Thermodynamic Environment Research Articles

The Sensitivity of the Northeast Colorado Thunderstorm Environment to Upstream Surface Conditions

Abstract Statistical evidence supports a relationship between the warm-season thermodynamic environment in northeastern Colorado and the antecedent snow conditions in the upstream higher elevations. Above-normal snow storage and longevity reduces the sensible heating of the elevated terrain by increasing the albedo and exerting a stabilizing influence upon the surface layer. It follows that the residual spring snowpack of the Colorado Rockies represents an energy sink to the prevailing west to southwesterly winds of the free atmosphere, which traverse the uplift of the Western Plateau. The abrupt drop in elevation along the Front Range provides for a setting in which a thermal perturbation induced by mountain snow may be communicated to the downstream plains by cooling and perhaps stratifying the mobile elevated residual layer. Statistical and observational evidence suggest that the May and June months that follow a robust and extended snowmelt season are characterized by cooler 700-mb temperatures over Denver, Colorado. The apparent moist convective response to a positive snowmelt anomaly in the lee plains region is for severe weather to develop on average 45 to 50 min earlier, during May and June, than observed during seasons having a negative snow anomaly. Conversely, late springs, which harbor below-average snow, subject the elevated terrain to greater sensible heating. During the May and June months of these “low snow years,” 700-mb environmental conditions over Denver tend to be warmer and drier. Occurring in tandem with these trends are a reduction in May and June rainfall and a lower number of severe hail days on the lee plains. Observations further suggest that during low snow years substantial precipitation deficits extend to synoptic-scale distances downstream.

Postcombustion Evolution of Soot Properties in an Aircraft Engine

Recent measurements have suggested that soot properties can evolve downstream of the combustor, changing the characteristics of aviation particulate matter emissions and possibly altering the subsequent atmospheric impacts. This paper addresses the potential for the postcombustion thermodynamic environment to influence aircraft nonvolatile particulate matter emissions. Microphysical processes and interactions with gas phase species have been modeled for temperatures and pressures representative of in-service engines. Time-scale arguments are used to evaluate the relative contributions that various phenomena may make to the evolution of soot, including coagulation growth, ion-soot attachment, and vapor condensation. Then a higher-fidelity microphysics kinetic is employed to estimate the extent to which soot properties evolve as a result of these processes. Results suggest that limited opportunities exist for the modification of the size distribution of the soot, its charge distribution, or its volatile content, leading to the conclusion that the characteristics of the turbine and nozzle of an aircraft engine have little or no influence on aircraft nonvolatile emissions. Combustor processing determines the properties of soot particulate matter emissions from aircraft engines, setting the stage for interactions with gaseous emissions and development as cloud condensation nuclei in the exhaust plume.

Analysis of large‐scale conditions associated with convection over the Indian monsoon region

AbstractThis paper examines the thermodynamics of the atmosphere in relation to occurrence of convective rainfall over the Indian region. Various thermodynamical and kinematic parameters such as convective available potential energy (CAPE), convective inhibition energy (CINE), equivalent potential temperature and relative vorticity field are computed based on model analysis field of the limited area model of India Meteorological Department. The data period of this study is from 15 March 2001 to 28 February 2002. The study shows that the spatial distribution of CAPE in pre‐monsoon and monsoon season exhibits a belt of high CAPE along the east coast extending north up to Gangetic West Bengal and another zone of high CAPE along the southwest coast. During post‐monsoon and winter seasons, the belt of maximum CAPE shifts over the extreme southern Peninsula. The annual variation of CAPE over all stations is bi‐modal in nature with twin peaks each at the beginning and at the end of the monsoon season. Magnitude of CINE on the other hand is nearly zero during the monsoon season. The result shows that presence of strong thermodynamic environment is not sufficient for the occurrence of deep convection. Factors like proper dynamic conditions play a very important role in controlling the occurrence of deep convection. The surface warming during the pre‐monsoon season destabilizes the atmosphere, but large‐scale moist convection, generally, does not take place until CINE becomes close to zero (in the monsoon season). Preceding the monsoon season, the land sea contrast heating strengthens circulation in the lower atmosphere, which in turn leads to the moistening and destabilization of lapse rate. This process starts about a month prior to the actual start of the monsoon rainfall, allowing moisture to build up to lead to the widespread monsoon rainfall. During winter months, rainfall occurs over northwest India in association with high negative value of CINE and over southern Peninsula in association with high value of CAPE. Substantial changes are noticed in the equivalent potential temperature (θe) and vorticity profiles between days with and without rainfall over northwest India (Delhi). The results suggest that the increase in moisture in association with the occurrence of rainfall over Delhi is of advective origin. Where as, other regions (Kolkata, Mumbai and Thiruvananthapuram) display relatively smaller changes in the vertical profiles of θe, indicating an in situ presence of moisture. Copyright © 2007 Royal Meteorological Society

