WRF Model Simulations Research Articles

Atmospheric black carbon observations and its valley-mountain dynamics: Eastern cordillera of the central Andes of Peru

Glacial bodies in the Peruvian Andes Mountains store and supply freshwater to hundreds of thousands of people in central Peru. Atmospheric black carbon (BC) is known to accelerate melting of snow and ice, in addition to contributing to air pollution and the health of people. Currently there is limited understanding on the sources and temporal variability of BC in valley and mountain environments in Peru. To address this problem, this study combined surface observations of BC collected during 2022–2023 with WRF model simulations and HYSPLIT trajectories to analyze the dispersion and sources of BC in valley and high elevation environments and the associated local atmospheric circulations. Results show high BC concentrations are associated with the valley-mountain wind system that occurs on both sides of the Huaytapallana mountain range. A pronounced circulation occurs on the western slopes of Huaytapallana when concentrations of BC increase during daylight hours, which transports atmospheric pollutants from cities in the Mantaro River Valley to the Huaytapallana mountain range. Low concentrations of BC are associated with circulations from the east that are channeled by the pronounced ravines of the Andes-Amazon transition. On average, during the season of highest BC concentrations (July–November), the relative contributions of fossil fuels are dominant to biomass burning at the valley observatory and are slightly lower at the Huaytapallana observatory. These results demonstrate the need to promote mitigation actions to reduce emissions of BC and air pollution associated with forest fires and local anthropogenic activity.

WRF Model Simulation of the Active and Break Spells of Summer Monsoon over Bangladesh during 2011 and 2013

In this paper, the weather systems associated with the active and break spells of summer monsoon over Bangladesh have been studied using the WRF model as a regional climate model for two selected years 2011 (excess rainfall year) and 2013 (deficit rainfall year). The active and break spells for two selected years were identified using daily observed rainfall data from Bangladesh Meteorological Department (BMD). The model simulations for each year were made for the period of 1st May to 30th September with a single domain of 30 km resolution and 19 vertical levels. FNL 1°×1° resolution data was used for model simulation. The model-simulated daily rainfall, Sea Level Pressure (SLP), and the wind at 850 hPa, and 200 hPa were compared with TRMM, and ERA-Interim data, respectively. This study reveals that the increase in rainfall is concurrent with a south-westerly wind and the decrease of rainfall simultaneously occurs with a south-easterly wind over the country. A cyclonic circulation (850 hPa) was found over the southwestern part of Bangladesh during active days and the south-easterly wind was found (850 hPa) during break days. The easterly wind was found (200 hPa) during active days and break days. The lowest SLP was found over the southern part during active days (992-995 hPa) compared to break days (998-100 8hPa). Outgoing Longwave Radiation was found high (250-275 W/m²) during break days compared to Active days (220-270 W/m²). The WRF model can reasonably capture well the active and break phases of summer monsoon for the years 2011 and 2013.
 The Dhaka University Journal of Earth and Environmental Sciences, Vol. 11(2), 2022: P 43-52

Analysis of the driving factors of precipitation change during the development of the Jing-Jin-Ji urban agglomeration

Urbanization has an important impact on local precipitation changes. Previous studies of urban precipitation have largely considered single precipitation events, with few long-term statistical studies based on multiple samples. Therefore, based on WRF model simulations, this study extracted the latent heat flux, sensible heat flux, net radiation, friction velocity, albedo, and precipitation of 156 urban units in Beijing-Tianjin-Hebei (Jing-Jin-Ji) urban agglomeration for the summers of 2001–2019, and identified the main factors driving long-term urban precipitation changes based on multiple samples. There were three key results. 1) The factors that had the greatest impact on urban precipitation changes in Jing-Jin-Ji were the sensible heat flux, latent heat flux, and friction velocity. The latent heat flux was the leading factor driving precipitation changes during the urbanization of Jing-Jin-Ji. 2) In the different stages of urbanization, there were different factors affecting urban precipitation changes. In highly urbanized cities, the sensible heat flux had the greatest impact on precipitation changes, followed by the friction velocity. For medium-sized cities, the friction velocity was the most important variable. For small cities, the latent heat flux was the most important factor. 3) The friction velocity was the leading factor in precipitation change at the 0.95 quantile in Jing-Jin-Ji, and was therefore the most important factor driving changes in urban extreme precipitation. The results provide a theoretical basis for the formulation of climate policies, and disaster prevention and mitigation in Jing-Jin-Ji. They will also assist in promoting the construction of an ecological civilization and building safe and livable cities.

