, with little extra computational cost.

Variability in surface meteorology and air‐sea fluxes due to cumulus convective systems observed during CINDY/DYNAMO

AbstractThis study examines the variability in surface meteorological parameters and air‐sea heat fluxes due to cold pools emanating from cumulus convective systems observed over the tropical Indian Ocean in November 2011. In particular, this study focuses on convective systems that are spatially smaller than mesoscale convective systems in a southeasterly trade wind environment. Composite analyses of convectively active periods show an increase in the sensible heat flux by 15–20 W m−2 that is primarily attributed to an increase in the difference between the surface air temperature and sea surface temperature and an increase in the latent heat flux by 30–70 W m−2 due to enhanced surface wind speeds. A succession of convectively active periods leads to a greater influence than those occurring independently. The direction of the surface wind velocity anomaly due to cold pools tends to be close to that of the environmental wind velocity, resulting in an efficient enhancement of wind speed. This study also demonstrates the close relation between cold pool intensities and convective activity. In particular, two measures of cold pool intensity, a minimum surface air temperature and a maximum amount of surface wind speed enhancement, are correlated with each other and with the convective activity around the observation point measured by radar‐estimated rainfall and radar echo coverage.

Squall-Line Intensification via Hydrometeor Recirculation

Abstract Many studies have demonstrated the intimate connection between microphysics and deep moist convection, especially for squall lines via cold pool pathways. The present study examines four numerically simulated idealized squall lines using the Regional Atmospheric Modeling System (RAMS) and includes a control simulation that uses full two-moment microphysics and three sensitivity experiments that vary the mean diameter of the hail hydrometeor size distribution. Results suggest that a circulation centered at the freezing level supports midlevel convective updraft invigoration through increased latent heating. The circulation begins with hail hydrometeors that initiate within the convective updraft above the freezing level and are then ejected upshear because of the front-to-rear flow of the squall line. As the hail falls below the freezing level, the rear-inflow jet (RIJ) advects the hail hydrometeors downshear and into the upshear flank of the midlevel convective updraft. Because the advection occurs below the freezing level, some of the hail melts and sheds raindrops. The addition of hail and rain to the updraft increases latent heating owing to both an enhancement in riming and vapor deposition onto hail and rain. The increase in latent heating enhances buoyancy within the updraft, which leads to an increase in precipitation and cold pool intensity that promote a positive feedback on squall-line strength. The upshear-tilted simulated squall lines in this study indicate that as hail size is decreased, squall lines are invigorated through the recirculation mechanism.

Sensitivity of a Simulated Midlatitude Squall Line to Parameterization of Raindrop Breakup

Abstract This paper describes idealized simulations of a squall line observed on 20 June 2007, in central Oklahoma. Results are compared with measurements from dual-polarization radar and surface disdrometer. The baseline model configuration qualitatively reproduces key storm features, but underpredicts precipitation rates and generally overpredicts median volume raindrop diameter. The sensitivity of model simulations to parameterization of raindrop breakup is tested under different low-level (0–2.5 km) environmental vertical wind shears. Storm characteristics exhibit considerable sensitivity to the parameterization of breakup, especially for moderate (0.0048 s−1) shear. Simulations with more efficient breakup tend to have higher domain-mean precipitation rates under both moderate and higher (0.0064 s−1) shear, despite the smaller mean drop size and hence lower mass-weighted fall speed and higher evaporation rate for a given rainwater content. In these runs, higher evaporation leads to stronger cold pools, faster propagation, larger storm size, greater updraft mass flux (but weaker convective updrafts at mid- and upper levels), and greater total condensation that compensates for the increased evaporation to give more surface precipitation. The impact of drop breakup on mass-weighted fall speed is also important and leads to a nonmonotonic response of storm characteristics (surface precipitation, cold pool strength, etc.) to changes in breakup efficiency under moderate wind shear. In contrast, the response is generally monotonic at higher wind shear. Interactions between drop breakup, convective dynamics, cold pool intensity, and low-level environmental wind shear are also described in the context of “Rotunno–Klemp–Weisman (RKW) theory,” which addresses how density currents evolve in sheared environments.