Satellite‐based assessment of marine low cloud variability associated with aerosol, atmospheric stability, and the diurnal cycle

This study examines variability in marine low cloud properties derived from semiglobal observations by the Tropical Rainfall Measuring Mission (TRMM) satellite, as linked to the aerosol index (AI) and lower‐tropospheric stability (LTS). AI is derived from the Moderate Resolution Imaging Spectroradiometer (Terra MODIS) sensor and the Goddard Chemistry Aerosol Radiation and Transportation (GOCART) model and is used to represent column‐integrated aerosol concentrations. LTS is derived from the NCEP/NCAR reanalysis and represents the background thermodynamic environment in which the clouds form. Global statistics reveal that cloud droplet size tends to be smallest in polluted (high‐AI) and strong inversion (high‐LTS) environments. Statistical quantification shows that cloud droplet size is better correlated with AI than it is with LTS. Simultaneously, the cloud liquid water path (CLWP) tends to decrease as AI increases. This correlation does not support the hypothesis or assumption that constant or increased CLWP is associated with high aerosol concentrations. Global variability in corrected cloud albedo (CCA), the product of cloud optical depth and cloud fraction, is very well explained by LTS, while both AI and LTS are needed to explain local variability in CCA. Most of the local correlations between AI and cloud properties are similar to the results from the global statistics, while weak anomalous aerosol‐cloud correlations appear locally in the regions where simultaneous high (low) AI and low (high) LTS compensate each other. Daytime diurnal cycles explain additional variability in cloud properties. CCA has the largest diurnal cycle in high‐LTS regions. Cloud droplet size and CLWP have weak diurnal cycles that differ between clean and polluted environments. The combined results suggest that investigations of marine low cloud radiative forcing and its relationship to hypothesized aerosol indirect effects must consider the combined effects of aerosols, thermodynamics, and the diurnal cycle.

GOES Climatology and Analysis of Thunderstorms with Enhanced 3.9-μm Reflectivity

Abstract By combining observations from the Geostationary Operational Environmental Satellite (GOES) 3.9- and 10.7-μm channels, the reflected component of the 3.9-μm radiance can be isolated. In this paper, these 3.9-μm reflectivity measurements of thunderstorm tops are studied in terms of their climatological values and their utility in diagnosing cloud-top microphysical structure. These measurements provide information about internal thunderstorm processes, including updraft strength, and may be useful for severe weather nowcasting. Three years of summertime thunderstorm-top 3.9-μm reflectivity values are analyzed to produce maps of climatological means across the United States. Maxima occur in the high plains and Rocky Mountain regions, while lower values are observed over much of the eastern United States. A simple model is used to establish a relationship between 3.9-μm reflectivity and ice crystal size at cloud top. As the mean diameter of a cloud-top ice crystal distribution decreases, more solar radiation near 3.9 μm is reflected. Using the North American Regional Reanalysis dataset, the thermodynamic environment that favors thunderstorms with large 3.9-μm reflectivity values is identified. In the high plains and mountains, environments with relatively dry boundary layers, steep lapse rates, and large vertical shear values favor thunderstorms with enhanced 3.9-μm reflectivity. Thunderstorm processes that lead to small ice crystals at cloud top are discussed, and a possible relationship between updraft strength and 3.9-μm reflectivity is presented.

Cirrus Mammatus Properties Derived from an Extended Remote Sensing Dataset

Abstract The first quantitative and statistical evaluation of cirrus mammatus clouds based on wavelet analysis of remote sensing data is made by analyzing the University of Utah Facility for Atmospheric Remote Sensing (FARS) 10-yr high-cloud dataset. First, a case study of cirrus mammata combining a high-resolution lidar system and a W-band Doppler radar is presented, yielding an assessment of the thermodynamic environment and dynamic mechanisms. Then, 25 cirrus mammatus cases selected from the FARS lidar dataset are used to disclose their characteristic environmental conditions, and vertical and length scales. The results show that cirrus mammata occur in the transition zone from moist (cloudy) to dry air layers with weak wind shear, which suggests that cloud-induced thermal structures play a key role in their formation. Their maximum vertical and horizontal length scales vary from 0.3 to 1.1 km and 0.5 to 8.0 km, respectively. It is also found that small-scale structures develop between the large-scale protuberances. The spectral slopes of the lidar-returned power and mean radar Doppler velocity data extracted from the cirrus cloud-base region further indicate the presence of developed three-dimensional, locally isotropic, homogeneous turbulence generated by buoyancy. Finally, comparisons of anvil and cirrus mammata are made. Although both are generated in a similar environment, cirrus mammata generally do not form fallout fronts like their anvil counterparts, and so do not have their smooth and beautiful outlines.

