Capping Inversion Research Articles

Boundary Layer Structures Over the Northwest Atlantic Derived From Airborne High Spectral Resolution Lidar and Dropsonde Measurements During the ACTIVATE Campaign

AbstractThe Planetary Boundary Layer height (PBLH) is essential for studying PBL and ocean‐atmosphere interactions. Marine PBL is usually defined to include a mixed layer (ML) and a capping inversion layer. The ML height (MLH) estimated from the measurements of aerosol backscatter by a lidar was usually compared with PBLH determined from radiosondes/dropsondes in the past, as the PBLH is usually similar to MLH in nature. However, PBLH can be much greater than MLH for decoupled PBL. Here we evaluate the retrieved MLH from an airborne lidar (HSRL‐2) by utilizing 506 co‐located dropsondes during the ACTIVATE field campaign over the Northwest Atlantic from 2020 to 2022. First, we define and determine the MLH and PBLH from the temperature and humidity profiles of each dropsonde, and find that the MLH values from HSRL‐2 and dropsondes agree well with each other, with a coefficient of determination of 0.66 and median difference of 18 m. In contrast, the HSRL‐2 MLH data do not correspond to dropsonde‐derived PBLH, with a median difference of −47 m. Therefore, we modify the current operational and automated HSRL‐2 wavelet‐based algorithm for PBLH retrieval, decreasing the median difference significantly to −8 m. Further data analysis indicates that these conclusions remain the same for cases with higher or lower cloud fractions, and for decoupled PBLs. These results demonstrate the potential of using HSRL‐2 aerosol backscatter data to estimate both marine MLH and PBLH and suggest that lidar‐derived MLH should be compared with radiosonde/dropsonde‐determined MLH (not PBLH) in general.

The multi-scale coupled model: a new framework capturing wind farm–atmosphere interaction and global blockage effects

Abstract. The growth in the number and size of wind energy projects in the last decade has revealed structural limitations in the current approach adopted by the wind industry to assess potential wind farm sites. These limitations are the result of neglecting the mutual interaction of large wind farms and the thermally stratified atmospheric boundary layer. While currently available analytical models are sufficiently accurate to conduct site assessments for isolated rotors or small wind turbine clusters, the wind farm's interaction with the atmosphere cannot be neglected for large-size arrays. Specifically, the wind farm displaces the boundary layer vertically, triggering atmospheric gravity waves that induce large-scale horizontal pressure gradients. These perturbations in pressure alter the velocity field at the turbine locations, ultimately affecting global wind farm power production. The implication of such dynamics can also produce an extended blockage region upstream of the first turbines and a favorable pressure gradient inside the wind farm. In this paper, we present the multi-scale coupled (MSC) model, a novel approach that allows the simultaneous prediction of micro-scale effects occurring at the wind turbine scale, such as individual wake interactions and rotor induction, and meso-scale phenomena occurring at the wind farm scale and larger, such as atmospheric gravity waves. This is achieved by evaluating wake models on a spatially heterogeneous background velocity field obtained from a reduced-order meso-scale model. Verification of the MSC model is performed against two large-eddy simulations (LESs) with similar average inflow velocity profiles and a different capping inversion strength, so that two distinct interfacial gravity wave regimes are produced, i.e. subcritical and supercritical. Interfacial waves can produce high blockage in the first case, as they are allowed to propagate upstream. On the other hand, in the supercritical regime their propagation speed is less than their advection velocity, and upstream blockage is only operated by internal waves. The MSC model not only proves to successfully capture both local induction and global blockage effects in the two analyzed regimes, but also captures the interaction between the wind farm and gravity waves, underestimating wind farm power by about only 2 % compared with the LES results. Conversely, wake models alone cannot distinguish between differences in thermal stratification, even if combined with a local induction model. Specifically, they are affected by a first-row overprediction bias that leads to an overestimation of the wind farm power by 13 % to 20 % for the modeled regimes.