PostWRF: Interactive tools for the visualization of the WRF and ERA5 model outputs

PostWRF is an open-source software toolkit to facilitate the main visualization tasks and data handling for the WRF model outputs. The toolkit is mostly written in NCL and Shell, with a namelist that resembles the WRF or WPS namelists. Besides the visualizations, PostWRF provides WRF-NetCDF to GeoTIFF conversion for GIS applications, ERA5-NetCDF reanalysis data plotting and extraction, and preparation and plotting of the RTTOV model inputs (from WRF) and outputs. The primary purpose of PostWRF is to benefit the environmental researchers (both experienced and inexperienced) to make use of the WRF model simulations, in a straightforward and efficient way, without dealing with coding and syntax errors. Since the WRF model simulates most aspects of a full atmospheric model in the regional scale, the toolkit can also be used as an educational aid in meteorological and environmental science. PostWRF is freely available on GitHub (https://github.com/anikfal/PostWRF), provided with HTML documentations (https://postwrf.readthedocs.io/en/master) and guided examples.

Evaluation of the Performance of the WRF Model in a Hyper-Arid Environment: A Sensitivity Study

Accurate simulation of boundary layer surface meteorological parameters is essential to achieve good forecasting of weather and atmospheric dispersion. This paper is devoted to a model sensitivity study over a coastal hyper-arid region in the western desert of the United Arab Emirates. This region hosts the Barakah Nuclear Power Plant (BNPP), making it vital to correctly simulate local weather conditions for emergency response in case of an accidental release. We conducted a series of high-resolution WRF model simulations using different combinations of physical schemes for the months January 2019 and June 2019. The simulated results were verified against in-situ meteorological surface measurements available offshore, nearshore, and inland at 12 stations. Several statistical metrics were calculated to rank the performance of the different simulations and a near-to-optimal set of physics options that enhance the performance of a WRF model over different locations in this region has been selected. Additionally, we found that the WRF model performed better in inland locations compared to offshore or nearshore locations, suggesting the important role of dynamical SSTs in mesoscale models. Moreover, morning periods were better simulated than evening ones. The impact of nudging towards station observations resulted in an overall reduction in model errors by 5–15%, which was more marked at offshore and nearshore locations. The sensitivity to grid cell resolution indicated that a spatial resolution of 1 km led to better performance compared to coarser spatial resolutions, highlighting the advantage of high-resolution simulations in which the mesoscale coastal circulation is better resolved.

Impact of COVID-19 lockdown on the atmospheric boundary layer and instability process over Indian region

The abrupt reduction in the human activities during the first lockdown of the COVID-19 pandemic created unprecedented changes in the background atmospheric conditions. Several studies reported the anthropogenic and air quality changes observed during the lockdown. However, no attempts are made to investigate the lockdown effects on the Atmospheric Boundary Layer (ABL) and background instability processes. In this study, we assess the lockdown impacts on the ABL altitude and instability parameters (Convective Available Potential Energy (CAPE) and Convective Inhibition Energy (CINE)) using WRF model simulations. Results showed a unique footprint of COVID-19 lockdown in all these parameters. Increase in the visibility, surface temperature and wind speed and decrease in relative humidity during the lockdown is noticed. However, these responses are not uniform throughout India and are significant in the inland compared to the coastal regions. The spatial variation of temperature (wind speed) and relative humidity shows an increase and decrease over the Indo Gangetic Plain (IGP) and central parts of India by 20% (100%) and 40%, respectively. Increase (80%) in the ABL altitude is larger over the IGP and central parts of India during lockdown of 2020 compared to similar time period in 2015–2019. This increase is attributed to the stronger insolation due to absence of anthropogenic activity and other background conditions. At the same time, CAPE decreased by 98% in the IGP and central parts of India, where it shows an increase in other parts of India. A prominent strengthening of CINE in the IGP and a weakening elsewhere is also noticed. These changes in CAPE and CINE are mainly attributed to the dearth of saturation in lower troposphere levels, which prevented the development of strong adiabatic ascent during the lockdown. These results provide a comprehensive observation and model-based insight for lockdown induced changes in the meteorological and thermo-dynamical parameters.

