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John M. Lanicci
and
Thomas T. Warner

Abstract

A synoptic climatology of the atmospheric conditions associated with the creation of the elevated mixed-layer inversion, or lid, over the southern Great Plains of the U.S. (defined as Kansas, Oklahoma, Texas, and portions of the surrounding states) during four spring (April, May, June) seasons from 1983 through 1986 is presented. The lid sounding, also known as a type 1 tornado sounding, is created through the superposition of a potentially warm, nearly dry-adiabatic elevated mixed layer (EML) over a moist, potentially unstable layer. This study examines the situations which are favorable and unfavorable for creation of the lid stratification, using EML and lid occurrence statistics and analyses of various parameters associated with the EML and lid. In addition, we define a set of synoptic types that prevail in this region during the three-month period. The synoptic types are categorized by simultaneously examining the surface isobaric patterns and the predominant 500-mb flow direction over the study region, and designating the flow as either “favorable” or “unfavorable” for lid formation based on the implied thermal advection in the layer and the number of lid soundings observed over the region.

Our analysis reveals that a typical lid covers only about 20% to 25% of the southern Great Plains, and that a lid coverage greater than 50% occurred on fewer than 2% of the study days. High lid-frequency values expand northward during the season, and the maximum-frequency axis shifts westward. We show that this seasonal change is primarily caused by the northward expansion of the EML-source region from Mexico into the central Rockies and Great Basin, and a westward shift in the mean low-level moist axis. The westward shift in the low-level moist axis is related to the westward expansion of the Bermuda anticyclone. We find that the relative airstream configuration associated with the classic models of lid formation and severe weather over this region occurs most frequently in April and May, and corresponds to a flow type associated with southerly low-level flow and southwest flow aloft. As the season progresses, the expansion of both the EML-source region and low-level moist areas allows the lid to be created with a variety of additional flow configurations. In addition to the classic southwesterly midtropospheric flow type, these configurations include northwest and anticyclonic 500-mb flows by May and June. The dominance in late spring of flow types associated with large scale subsidence leads to an airstream configuration in which the inversion base sinks and the lid strengthens downstream from the source region. This is in stark contrast to the classic lid model where the inversion base rises and the lid weakens in an environment of large-scale vertical ascent.

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John M. Lanicci
and
Thomas T. Warner

Abstract

This study documents the cycle of lid formation and dissipation over the central U.S. during the spring season(defined as April, May, and June). The primary area of interest is Kansas, Oklahoma, and Texas; however, thestudy encompasses the surrounding states and the source regions for the elevated mixed layer, such as thewestern U.S. and northern Mexico. The database includes conventional surface and rawinsonde observations,as well as derived parameters that define the lid structure. We examine the temporal and spatial variability oflid occurrence and the associated surface/500-mb synoptic patterns to determine the periodicity of lid occurrence,seasonal tendencies, and relationships between different slages of the lid cycle and specific synoptic flow types.

Our results indicate that the lid cycle has a mean period of about one week. Synoptic typing shows that thereare basically two types of lid cycles: one that begins with a surface high pressure incursion into the southernPlains, and one that begins with a weak southerly surface flow. The first type of lid cycle represents about 60%of the total occurrences and appears throughout the entire season. It is of longer duration than the second andis associated with the progression of strong baroclinie waves in the we~teriies over the study area. The secondtype appears around mid-May, and subsequently becomes as frequent as the first type. It is typically associatedwith weak low4evel flow and subtropical circulations that exis~ over the region in late spring and summer afterthe polar jet has relreated northward. We define a four-phase composite of the chronology of the lid cycle..Analyses of composited synoptic-flow types to represent the various stages in each type of lid cycle are presented,and we examine several of these composites to identify geographically favored zones for initiation of deep convection.

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John M. Lanicci
and
Thomas T. Warner

Abstract

This study investigates the relationships between the occurrence of the lid (also known as a type 1 tornado sounding) and the occurrence of severe storms over Kansas, Oklahoma, and Texas during the spring season (defined as the months of April, May, and June). The period of this study covers four seasons from 1983 through 1986. The size distribution of severe storm events is examined in relation to the occurrence/size of the antecedent lid over the study region. Days in which no severe weather was observed over the region (defined as “non-event” days) are included in order to examine the occurrence/size of the lid on these days as well. The relationships between the occurrence/size of severe-storm outbreaks and the antecedent lid are also examined using conceptual models of the life cycle of lid development and dissipation, for both early and late spring. Composite mean analyses of key meteorological parameters and geographic frequency composites of the elevated mixed layer, buoyant instability (as defined by an unstable value of the Lifted Index or buoyancy term in the Lid Strength Index), and severe-weather events are constructed for different stages of lid development. These composites are then utilized to determine geographic relationships among these parameters.