Identification of Convective Boundary Layer Depth over the Great Lakes Region Using Aircraft Observations: A Comparison of Various Methods

Abstract This study evaluates the methods of identifying the height zi of the top of the convective boundary layer (CBL) during winter (December and January) over the Great Lakes and nearby land areas using observations taken by the University of Wyoming King Air research aircraft during the Lake-Induced Convection Experiment (1997/98) and Ontario Winter Lake-effect Systems (2013/14) field campaigns. Since CBLs facilitate vertical mixing near the surface, the most direct measurement of zi is that above which the vertical velocity turbulent fluctuations are weak or absent. Thus, we use zi from the turbulence method as the “reference value” to which zi from other methods, based on bulk Richardson number (Rib), liquid water content, and vertical gradients of potential temperature, relative humidity, and water vapor mixing ratio, are compared. The potential temperature gradient method using a threshold value of 0.015 K m−1 for soundings over land and 0.011 K m−1 for soundings over lake provided the estimates of zi that are most consistent with the turbulence method. The Rib threshold-based method, commonly used in numerical simulation studies, underestimated zi. Analyzing the methods’ performance on the averaging window zavg we recommend using zavg = 20 or 50 m for zi estimations for lake-effect boundary layers. The present dataset consists of both cloudy and cloud-free boundary layers, some having decoupled boundary layers above the inversion top. Because cases of decoupled boundary layers appear to be formed by nearby synoptic storms, we recommend use of the more general term, elevated mixed layers. Significance Statement The depth zi of the convective atmospheric boundary layer (CBL) strongly influences precipitation rates during lake-effect snowstorms (LES). However, various zi approximation methods produce significantly different results. This study utilizes extensive concurrently collected observations by project aircraft during two LES field studies [Lake-Induced Convection Experiment (Lake-ICE) and OWLeS] to assess how zi from common estimation methods compare with “reference” zi derived from turbulent fluctuations, a direct measure of CBL mixing. For soundings taken both over land and lake; with cloudy or cloud-free conditions, potential temperature gradient (PTG) methods provided the best agreement with the reference zi. A method commonly employed in numerical simulations performed relatively poorly. Interestingly, the PTG method worked equally well for “coupled” and elevated decoupled CBLs, commonly associated with nearby cyclones.

Planetary Boundary Layer Flow over Complex Terrain during a Cold Surge Event: A Case Study

Planetary boundary layer (PBL) flow over complex terrain during a cold surge event was investigated using 3-hourly radiosonde measurements in upwind, near ridge, and downwind locations of mountains in the northeastern part of Republic of Korea and using a high-resolution (333-m) numerical simulation. A cold surge occurred on 23 January 2018 and lasted for 4 days. We analyzed onset day of the cold surge when air temperature dropped rapidly. Analysis of the radiosonde data shows that the PBL was characterized by an adiabatic layer with strong capping inversion in early morning and evening as well as during daytime in the upwind and near-ridge sites. The PBL flow at the near-ridge site was strongest among three sites except at 0600 local standard time (LST) when the PBL flow in the lee was strongest. We performed high-resolution (333-m) numerical simulations using the Weather Research and Forecasting (WRF) model. The adiabatic PBL in the upwind site at 0600 LST was simulated, although its depth was underestimated. The model reproduced the strong low-level wind at 0600 LST and large wind shear during the daytime in the lee, but it did not capture the exact timing of the large wind shear. The model showed overall good performance in simulating the vertical profile of the virtual potential temperature and wind below 2 km above ground level at the three sites, with a high index of agreement (IOA) except for the wind at 1200 and 1500 LST in the lee. To examine the cause for the different behavior of PBL flow in the lee of mountains between 0600 LST and the daytime, we calculated the Froude number for PBL flow using radiosonde measurements based on reduced gravity shallow water (RGSW) theory. At 0600 LST, the upwind Froude number F0 was close to 1, while during the daytime, it was much lower than 1. The observed lee flow behavior was consistent with the flow regime change of a single layer over an obstacle with changing F0; the flow with a propagating lee jump changes into that with a stationary lee jump with decreasing F0. Numerical simulation shows that the steepening of streamlines of lee-wave field leads to a jump-like structure in the lee of mountains during the daytime.