The Impacts of Microphysics and Terminal Velocities of Graupel/Hail on the Rainfall of Typhoon Fitow (2013) as Seen From the WRF Model Simulations With Several Microphysics Schemes

AbstractThis study investigated cloud microphysical processes, and the upward and downward movement of precipitating particles with the Weather Research and Forecasting model during Typhoon Fitow (2013). The Purdue‐Lin single‐moment scheme and the Milbrandt‐Yau and National Severe Storms Laboratory (NSSL) double‐moment microphysics schemes were utilized. The area‐mean rainfall simulated by the three schemes shared similar magnitudes and variations. The area‐mean rain rates were insensitive to microphysics schemes, as they were mainly determined by large‐scale water vapor convergence. However, the local rainfall intensity was sensitive to the schemes and graupel and hail parameterizations. This indicates great impacts of microphysics and terminal velocities of graupel/hail on the local rainfall intensity. Upward‐moving graupel had a stronger net source compared to the downward‐moving graupel over the spiral rainband in the Milbrandt‐Yau simulation due to the low terminal velocity; whereas the net source was much weaker for upward‐moving graupel than for downward‐moving graupel over the eyewall in the NSSL simulation due to the large terminal velocity. Decreasing the terminal velocities of graupel and hail (if hail exists) in the mid and lower troposphere significantly reduced the maximum local rainfall intensity via increasing the sources of upward‐moving graupel. This is because the upward‐moving graupel is prone to remaining in the clouds being advected out of the updraft region instead of directly converting to raindrops as the sinks of graupel. The microphysics and the terminal velocities were together responsible for the local rainfall intensity.

Understanding the rapid intensification of tropical cyclone Titli using Hurricane WRF model simulations

Understanding Rapid Intensification (RI) is crucial for improving the Tropical Cyclone (TC) intensity forecast skill and TC induced disaster preparedness. The physical processes that lead to RI are not well studied over the North Indian Ocean (NIO) region. Two TC cases, i.e., Titli (exhibited RI) and Phethai (not exhibited RI) from the 2018 post-monsoon season are considered to understand the environmental conditions responsible for making RI happen. The Hurricane Weather Research and Forecasting (HWRF) model produced simulations for Titli and Phethai are analyzed to understand the intensification changes. The purpose of this study is to investigate the environmental and storm structure characteristics that led to the RI of TC Titli when compared to TC Phethai. The Phethai and Titli simulations are initially validated against the best estimations by India Meteorological Department (IMD) and compared in terms of the environmental vertical wind shear as this parameter is often negatively correlated with TC intensification. Area averages of the deep-layer and mid-layer vertical wind shear over the TC environment suggest that shear did not affect the intensification of the Titli simulation. Initially, Titli contained higher magnitudes of relative humidity throughout the troposphere within the storm’s circulation. This steered to a higher upward mass flux in the troposphere and rapid intensification of relative vorticity (circulation) in the middle troposphere prior to any significant change ensued at the surface. After a deep vortex was established, the mass flux (and lower-tropospheric convergence) continued to rise, leading to RI. These results offer a basis for future research to understand better and forecast the development of RI.

Examining Terrain Effects on an Upstate New York Tornado Event Utilizing a High-Resolution Model Simulation

AbstractThe complex terrain at the intersection of the Mohawk and Hudson valleys of New York has an impact on the development and evolution of severe convection in the region. Specifically, previous research has concluded that terrain-channeled flow in the Mohawk and Hudson valleys likely contributes to increased low-level wind shear and instability in the valleys during severe weather events such as the historic 31 May 1998 event that produced a strong (F3) tornado in Mechanicville, New York.The goal of this study is to further examine the impact of terrain channeling on severe convection by analyzing a high-resolution WRF model simulation of the 31 May 1998 event. Results from the simulation suggest that terrain-channeled flow resulted in the localized formation of an enhanced low-level moisture gradient, resembling a dryline, at the intersection of the Mohawk and Hudson valleys. East of this boundary, the environment was characterized by stronger low-level wind shear and greater low-level moisture and instability, increasing tornadogenesis potential. A simulated supercell intensified after crossing the boundary, as the larger instability and streamwise vorticity of the low-level inflow was ingested into the supercell updraft. These results suggest that terrain can have a key role in producing mesoscale inhomogeneities that impact the evolution of severe convection. Recognition of these terrain-induced boundaries may help in anticipating where the risk of severe weather may be locally enhanced.