Surface Cold Pools in the Outer Rainbands of Tropical Storm Hanna (2008) Near Landfall

Surface mesonet observations were used to document the structure of prominent cold pools associated with two convective outer rainbands of Tropical Storm Hanna on 5 September 2008. The developing rainbands were located ~400 km north of the storm center as they crossed the Carolina coastline and passed over the mesonet. The combination of moderate CAPE, moderate low-level cross-band vertical shear, and dry midlevel air provided an environment supportive of surface cold pool formation and long-lived quasi-linear convection. Both rainbands exhibited multiple outward-tilting convective cells and discrete line segments centered within a narrow zone of nearly continuous stratiform precipitation. The mesonet observations provided a unique along- and cross-band perspective of discrete cold pools situated beneath and behind the convection portions of each rainband. Prominent cold pools extended 40–80 km behind the rainband’s leading edge and exhibited maximum potential temperature, mixing ratio, and equivalent potential temperature deficits of 2–4 K, 1–2 g kg−1, and 4–10 K, respectively. In the cross-band direction, convergence of storm-relative inflow along the cold pool leading edge was coincident with a modest high pressure anomaly, while inflow divergence prevailed through the cold pool and rainfall maxima. Several cold pools expanded along-band while being advected downband by the prevailing cyclonic flow. Cold pool wakes were observed more than 50 km behind the rainband leading edge and up to 20 km downband from intense convection. Variations in cold pool intensity were not well correlated with convective intensity, rainfall rate, or the degree of midlevel dryness. Implications of prominent cold pools on tropical cyclone convection, size, and intensity are discussed.

Observations of a Squall Line and Its Near Environment Using High-Frequency Rawinsonde Launches during VORTEX2

Abstract Rawinsonde data were collected before and during passage of a squall line in Oklahoma on 15 May 2009 during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Nine soundings were released within 3 h, allowing for unprecedented analysis of the squall line’s internal structure and nearby environment. Four soundings were released in the prestorm environment and they document the following features: low-level cooling associated with the reduction of solar isolation by a cirrus anvil; abrupt warming (1.5 K in 30 min) above the boundary layer, which is probably attributable to a gravity wave; increases in both low-level and deep-layer vertical wind shear within 100 km of the squall line; and evidence of ascent extending at least 75 km ahead of the squall line. The next sounding was released ∼5 km ahead of the squall line’s gust front; it documented a moist absolutely unstable layer within a 2-km-deep layer of ascent, with vertical air velocity of approximately 6 m s−1. Another sounding was released after the gust front passed but before precipitation began; this sounding showed the cold pool to be ∼4 km deep, with a cold pool intensity C ≈ 35 m s−1, even though this sounding was located only 8 km behind the surface gust front. The final three soundings were released in the trailing stratiform region of the squall line, and they showed typical features such as: “onion”-shaped soundings, nearly uniform equivalent potential temperature over a deep layer, and an elevated rear inflow jet. The cold pool was 4.7 km deep in the trailing stratiform region, and extended ∼1 km above the melting level, suggesting that sublimation was a contributor to cold pool development. A mesoscale analysis of the sounding data shows an upshear tilt to the squall line, which is consistent with the cold pool intensity C being much larger than a measure of environmental vertical wind shear ΔU. This dataset should be useful for evaluating cloud-scale numerical model simulations and analytic theory, but the authors argue that additional observations of this type should be collected in future field projects.

The Impact of Spatial Variations of Low-Level Stability on the Life Cycle of a Simulated Supercell Storm