Elevated Mixed Layers during Great Lake Lake-Effect Events: An Investigation and Case Study from OWLeS

Abstract Data from rawinsondes launched during intensive observation periods (IOPs) of the Ontario Winter Lake-Effect Systems (OWLeS) field project reveal that elevated mixed layers (EMLs) in the lower troposphere were relatively common near Lake Ontario during OWLeS lake-effect events. Conservatively, EMLs exist in 193 of the 290 OWLeS IOP soundings. The distribution of EML base pressure derived from the OWLeS IOP soundings reveals two classes of EML, one that has a relatively low-elevation base (900–750 hPa) and one that has a relatively high-elevation base (750–500 hPa). It is hypothesized that the former class of EML, which is the focus of this research, is, at times, the result of mesoscale processes related to individual Great Lakes. WRF reanalysis fields from a case study during the OWLeS field project provide evidence of two means by which low-elevation base EMLs can originate from the lake-effect boundary layer convection and associated mesoscale circulations. First, such EMLs can form within the upper-level outflow branches of mesoscale solenoidal circulations. Evacuated Great Lakes–modified convective boundary layer air aloft then lies above ambient air of a greater static stability, forming EMLs. Second, such EMLs can form in the absence of a mesoscale solenoidal circulation when Great Lake–modified convective boundary layers overrun ambient air of a greater density. The reanalysis fields show that EMLs and layers of reduced static stability tied to Great Lakes–modified convective boundary layers can extend downwind for hundreds of kilometers from their areas of formation. Operational implications and avenues for future research are discussed.

A Novel Framework for Spatiotemporal Analysis of Temperature Profiles Applied to Europe

Abstract Vertical temperature profiles influence the wind power generation of large offshore wind farms through stability-dependent effects such as blockage and gravity waves. However, numerical tools that are used to model these effects are often computationally too expensive to cover the large variety of atmospheric states occurring over time. Generally, an informed decision about which representative nonidealized situations to simulate is missing because of the lack of easily available information on representative vertical profiles, taking into account their spatiotemporal variability. Therefore, we present a novel framework that allows a smart selection of vertical temperature profiles. The framework consists of an improved analytical temperature model for the atmospheric boundary layer and lower troposphere, a subsequent clustering of these profiles to identify representatives, and last, a determination of areas with similar spatiotemporal characteristics of vertical profiles. When applying this framework on European ERA5 data, physically realistic representatives were identified for Europe, excluding the Mediterranean. Two or three profiles were found to be dominant for the open ocean, whereas more profiles prevail for land. Over the open ocean, weak temperature gradients in the boundary layer and a clear capping inversions are widespread, and stable profiles are absent except in the region of the East Icelandic Current. Interestingly, according to the ERA5 data, at its resolution, coastal areas and seas surrounded by land behave more similar to the land areas than to the open ocean, implying that a larger set of model integrations are needed for these areas to obtain representative results for offshore wind power assessments in comparison with the open ocean. Significance Statement Numerical tools used to simulate the effect of large, offshore wind farms on neighboring farms and the atmosphere are very expensive. Therefore, they can only be computed for a limited number of cases. As temperature is one of the most important parameters in these kinds of simulations, this work provides a new vertical temperature model and an analysis framework that allows for a smart selection of these cases such that they ideally represent the full variation of the atmosphere’s temperature profiles.

Subseasonal Variation Characteristics of Low-Cloud Fraction in Southeastern and Northwestern North Pacific

The subseasonal variability of the low-cloud fraction (LCF) over the southeastern North Pacific (SENP) and northwestern North Pacific (NWNP) was studied using satellite observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. It is found that subseasonal variability of the LCF was closely related to variations in the estimated inversion strength (EIS), sea surface wind speed (SSW), sensible heat flux (SHF), sea surface temperature (SST), surface temperature advection (Tadv), relative humidity (RH), surface level pressure (SLP) and surface air temperature (SAT). An increase in the LCF over the SENP is associated with the development of an anomalous anticyclonic circulation, which is located on the west coast of America. The cold advection, together with the subsidence warming associated with the anticyclonic circulation, strengthens the temperature inversion, favoring the development of the LCF. In the NWNP, the maximum LCF anomaly was also correlated with the stable boundary layer. The southerly wind blows airflow over the Kuroshio Extension from the subtropics, which brings warm and moist air. When air flows to the colder sea surface, it is cooled and condensed by the intensified heat exchange. A lead-lag composite analysis indicates that the mechanisms are different between the SENP and the NWNP, possibly due to the different types of low-level clouds over these two regions. In the SENP, the trade cumulus dominates under a strong capping inversion over the subtropics, whereas fog and stratus often occur under a shallow capping inversion in the NWNP. The effects of atmospheric circulation are also discussed.