Moisture sources and transport during an extreme rainfall event over the Limpopo River Basin, southern Africa

Intermittent heavy rainfall over an eleven day period (11–21 January 2013) led to devastating floods over parts of South Africa and southern Mozambique, with regions within the Limpopo River Basin (LRB) being the worst affected. Previous observational work, based on an Eulerian analysis and low resolution data, indicated that moisture emanating from anomalously warm waters off the Angolan coast played a very important role in the evolution of this heavy rainfall event. However, there has been some debate about this moisture source and the general synoptic settings of the event that produced the heavy rainfall in the LRB. In this study, an analysis of the event using WRF model (12 km resolution) simulations, reanalyses, SAWS synoptic maps and NASA satellite images indicated that a tropical low was largely responsible for the floods which occurred over the LRB, particularly between 17 and 21 January. Moisture sources and transport are determined using the HYSPLIT model forced with WRF output and reveal a new perspective on the complex evolution of the event. During the early parts of the event, there were two core moisture source regions: i) a continental source with moisture being supplied from the Congo Basin region and, ii) a local oceanic region with moisture originating from the Agulhas Current and the Mozambique Channel. As the event progressed, the dominant moisture input was from parts of the South Indian Ocean including off the coast of Tanzania/Mozambique and to the east of Madagascar. Two days showed a moisture contribution from the tropical South Atlantic Ocean. A WRF sensitivity study with the observed warm SST anomaly off Angola replaced by climatological SST there indicated a reduction of moisture from the tropical South East Atlantic Ocean in the beginning and towards the end of the event, when there was some of the heaviest rainfall over the LRB, thereby reinforcing the suggestion that there was also some moisture being sourced from the tropical South East Atlantic during part of the observed event. When calculated over the entire 11–21 January 2013 event, there were four moisture sources, the largest contribution of which was from the Agulhas Current/Mozambique Channel and the South Indian Ocean, accounting for 53% of the moisture, followed by the continental source (25%). The tropical western Indian Ocean had the third largest contribution, at 17%, and the midlatitude South Atlantic Ocean (5%) contributed the least.

Physical and Dynamical Characteristics of Thunderstorms Over Bangladesh Based on Radar, Satellite, Upper-Air Observations, and WRF Model Simulations

Physical and Dynamical Characteristics of Thunderstorms Over Bangladesh Based on Radar, Satellite, Upper-Air Observations, and WRF Model Simulations

Uncertainty of land surface model and land use data on WRF model simulations over China

The land–atmosphere interaction has been considered one of the most important part for weather prediction and climate modeling. To evaluate the uncertainty coming from land surface models (LSMs) and land use (LU) data in WRF simulated climatology over China, we have conducted fifteen 10-year simulations from 1996 to 2005 with three LSMs (NOAH, CLM and RUC) and five LU data sets (MODIS, HYDE, HH, RF and CESM). Compared to the MODIS, the most major differences for HYDE, HH and RF include the reduction of the barren or sparsely vegetated area and the CESM map shows the largest arid and semi-arid area. Based on performance evaluation of WRF model, the uncertainties of LSMs and LU data are analyzed in a three-dimension aspect: the magnitudes of response, spatial and temporal patterns. The impact of LSM and LU data is statistically significant in some regions and the LSM effect is substantially higher than the LU data especially for precipitation. The temporal effect of combinations of LSM and LU data varied across regions. For temperature, we find that the effects of LSMs and LU data on the spatial pattern and magnitude are one order smaller than those on temporal pattern, and the uncertainties from LSMs and LU datasets are as the same order when considering the temporal and spatial patterns. The results also indicate that the uncertainty of LU data on precipitation is much smaller than that of LSMs on magnitude and spatial patterns. These findings reflect that the relative importance of LSMs and LU data in the WRF climate modeling largely depends on the specific LSM.