The results show that the size distribution of severe-weather events has a peak at the 1600 km2 (a 40- × 40- km grid square) category; this peak strengthens during the season (especially from May to June). Also found is a relationship between the occurrence and size of the lid at 1200 UTC and the occurrence and size of subsequent severe-storm events, where this relationship is most well-defined in April and deteriorates rapidly from May to June. We hypothesize that the deterioration of this relationship is due to factors such as the increasing horizontal extent of the low-level moist layer (and buoyant instability) during the spring, and changes in the synoptic-scale circulation from baroclinic waves in the westerlies to subtropical anticyclones and weak cyclonic disturbances. In some late spring synoptic patterns, severe weather is not only associated with the presence of a widespread lid, but also is found to exist in an environment containing a variety of sounding types; these include lid soundings, uncapped soundings (with a zero or negative lid strength term), and even a subsidence-type sounding that is buoyantly unstable. In such environments, features such as the low-level jet and surface troughs may provide a sufficient lifting mechanism to allow the development of deep convection to occur.

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John M. Lanicci
and
Thomas T. Warner

Abstract

A case of lid formation and evolution over the southern Great Plains during early spring is analyzed using conventional data analysis and a 120-h simulation from the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model, Version 4 (MM4). The authors begin with a brief discussion of the evolution of the synoptic-scale circulation during the different phases of the lid development cycle in this case and follow with a description of MM4 as it was configured for this study. The model results are then used to supplement the data analyses in an examination of how the synoptic-scale circulation evolves and how it influences the physical and dynamical processes acting on the elevated mixed layer (EML) and low-level moist-layer source regions of the North American plateau and the Gulf of Mexico, respectively. The importance of choosing an appropriately large model domain and long simulation period in order to document properly the sequence of events, focusing primarily on the precursors to, and genesis of, the lid environment is stressed. The 120-h simulation allows one to focus on the ways in which synoptic and mesoscale processes interact to produce the observed mesoscale lid features such as inversions, moist layers, and EMLs over Texas, Oklahoma, and the adjacent coastal region.

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Roy M. Rasmussen
,
Piotr Smolarkiewicz
, and
John Warner

Abstract

This paper presents a detailed comparison study of three-dimensional model results with an aircraft wind field mapping for the island of Hawaii. Model runs were initialized using an aircraft sounding from 1 August 1985, and detailed predictions from the model are compared with observations from that day.

The strength and location of the upwind convergence zone were well simulated, as well as the strong deflection and deceleration of the flow around the island and the geometry and location of the upstream cloud bands. The good agreement between the model results and observations supports the results of our previous study in which we show that the flow pattern and associated cloud processes around the island of Hawaii can be understood by considering the flow of a stably stratified fluid around a large three-dimensional obstacle.

Model runs with different wind directions showed that increasing northerly tradewind flow resulted in the band clouds moving closer to the shore line, and the large scale flow pattern rotating counterclockwise. Model results were also compared with various aspects of the island climatology, and good agreement was found in both the temporal and spatial distribution of precipitation on the island.

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John M. Lanicci
,
Toby N. Carlson
, and
Thomas T. Warner

Abstract

This study examines the influence of differences in ground moisture over the southern Great Plairs and the Mexican plateau on the formation and evolution of the dryline, the elevated mixed layer, and the local planetary boundary layer. These features are examined in a series of numerical experiments in which dry and wet surface conditions over the southern plains and Mexico are simulated by the model

Results of the numerical simulations show that the dry soil conditions of northern Mexico are critical to the formation of the lid, and the variable soil conditions of the southern Great Plains are important for the processes of differential surface heating and generation of low-level instability through strong surface evaporation. The processes interact dynamically to alter the prestorm conditions and subsequent convective patterns observed over Texas and Oklahoma in the SESAME IV case.

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Stephen M. Sekelsky
,
Warner L. Ecklund
,
John M. Firda
,
Kenneth S. Gage
, and
Robert E. McIntosh

Abstract

Multifrequency radar measurements collected at 2.8 (S band), 33.12 (Ka band), and 94.92 GHz (W band) are processed using a neural network to estimate median particle size and peak number concentration in ice-phase clouds composed of dry crystals or aggregates. The model data used to train the neural network assume a gamma particle size distribution function and a size–density relationship having decreasing density with size. Results for the available frequency combinations show sensitivity to particle size for distributions with median volume diameters greater than approximately 0.2 mm.

Measurements are presented from the Maritime Continent Thunderstorm Experiment, which was held near Darwin, Australia, during November and December 1995. The University of Massachusetts—Amherst 33.12/94.92-GHz Cloud Profiling Radar System, the NOAA 2.8-GHz profiler, and other sensors were clustered near the village of Garden Point, Melville Island, where numerous convective storms were observed. Attenuation losses by the NOAA radar signal are small over the pathlengths considered so the cloud-top reflectivity values at 2.8 GHz are used to remove propagation path losses from the higher-frequency measurements. The 2.8-GHz measurements also permit estimation of larger particle diameters than is possible using only 33.12 and 94.92 GHz. The results suggest that the median particle size tends to decrease with height for stratiform cloud cases. However, this trend is not observed for convective cloud cases where measurements indicate that large particles can exist even near the cloud top.