Modelling the impact of trapped lee waves on offshore wind farm power output

Abstract. Mesoscale meteorological phenomena, including atmospheric gravity waves (AGWs) and including trapped lee waves (TLWs), can result from flow over topography or coastal transition in the presence of stable atmospheric stratification, particularly with strong capping inversions. Satellite images show that topographically forced TLWs frequently occur around near-coastal offshore wind farms. Yet current understanding of how they interact with individual turbines and whole farm energy output is limited. This parametric study investigates the potential impact of TLWs on a UK near-coastal offshore wind farm, Westermost Rough (WMR), resulting from westerly–southwesterly flow over topography in the southeast of England. Computational fluid dynamics (CFD) modelling (using Ansys CFX) of TLW situations based on real atmospheric conditions at WMR was used to better understand turbine level and whole wind farm performance in this parametric study based on real inflow conditions. These simulations indicated that TLWs have the potential to significantly alter the wind speeds experienced by and the resultant power output of individual turbines and the whole wind farm. The location of the wind farm in the TLW wave cycle was an important factor in determining the magnitude of TLW impacts, given the expected wavelength of the TLW. Where the TLW trough was coincident with the wind farm, the turbine wind speeds and power outputs were more substantially reduced compared with when the TLW peak was coincident with the location of the wind farm. These reductions were mediated by turbine wind speeds and wake losses being superimposed on the TLW. However, the same initial flow conditions interacting with topography under different atmospheric stability settings produce differing near-wind-farm flow. Factors influencing the flow within the wind farm under the different stability conditions include differing, hill and coastal transition recovery, wind farm blockage effects, and wake recovery. Determining how much of the differences in wind speed and power output in the wind farm resulted from the TLW is an area for future development.

Exploring Atmospheric Boundary Layer Depth Variability in Frontal Environments Over an Arid Region

Details about atmospheric boundary layer (ABL) dynamics under advection over arid regions remain unexplored. Most numerical weather prediction (NWP) models strictly rely on ABL parametrization schemes under steady-state assumptions while observation-based research also considers horizontally homogeneous atmospheric conditions for estimating ABL depth (zi) growth rates. However, how different frontal systems modify ABL thermodynamic features, including zi, remains sparse in the literature. This work investigates how synoptic events impact daytime zi variability in different seasons and provides new insights on front-relative zi variability over two sites located in an arid region. Radiosonde-derived thermodynamic profiles obtained during more than 40 synoptic events in 2020 from two sites (Amarillo and Midland, Texas) have been used. Individual soundings are divided into a three-day period: prefrontal, frontal, and postfrontal. The ABL regimes and associated soundings are further classified into four categories: (1) Type-I with elevated mixed layer (EML) only, (2) Type-II with dryline only, (3) Type-III with both EML and dryline, and (4) Type-IV without dryline or EML. Results suggest that zi decreases substantially during frontal passages and is shallower in the cold sector than the warm sector. We also find that the zi variability in the postfrontal airmass is more uniform throughout the year compared to the zi in the prefrontal airmass indicating complexities associated with airmass advection. Regression analyses comparing frontal strength and observed Δzi (i.e., frontal contrasts in zi estimated via zi in warm sector minus zi in cold sector) do not yield any correlations, which suggests that advection from frontal passages has considerable influence in governing zi variability unlike in the conditions when zi is mainly dependent on surface forcings. By explaining how airmass exchange associated with frontal environments impacts overall ABL dynamics, new parametrizations for ABL modelling can be developed with an emphasis on zi advection.