A process-based anatomy of Mediterranean cyclones: from baroclinic lows to tropical-like systems

Abstract. In this study, we address the question of the atmospheric processes that turn Mediterranean cyclones into severe storms. Our approach applies online potential vorticity (PV) budget diagnostics and piecewise PV inversion to WRF model simulations of the mature stage of 100 intense Mediterranean cyclones. We quantify the relative contributions of different processes to cyclone development and therefore deliver, for the first time, a comprehensive insight into the variety of cyclonic systems that develop in the Mediterranean from the perspective of cyclone dynamics. In particular, we show that all 100 cyclones are systematically influenced by two main PV anomalies: a major anomaly in the upper troposphere, related to the baroclinic forcing of cyclone development, and a minor anomaly in the lower troposphere, related to diabatic processes and momentum forcing of wind. Among the diabatic processes, latent heat is shown to act as the main PV source (reinforcing cyclones), being partly balanced by PV sinks of temperature diffusion and radiative cooling (weakening cyclones). Momentum forcing is shown to have an ambiguous feedback, able to reinforce and weaken cyclones while in certain cases playing an important role in cyclone development. Piecewise PV inversion shows that most cyclones develop due to the combined effect of both baroclinic and diabatic forcing, i.e. due to both PV anomalies. However, the stronger the baroclinic forcing, the less a cyclone is found to develop due to diabatic processes. Several pairs of exemplary cases are used to illustrate the variety of contributions of atmospheric processes to the development of Mediterranean cyclones: (i) cases where both baroclinic and diabatic processes contribute to cyclone development; (ii) cases that mainly developed due to latent-heat release; (iii) cases developing in the wake of the Alps; and (iv) two unusual cases, one where momentum forcing dominates cyclone development and the other presenting a dual-surface pressure centre. Finally, we focus on 10 medicane cases (i.e. tropical-like cyclones). In contrast to their tropical counterparts – but in accordance with most intense Mediterranean cyclones – most medicanes are shown to develop under the influence of both baroclinic and diabatic processes. In discussion of medicane-driving processes, we highlight the need for a physical definition of these systems.

Initialization of WRF Model Simulations With Sentinel-1 Wind Speed for Severe Weather Events

The model initialization with high-resolution SAR wind data provided by the Sentinel-1 mission and its impact on the meteorological model WRF-ARW simulations is discussed. The activity is performed within the Horizon 2020 CEASELESS project, focusing on one of the target areas, the northern Adriatic Sea (northern-central Mediterranean). The Sentinel-1 SAR wind is ingested into LAPS, a numerical system developed at NOAA, specifically designed for data analysis and nowcasting issues, since it has the advantage of being faster and less computational demanding than advanced data assimilation methods. Here, LAPS analyses are used to perform a smarter initialization of the WRF-ARW model simulations than using simply global model fields. The impact of the Sentinel-1 SAR wind on the model simulations is evaluated for twenty cases, ranging through several atmospheric conditions occurring in different seasons of the years 2014–2018. For each case study, a reference WRF-ARW simulation is forced with GFS analysis and forecasts used as initial and boundary conditions, respectively. Additional model runs are initialized with the LAPS analyses, which include the information of Sentinel-1 SAR wind, METAR data and the SEVIRI/MSG (Eumetsat) brightness temperature. A statistical evaluation of the WRF-ARW simulations is performed versus an independent set of surface records, provided by the Friuli Venezia Giulia regional station network (northeastern Italy), and METAR data. The comparison is performed for 10 m wind, 2 m air and dew point temperature. The results show a positive, albeit modest, impact on the WRF model simulations initialized with the LAPS analyses. The initialization with the Sentinel-1 SAR wind show benefits for all surface variables. Finally, a Mediterranean tropical-like cyclone (Medicane), occurred in the Ionian Sea in November 2017, is considered in order to show how the use of Sentinel wind data can contribute to a better analysis and simulation of severe weather episodes in the Mediterranean. The improvement in the simulation of the pressure minimum location is remarkable.