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Christie A. Hegermiller
,
John C. Warner
,
Maitane Olabarrieta
, and
Christopher R. Sherwood

Abstract

Hurricanes interact with the Gulf Stream in the South Atlantic Bight (SAB) through a wide variety of processes, which are crucial to understand for prediction of open-ocean and coastal hazards during storms. However, it remains unclear how waves are modified by large-scale ocean currents under storm conditions, when waves are aligned with the storm-driven circulation and tightly coupled to the overlying wind field. Hurricane Matthew (2016) impacted the U.S. Southeast coast, causing extensive coastal change due to large waves and elevated water levels. The hurricane traveled on the continental shelf parallel to the SAB coastline, with the right side of the hurricane directly over the Gulf Stream. Using the Coupled Ocean–Atmosphere–Wave–Sediment Transport modeling system, we investigate wave–current interaction between Hurricane Matthew and the Gulf Stream. The model simulates ocean currents and waves over a grid encompassing the U.S. East Coast, with varied coupling of the hydrodynamic and wave components to isolate the effect of the currents on the waves, and the effect of the Gulf Stream relative to storm-driven circulation. The Gulf Stream modifies the direction of the storm-driven currents beneath the right side of the hurricane. Waves transitioned from following currents that result in wave lengthening, through negative current gradients that result in wave steepening and dissipation. Wave–current interaction over the Gulf Stream modified maximum coastal total water levels and changed incident wave directions at the coast by up to 20°, with strong implications for the morphodynamic response and stability of the coast to the hurricane.

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Joseph B. Zambon
,
Ruoying He
,
John C. Warner
, and
Christie A. Hegermiller

Abstract

Hurricane Florence (2018) devastated the coastal communities of the Carolinas through heavy rainfall that resulted in massive flooding. Florence was characterized by an abrupt reduction in intensity (Saffir–Simpson category 4 to category 1) just prior to landfall and synoptic-scale interactions that stalled the storm over the Carolinas for several days. We conducted a series of numerical modeling experiments in coupled and uncoupled configurations to examine the impact of sea surface temperature (SST) and ocean waves on storm characteristics. In addition to experiments using a fully coupled atmosphere–ocean–wave model, we introduced the capability of the atmospheric model to modulate wind stress and surface fluxes by ocean waves through data from an uncoupled wave model. We examined these experiments by comparing track, intensity, strength, SST, storm structure, wave height, surface roughness, heat fluxes, and precipitation in order to determine the impacts of resolving ocean conditions with varying degrees of coupling. We found differences in the storm’s intensity and strength, with the best correlation coefficient of intensity (r = 0.89) and strength (r = 0.95) coming from the fully coupled simulations. Further analysis into surface roughness parameterizations added to the atmospheric model revealed differences in the spatial distribution and magnitude of the largest roughness lengths. Adding ocean and wave features to the model further modified the fluxes due to more realistic cooling beneath the storm, which in turn modified the precipitation field. Our experiments highlight significant differences in how air–sea processes impact hurricane modeling. The storm characteristics of track, intensity, strength, and precipitation at landfall are crucial to predictability and forecasting of future landfalling hurricanes.

Open access
Kinya Toride
,
Yoshihiko Iseri
,
Michael D. Warner
,
Chris D. Frans
,
Angela M. Duren
,
John F. England
, and
M. Levent Kavvas

Abstract

The concept of probable maximum precipitation (PMP) is widely used for the design and risk assessment of water resource infrastructure. Despite its importance, past attempts to estimate PMP have not investigated the realism of design maximum storms from a meteorological perspective. This study investigates estimating PMP with realistically maximized storms in a Pacific Northwest region dominated by atmospheric rivers (ARs) using numerical weather models (NWMs). The moisture maximization and storm transposition methods used in NWM-based PMP estimates are examined. We use integrated water vapor transport as a criterion to modify water vapor only at the modeling boundary crossing the path of ARs, whereas existing methods maximize relative humidity at all initial/boundary conditions. It is found that saturation of the entire modeling boundaries can produce unrealistic atmospheric conditions and does not necessarily maximize precipitation over a watershed due to storm structure, stability, and topography. The proposed method creates more realistic atmospheric fields and more severe precipitation. The simultaneous optimization of moisture content and location of storms is also considered to rigorously estimate the most extreme precipitation. Among the 20 most severe storms during 1980–2016, the AR event during 5–9 February 1996 produces the largest 72-h basin-average precipitation when maximized with our method (defined as PMP of this study), in which precipitation is intensified by 1.9 times with a 0.7° shift south and a 30% increase in AR moisture. The 24-, 48-, and 72-h PMP estimates are found to be at least 70 mm lower than the Hydrometeorological Reports estimates regardless of duration.

Open access