Evolution of Convective Energy and Inhibition before Instances of Large CAPE

Abstract We analyze the evolution of convective available potential energy (CAPE) and convective inhibition (CIN) in the days leading up to episodes of high CAPE in North America. The widely accepted theory for CAPE buildup, known as the advection hypothesis, states that high moist static energy (MSE) parcels of air moving north from the Gulf of Mexico become trapped under warm but dry parcels moving east from over elevated dry terrain. If and when the resulting CIN erodes, severe convection can occur due to the large energy difference between the boundary layer parcels and cool air aloft. However, our results, obtained via backward Lagrangian tracking of parcels at locations of peak CAPE, show that large values of CAPE are generated mainly via boundary layer moistening in the days leading up to the time of peak CAPE, and that a large portion of this moisture buildup happens on the day of peak CAPE. On the other hand, the free-tropospheric temperature above these tracked parcels rarely changes significantly over the days leading up to such occurrences. In addition, the CIN that allows for this buildup of CAPE arises mostly from unusually strong boundary layer cooling the night before peak CAPE, and has a contribution from differential advection of unusually warm air above the boundary layer to form a capping inversion. These results have important implications for the climatology of severe convective events, as it emphasizes the role of surface properties and their gradients in the frequency and intensity of high CAPE occurrences. Significance Statement Severe convective events, such as thunderstorms, tornadoes, and hail storms, are among the most deadly and destructive weather systems. Although forecasters are quite good at predicting the probability of these events a few days in advance, there is currently no reliable seasonal prediction method of severe convection. We show that the buildup of energy for severe convection relies on both strong surface evaporation during the day of peak energy and anomalous cooling the night before. This progress represents a step toward understanding what controls the frequency of severe convective events on seasonal and longer time scales, including the effect of greenhouse gas–induced climate change.

Investigation of Atmospheric Dynamic and Thermodynamic Structures of Typhoon Sinlaku (2020) from High-Resolution Dropsonde and Two-Way Rawinsonde Measurements

Profiling the vertical atmospheric structure for typhoons remains challenging. Here, the atmospheric dynamic and thermodynamic structures were investigated during the passage of Typhoon Sinlaku (2020) over Xisha Islands in the South China Sea for the period 28 July to 2 August 2020, mainly based on two-way rawinsonde and dropsonde measurements in combination with surface-based automatic weather station observations, disdrometer measurements, and Himawari-8 geostationary satellite images. The study period was divided to three stages: the formation stage of tropical depression (pre-TD), tropical depression (TD), and tropical storm (TS). The wind speed and local vertical wind shear reached the maximum value at 3 km above mean sea level (AMSL) before the typhoon approached the Xisha islands. Pseudo-equivalent potential temperature (θse) was found to decrease with the altitude below 2 km AMSL; temperature inversions occurred frequently within this altitude range, particularly during the TS stage. This seemed a typical capping inversion that indicated a downward motion above 2 km AMSL. The temperature increased slightly with the development of Typhoon Sinlaku (2020) at altitudes of 8–10 km AMSL. This indicated that our observations presumably captured the air mass warmed by the condensation, which was a good signature of an upper air in the tropical cyclone. In addition, wind speed (particularly in the lower stratosphere), specific humidity, and equivalent potential temperature escalated significantly when the tropical depression strengthened into Typhoon Sinlaku (2020), which indicated that the typhoon constantly obtained energy from the sea surface during its passage over the study region. The thermodynamic and dynamic structures of atmosphere advance our understanding of the inner structure of typhoons during the different evolutionary stages.

Response of Moist Convection to the Spectral Feature of the Surface Flux Field at Model Resolution Across the Gray Zone