Initiation of an Elevated Mesoscale Convective System With the Influence of Complex Terrain During Meiyu Season

AbstractA mesoscale convective system (MCS) was initiated on the lee side of the Wuling Mountain in central‐east China in the late afternoon (1700 LT) on June 27, 2016, and led to an early morning precipitation peak. We investigated the convective initiation (CI) mechanism of this MCS using radars and automatic weather station observations, WRF model simulation, and analysis from the radar data assimilation system Variational Doppler Radar Analysis System. It was found that the convection initiation was elevated as indicated by an elevated convergence zone and updraft located above 1,625 m from the ground. A zero convective inhibition layer was located at 850 hPa, over a stable boundary layer (BL) resulting from the daytime rainfall in the plain region, and provided a favorable condition for the elevated CI. Our study suggested that the transition of mountain‐plain thermal wind near sunset was the trigger for the convective initiation. An early afternoon mountain‐plain thermal circulation was disrupted when the east‐facing lee side slope was in the shadow of sunshine after 4 p.m. As a result, a local‐scale northwest downslope wind accelerated and led to a downstream convergence above the stable BL, where the elevated convection was initiated near 1,625 m above ground level. By combining the model simulation, the analysis from data assimilation, and the observations, this study is the first to provide a detailed analysis of CI mechanism of elevated convection in a complex topography region during the Meiyu season in China.

Hydrometeor Budget of the Meiyu Frontal Rainstorms Associated With Two Different Atmospheric Circulation Patterns

AbstractCharacteristics of hydrometeor budget and the microphysical processes responsible for heavy precipitation are studied based on the WRF model simulations of two representative Meiyu frontal rainstorms that are associated with two distinct atmospheric circulation patterns. Case 1 is characterized by the coupling of the Eastward Propagating Mesoscale Vortex (EPMV) and Meiyu front, while Case 2 is dominated by the interaction between the Low‐Level Wind Shear (LLWS) and Meiyu front. The temporal and spatial characteristics of the hydrometeor budget are validated against observations and assimilation products including those obtained during the 2018 Integrative Monsoon Frontal Rainfall Experiment (IMFRE) campaign and discussed in the context of contrasting the precipitation intensification and dissipation stage. Specifically, the ice‐dependent cloud processes, rather than the liquid‐dependent cloud processes, are predominantly responsible for the variation of precipitation. These terms include the deposition from water vapor to the ice phase hydrometeors, the accretion from cloud liquid water to the ice phase hydrometeors in the upper troposphere, and the melting of the ice phase hydrometeors into raindrops in the mid‐lower troposphere. Then three major ice cloud conversion pathways and two minor warm cloud conversion pathways for the formation of raindrops are extracted from the overall microphysical processes active in both Case 1 and Case 2. One of the key findings is that ice‐dependent cloud processes are significantly more active in the case characterized by the coupling of EPMV and Meiyu front, and this difference is at least partly explained by the differences in dynamical and thermodynamic conditions dominated by the circulation patterns.

Comparative study on WRF model simulations from the viewpoint of optimum ship routing

In addition to other studies focused on the WRF performance of rainfall, temperature, and precipitation, the present study provides practical suggestions for ship operators on choosing grid point value (GPV) datasets and WRF modeling approaches to help make better decisions on weather routing by estimating strong wind with a higher accuracy. Two different GPV datasets, the NCEP-FNL and ERA-Interim, are used for WRF modeling. Various modeling approaches, which include applying different physics options and choosing whether to adopt the four-dimensional-data-assimilation, are employed for numerical simulation. Then, on-board measurements of a bulk carrier has been used to validate the calculated wind of eight rough-sea navigational cases. This occurs over a wide spatial range, covering ocean regions such as the Northwest Pacific Ocean, South of Australia, Tasman Sea, Southeast of Africa, and Southeast of America, as well as a wide temporal range of four years. Results of 64 WRF simulations show that ERA-Interim is suitable for longer periods, while NCEP-FNL can analyze extremely high winds. Additionally, grid nudging can also reduce the wind strength due to its smoothing effect. Finally, selections of these different GPV datasets and modeling approaches for ship weather routing are discussed, particularly for rough-sea navigations.