Abstract This study reports on the dynamical evolution of simulated, long-lived right-moving supercell storms in a high-CAPE, strongly sheared mesoscale environment, which initiate in a weakly capped region and subsequently move into a cold boundary layer (BL) and inversion region before dissipating. The storm simulations realistically approximate the main morphological features and evolution of the 22 May 1981 Binger, Oklahoma, supercell storm by employing time-varying inflow lateral boundary conditions for the storm-relative moving grid, which in turn are prescribed from a parent, fixed steady-state mesoscale analysis to approximate the observed inversion region to the east of the dryline on that day. A series of full life cycle storm simulations have been performed in which the magnitude of boundary layer coldness and the convective inhibition are varied to examine the ability of the storm to regenerate and sustain its main updraft as it moves into environments with increasing convective stability. The analysis of the simulations employs an empirical expression for the theoretical speed of the right-forward-flank outflow boundary relative to the ambient, low-level storm inflow that is consistent with simulated cold-pool boundary movement. The theoretical outflow boundary speed in the direction opposite to the ambient flow increases with an increasing cold-pool temperature deficit relative to the ambient BL temperature, and it decreases as ambient wind speed increases. The right-moving, classic (CL) phase of the simulated supercells is supported by increasing precipitation content and a stronger cold pool, which increases the right-moving cold-pool boundary speed against the constant ambient BL winds. The subsequent decrease of the ambient BL temperature with eastward storm movement decreases the cold-pool temperature deficit and reduces the outflow boundary speed against the ambient winds, progressing through a state of stagnation to an ultimate retrogression of the outflow boundary in the direction of the ambient flow. Onset of a transient, left-moving low-precipitation (LP) phase is initiated as the storm redevelops on the retrograding outflow boundary. The left-moving LP storm induces compensating downward motions in the inversion layer that desiccates the inflow, elevates the cloudy updraft parcel level of free convection (LFC), and leads to the final storm decay. The results demonstrate that inversion-region simulations support isolated, long-lived supercells. Both the degree of stratification and the coldness of the ambient BL regulate the cold-pool intensity and the strength and capacity of the outflow boundary to lift BL air through the LFC and thus regenerate convection, resulting in variation of supercell duration in the inversion region of approximately 1–2 h. In contrast, horizontally homogeneous conditions lacking an inversion region result in the development of secondary convection from the initial isolated supercell, followed by rapid upscale growth after 3 h to form a long-lived mesoscale convective system.

Sensitivity of quantitative precipitation forecast to soil moisture initialization and microphysics parametrization

In contrast to many other aspects of numerical weather prediction, quantitative precipitation forecasts did not gain much from a continuously improved physics over the past decades. A gap of knowledge remains as to how different processes contributing to the uncertainty in precipitation forecasts compare to each other. This research aims at gaining further insight into how two aspects influencing moist processes interrelate in atmospheric models. For this, high-resolution (3000 m) integrations with the non-hydrostatic Advanced Regional Prediction System were performed for two cases of extreme convection in Belgium, one driven by strong buoyancy and no vertical wind shear and the other driven by strong vertical wind shear and moderate buoyancy. Sensitivity experiments consisted of three integrations with different soil moisture initialization and three experiments with different size distribution assumptions in the microphysical parametrization scheme. The gain of spatially distributed soil moisture information was small in our simulations, but it was found important to have at least the mean soil moisture content right for the realistic simulation of cold-pool intensity. A mechanism, based on cold-pool and buoyancy interaction was proposed to explain the inverse relation between surface precipitation and soil moisture content. Changing the size distribution assumptions of the precipitating hydrometeors in the microphysics parametrization had a larger impact on the hydrometeor distribution than on surface precipitation. Weighting the size distribution assumptions of large hail to small graupel led to a poor representation of the general storm structure but did not affect the surface precipitation as much as was found in previous idealized studies. Copyright © 2010 Royal Meteorological Society

Effects of microphysical drop size distribution on tornadogenesis in supercell thunderstorms

Idealized simulations of tornadogenesis in supercell storms are performed using a grid of 100 m spacing. The cold pool intensity and low‐level storm dynamics are found to be very sensitive to the intercept parameters of rain and hail drop size distributions (DSD). DSDs favoring smaller (larger) hydrometeors result in stronger (weaker) cold pools due to enhanced (reduced) evaporative cooling/melting over a larger (smaller) geographic region. Sustained tornadic circulations of EF2 intensity are produced in two of the simulations with relatively weak cold pools. When the cold pool is strong, the updraft is tilted rearward by the strong, surging gust front, causing a disconnect between low‐level circulation centers near gust front and the mid‐level mesocyclone. Weaker cold pool cases have strong, sustained, vertical updrafts positioned near and above the low‐level circulation centers, providing strong dynamic lifting and vertical stretching to the low‐level parcels and favoring tornadogenesis.