AbstractThe Weather Research and Forecasting model is run at the effective resolution Δf across the gray zone, or the “terra incognita”, at which Δf ∼ l, where l is the scale of energy‐containing turbulent eddies. We examine the developments of afternoon moist convection over different multiscale features of the surface flux field as a function of the model grid spacing Δ that varies from 100 m to 1 km. Numerical experiments demonstrate that the model performs better under heterogeneous than homogeneous surface conditions in the gray zone in terms of the spatial distribution of moist convection. This improved performance is due to the explicit resolvability of the mesoscale temperature and humidity perturbations, which are critical for the growth of deep moist convection over the heterogeneous surface condition. However, under both heterogeneous and homogeneous surface conditions, the timing and intensity of moist convection vary with the model resolution. Compared with that in the large eddy simulation at Δ = 100 m, the earlier onset of moist convection at coarser resolutions appears to be related to the overestimated total buoyancy fluxes and total kinetic energy near the surface. That is, the more intense vertical mixing near the surface might induce a warmer surface layer, which yields individual cell updrafts strong enough to penetrate the capping inversion. In addition, misrepresentation of the diluting effects of entrainment on the penetrating updrafts in coarser‐resolution simulations could be responsible for the rapid development of deep convection instead of a gradual transition from shallow to deep moist convection.

Understanding the relationship of storm‐ to large‐scale environment in the monsoon trough region: results inferred from long‐term radar and reanalysis datasets

AbstractMonsoon trough (MT) convection exhibits multiscale spatial variability ranging from cellular storms to large organized convective systems. Understanding the relationships between storm‐scale convection and the large‐scale environment is crucial for accurately representing the properties and effect of convection in climate models. We investigate these relationships using nine wet seasons (June–September of 2009–2017) of S‐band radar observations over a tropical location (Kolkata) in the Indian summer monsoon trough region. A Lagrangian‐based objective cell‐tracking method, Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN), is applied to the volumetric reflectivity data to identify and track the convective storms. ERA5 hourly reanalysis products from the European Centre for Medium‐Range Weather Forecasts (ECMWF) are utilized for the field variables representing the large‐scale environment. We focus on the relationships of radar‐derived storm properties with large‐scale dynamic, thermodynamic, and stability variables such as convergence, mid‐level humidity, convective available potential energy (CAPE), and convective inhibition (CIN). Storm‐scale convection shows a selective nature, characterized as embedded cells in their unique environment. Furthermore, storms are classified as storm cells (simple and intense) and multicellular (linear and nonlinear) based on their geometrical properties and intensity. MT convection is predominantly composed of storm cells and characterized by an environment of moist mid‐level, low‐to‐moderate CAPE, low CIN, and strong dynamic forcing, that is, vertical ascent. In dry and weak dynamic forcing (i.e., subsiding atmosphere) with high CAPE and CIN, the highest rain intensity is associated with a few linear and nonlinear multicellular storms. In the development of such major linear and nonlinear multicellular storms over the eastern flank of the MT, the mid‐troposphere acts as a dry capping inversion. It is observed that the storm‐scale convection varies with the large‐scale dynamic forcing. During weak (strong) dynamic forcing conditions, the CAPE (mid‐level humidity) relates well with the observed storm‐scale convection.

The Multiscale Dynamics of the 29 June 2012 Super Derecho

The 29–30 June 2012 “super” derecho was, up until the 10 August 2020 “Iowa Derecho”, the most prolific derecho of modern times. While many of the synoptic-scale precursors to derecho events are understood, the multi-scale dynamics which likely distinguish derecho-producing events versus non-derecho events remain much more elusive. Using both observations and high-resolution WRF-ARW simulations, the sequence of adjustments that ultimately set up the pre-29 June derecho environment are examined. Planetary scale Rossby wave breaking occurred almost exactly two weeks before the super derecho on 15–16 June 2012 resulting in the development and intensification of a strong high-pressure system and mixed layer over the complex terrain of the western United States. A week after the initial Rossby wave break (~23 June), daily record-breaking temperatures began to dominate much of the central U.S. as the mixed layer/high pressure continued to strengthen. A second Rossby wave break on 26 June was crucial for detaching the mixed layer from the western U.S. elevated plateau, creating an elevated mixed layer that was rapidly deformed and propagated downstream to set up the derecho environment between 27–29 June. On 28 June, flow imbalance at the elevated mixed layer front resulted in highly ageostrophic circulations in the mid-levels, generating an along-stream mid-level jetlet which ultimately moved the elevated mixed layer and associated mesoscale front downstream across the Midwest and Mid-Atlantic. On the morning of 29 June, a well-defined corridor of both potential static instability and lowered inertial stability was set up across the Midwest and Mid-Atlantic states. This along with strong capping, a divergent polar jet entrance region to the north, and the highly imbalanced mid-level jetlet set the stage for this prolific severe convective event.