Quasi-stationary extreme rain produced by mesoscale convective system on the Mei-Yu front

A mesoscale convective system (MCS) occurred on the Mei-Yu front in the Jiang-Huai River Basin of China on 7–8 July 2007, which caused extreme rainfall. The MCS formed in an environment of moderate convective available potential energy, high precipitable water, and an almost unidirectional southwesterly low-level jet. The favorable environment for MCS initiation and development featured a low-level convergence between northeasterly wind north of the Mei-Yu front and warm-moist southwesterly airflow. The evaporative cooling generated cold outflow which continuously promoted new convection at the leading edge of the MCS. WRF model simulations reproduced the observed back-building initiation and upscale organization. The flow-parallel MCS was affected by the low-level jet, vertical wind shear, and near-surface cold outflow in a stable nocturnal planetary boundary layer. Stratiform precipitation was reinforced by the downstream propagation of cumulonimbus and advection of ice-phase hydrometeors by the mid-upper level wind. The quasi-stationarity was a product of subtle dynamical balance between the near-surface cold outflow and the background low-level southerly flow. Sensitivity experiments addressed the role of near-surface outflow and diurnal forcing in MCS organization.

Warm-Sector Heavy Rainfall in Southern China and Its WRF Simulation Evaluation: A Low-Level-Jet Perspective

Abstract Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within the warm sector hundreds of kilometers south of a front or without a front during April–June, characterized by poor predictability and a close relationship with low-level jets (LLJs). Based on 45 warm-sector heavy rainfall episodes in 2013 and 2014 in southern China, this study examines their general characteristics and evaluates the performance of convection-permitting WRF Model simulations from an LLJ perspective. The results show that 64% of the warm-sector heavy rainfall episodes are associated with an LLJ (LLJ type) and 36% are not (no-LLJ type). The LLJ type is distinct from the no-LLJ type, with large rainfall accumulation along the coastal area. It is more common for LLJs to occur at both 800 and 925 hPa in the LLJ type, where there is a wide 800-hPa LLJ west of Guangdong Province and two 925-hPa LLJs over Beibu Gulf and the South China Sea (SCS). The coastal convergence associated with the terminus of the LLJ on 925 hPa is conducive to the coastal rainfall. WRF generally presents lower QPF skill in the LLJ type than in the no-LLJ type, due to the severe underestimation of coastal rainfall. The QPF skill of the LLJ type is significantly correlated with the forecast accuracy of LLJs, especially at 925 hPa. The north bias of the simulated LLJ on 925 hPa over the SCS and the associated overestimation of wind speed below ~900 hPa over the inland region weaken the coastal convergence and eventually lead to the underestimation in coastal precipitation.

Investigating the Transition from Elevated Multicellular Convection to Surface-Based Supercells during the Tornado Outbreak of 24 August 2016 Using a WRF Model Simulation

Abstract On 24 August 2016, a tornado outbreak impacted Indiana, Ohio, and Ontario with 26 confirmed tornadoes. Elevated multicellular convection developed into surface-based supercells that produced several tornadoes, particularly near a differential heating boundary. This convective mode transition is of particular interest owing to its relatively rare occurrence. A WRF Model simulation accurately captures the environment and storm evolution during this outbreak. Trajectory analyses indicate that the multicellular updrafts were initially elevated. Since nearly all of the vertical wind shear was confined to the lowest 1 km, significant rotation did not develop via tilting of horizontal vorticity until the storms began ingesting near-surface air. Near-surface vertical wind shear decreased outside of cloud cover owing to vertical mixing, while it was preserved under the anvil, allowing for large values of 0–1-km storm-relative helicity to persist north of a differential heating boundary. Analysis of the perturbation pressure field from the WRF Model output indicates that the development of relatively large nonlinear vertical perturbation pressure gradients coincided with when near-surface air began to enter the updrafts, resulting in upward accelerations in the lowest 2 km, below the level of maximum rotation. In strengthening updrafts, upward-directed buoyancy perturbation pressure accelerations may have offset the downward-directed nonlinear perturbation pressure accelerations above the level of maximum rotation, allowing the updrafts to intensify further.