Multiscale Analysis of the 1 June 2011 Northeast U.S. Severe Weather Outbreak and Associated Springfield, Massachusetts Tornado

On 1 June 2011, the 12th killer tornado in New England since 1950 tracked 63 km (39 mi) from Westfield to Charlton, MA resulting in 3 fatalities, 200 injuries, and EF3 damage. At least fourteen supercells produced six confirmed tornadoes and six hail reports ≥7 cm (≥2.75 in) in diameter across eastern New York and New England. This paper takes a multiscale look at meteorological factors contributing to this event. The synoptic pattern evolution closely resembles the composite mean shown by Banacos and Ekster (2010) for significant severe weather events in the northeast United States associated with an elevated mixed layer (EML). The presence of an EML and rich boundary-layer moisture (surface dewpoints 20-22oC) supported surface-based CAPE >4000 J kg-1 by early afternoon on 1 June. A strengthening prefrontal trough within the moist and unstable boundary layer, together with increasing low and deep-layer shear created an environment favorable for tornadic supercells. In particular, storms moved into an environment with increasing values of 0-1-km AGL bulk shear, increasing storm relative helicity, and lower lifted condensation level heights. Values of these parameters were largely consistent with significant-tornado occurrence in proximity studies. Tornadic signatures in WSR-88D and experimental CASA 3-cm dual-polarization radar data are described, in addition to a remarkable three-body scatter spike associated with lofted debris near peak apparent intensity of the EF3 tornado.

The Role of Elevated Terrain and the Gulf of Mexico in the Production of Severe Local Storm Environments over North America

AbstractThe prevailing conceptual model for the production of severe local storm (SLS) environments over North America asserts that upstream elevated terrain and the Gulf of Mexico are both essential to their formation. This work tests this hypothesis using two prescribed-ocean climate model experiments with North American topography removed or the Gulf of Mexico converted to land and analyzes how SLS environments and associated synoptic-scale drivers (southerly Great Plains low-level jets, drylines, elevated mixed layers, and extratropical cyclones) change relative to a control historical run. Overall, SLS environments depend strongly on upstream elevated terrain but more weakly on the Gulf of Mexico. Removing elevated terrain substantially reduces SLS environments especially over the continental interior due to broad reductions in both thermodynamic instability and vertical wind shear, leaving a more zonally uniform residual distribution that is maximized near the Gulf coast and decays toward the continental interior. This response is associated with a strong reduction in synoptic-scale drivers and a cooler and drier mean-state atmosphere. Replacing the Gulf of Mexico with land modestly reduces SLS environments over the Great Plains (driven primarily thermodynamically) and increases them over the eastern United States (driven primarily kinematically), shifting the primary local maximum eastward into Illinois; it also eliminates the secondary, smaller local maximum over southern Texas. This response is associated with modest changes in synoptic-scale drivers and a warmer and drier lower troposphere. These experiments provide insight into the role of elevated terrain and the Gulf of Mexico in modifying the spatial distribution and seasonality of SLS environments.

Meteorological Observations of the Vento Norte Phenomenon in the Central Region of Rio Grande do Sul

Abstract During the cold season, episodes of a windstorm known as Vento Norte (VNOR) are frequently observed in the city of Santa Maria, situated in the central region of Rio Grande do Sul (RS). The onset of this windstorm is characterized by strong gusts with northerly component which are accompanied by a sharp increase in temperature and abrupt drying. A methodology based on the observed behavior of the standard deviation of temperature during VNOR was successful in identifying the onset and demise of the windstorm. Additionally, the research shows the results of different micrometeorological variables during the event. Early morning and evening atmospheric profiles exhibit an elevated mixed layer over central RS in association with the strong VNOR winds, indicating the presence of mechanically-forced mixing during nighttime hours.

The Dynamics of Observed Lee Waves over the Snæfellsnes Peninsula in Iceland

AbstractOn 20 October 2016, aircraft observations documented a significant train of lee waves above and downstream of the Snæfellsnes Peninsula on the west coast of Iceland. Simulations of this event with the Weather Research and Forecasting (WRF) Model provide an excellent representation of the observed structure of these mountain waves. The orographic features producing these waves are characterized by the isolated Snæfellsjökull volcano near the tip of the peninsula and a fairly uniform ridge along its spine. Sensitivity simulations with the WRF Model document that the observed wave train consists of a superposition of the waves produced individually by these two dominant orographic features. This behavior is consistent with idealized simulations of a flow over an isolated 3D mountain and over a 2D ridge, which reproduce the essential behavior of the observed waves and those captured in the WRF simulations. Linear analytic analysis confirms the importance of a strong inversion at the top on the boundary layer in promoting significant wave activity extending far downstream of the terrain. However, analysis of the forced and resonant modes for a two-layer atmosphere with a capping inversion suggest that this wave train may not be produced by resonant modes whose energy is trapped beneath the inversion. Rather, these appear to be vertically propagating modes with very small vertical group velocity that can persist far downstream of the mountain. These vertically propagating waves potentially provide a mechanism for producing near-resonant waves farther aloft due to interactions with a stable layer in the midtroposphere.

An Idealized Physical Model for the Severe Convective Storm Environmental Sounding

AbstractThis work develops a theoretical model for steady thermodynamic and kinematic profiles for severe convective storm environments, building off the two-layer static energy framework developed in work by Agard and Emanuel. The model is phrased in terms of static energy, and it allows for independent variation of the boundary layer and free troposphere separated by a capping inversion. An algorithm is presented to apply the model to generate a sounding for numerical simulations of severe convective storms, and the model is compared and contrasted with that of Weisman and Klemp. The model is then fit to a case-study sounding associated with the 3 May 1999 tornado outbreak, and its potential utility is demonstrated via idealized numerical simulation experiments. A long-lived supercell is successfully simulated with the historical sounding but not the analogous theoretical sounding. Two types of example experiments are then performed that do simulate a long-lived supercell: 1) a semitheoretical experiment in which a portion of the theoretical sounding is modified to match the real sounding (low-level moisture); 2) a fully theoretical experiment in which a model physical parameter is modified (free-tropospheric relative humidity). Overall, the construction of this minimal model is flexible and amenable to additional modifications as needed. The model offers a novel framework that may be useful for testing how severe convective storms depend on the vertical structure of the hydrostatic environment, as well as for linking variability in these environments to the physical processes that produce them within the climate system.

Insights into Supercells and Their Environments from Three Decades of Targeted Radiosonde Observations

AbstractHundreds of supercell proximity soundings obtained for field programs over the central United States are analyzed to reconcile differences in recent studies and to refine our knowledge of supercell environments. The large, storm-centric observation-based dataset and high vertical resolution of the sounding data provide an unprecedented look at supercell environments. Not surprisingly, storm-relative environmental helicity (SRH) is found to be larger in tornadic soundings than in nontornadic soundings. The primary finding that departs from previous studies is that storm-relative winds contribute substantially to the larger SRH. Stronger ground-relative winds and more rightward-deviant storm motions contribute to the larger storm-relative winds for the tornadic soundings. Spatial analyses of the soundings reveal lower near-ground pressure perturbations and stronger low- to midlevel cyclonic flow for the tornadic soundings, which suggests stronger mesocyclones, perhaps explaining the more rightward-deviant motions. Differences in the mean critical angle between the tornadic and nontornadic soundings are small and do not contribute to the larger mean SRH, but the tornadic soundings do have fewer instances of smaller (<60°) critical angles. Furthermore, the critical angle is shown to be a function of azimuth from the updraft. Other results include a low-to-the-ground (~250 m on average) hodograph kink for both the tornadic and nontornadic soundings and few notable differences in thermodynamic quantities, except for the expected lower LCLs related to higher RH for the tornadic soundings, somewhat smaller 0–3 km lapse rates in tornadic environments related to weaker/shallower capping inversions, and larger 0–3 km CAPE in near-field environments.