Diesel Spray Analysis by Means of Planar Laser-Induced Exciplex Fluorescence

The planar laser-induced exciplex fluorescence (PLIEF) technique has been used to visualize and measure the fuel/air concentration fields in both liquid and vapour phases of direct injection (DI) diesel sprays. Experiments have been performed in a hot rig, which provides a thermodynamic environment realistic for current DI diesel engines, using a blend of 90 per cent hexadecane, 9 per cent α-methyl-naphthalene, and 1 per cent N,N,N',N'-tetramethyl- p-phenylenediamine to separate spectrally the fluorescence signal from the liquid and vapour phases of the diesel. In this paper, measurements have been performed for three injection pressures and three combustion chamber density values. The results on penetration and fuel concentration in liquid and vapour phases are analysed, providing evidence of the similarities between diesel sprays and gaseous turbulent jets.

Influence of Cold Air Intrusions on the Wet Season Onset over Amazonia

Abstract Using 15-yr data from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-15), the authors found that rapid southeastward expansion of the rainy area from the western Amazon to southeastern Brazil is a result of midlatitude cold air intrusions. During austral spring, as the large-scale thermodynamic structure over Amazonia becomes destabilized, the incursions of extratropical cold air can trigger intense rainfall along the leading edge of northwest–southeast-oriented cold fronts east of the Andes. As these fronts penetrate into Amazonia, the northerly or northwesterly wind transports warm, moist air from the western Amazon to southeast Brazil. Moisture convergence consequently intensifies, resulting in northwest–southeast-elongated rainy areas. The latter contribute to the observed rapid, southeastward expansion of rainy areas shown in rainfall climatology during austral spring. The authors’ analysis suggests that cold air intrusions during austral spring collectively assist the transformation of large-scale thermodynamic and dynamic environments to those favorable for the wet season onsets. Each time the cold fronts pass by, they tend to increase the atmospheric humidity and the buoyancy of the lower troposphere, which destabilizes the atmosphere. In the upper troposphere, the cold air intrusions supply kinetic energy for the development of anticyclonic flow. Cold air intrusions in the transitional season are not different from those occurring immediately before the wet season onsets except that the latter occurs under a more humid and unstable atmospheric condition. Thus, cold air intrusions can trigger the wet season onsets only when atmospheric and land surface conditions are “ready” for the onset. Comparisons among early, normal, and late onsets on an interannual scale further suggest that more frequent and stronger cold air intrusions trigger the early onsets of wet seasons given suitable large-scale thermodynamic conditions. Likewise, less frequent and weaker cold air intrusions could delay the wet season onset even though the large-scale thermodynamic conditions appear to be favorable. Occasionally, strong unstable atmospheric thermodynamic conditions and northerly reversal of cross-equatorial flow can lead to wet season onsets without cold air intrusions. In such cases, enhanced precipitation is centered over central and eastern Amazon, and rainfall increases more gradually compared to the onset with cold air intrusions.

Structural modifications of smectites mechanically deformed under controlled conditions

AbstractSWy-1 and SAz-1 smectites and an Italian bentonite from Sardinia were mechanically deformed via high-energy ball milling for 20 h, in a controlled thermodynamic environment at constant temperature (25°C) under vacuum. The deformed smectites have a lower cation exchange capacity (CEC) and form thicker particles than the original ones, due to agglomeration of smectite crystallites. The 001 diffraction maximum shifted to lower d spacings, the intensity of the 060 reflection decreased and the background at 20-30°2θ increased, suggesting partial amorphization of the smectite. Moreover, the layer charge of the smectites decreased. The intensity of the complex stretching band at 3625 cm-1, and the AlAlOH, and AlFe3+OH bending bands at 916 cm-1 and 886 cm-1, respectively, decreased, while the band at AlMgOH bending at 849 cm-1, disappeared. Deformation mainly disrupted the octahedral sheet and preferentially destroyed those sites occupied by Mg cations, thus explaining the observed decrease in layer charge. Octahedral sites occupied by Fe were least affected. The disruption of the octahedral sheet is substantiated further by the almost total disappearance of the dehydroxylation peak, which is more pronounced in the Mg-rich SAz-1 smectite.

Thermal and structural properties of Ca-rich Montmorillonite mechanically deformed by compaction and shear

This research work reports on the thermal and structural properties of powdered Ca-rich montmorillonite induced by mechanical deformation, which involves simultaneous compression and shear in a controlled thermodynamic environment. X-ray powder diffraction (XRPD), infra-red (FT-IR) and thermal analyses (TG-DTG-DTA) were employed for structural and thermal characterization. Mechanical deformation performed up to 20 h induces significant physical and structural changes to Ca-rich montmorillonite as a function of time, leading to a progressive compaction and agglomeration of particles, reduction of crystallinity and an increase of microstrain, which affects both the tetrahedral and octahedral layers. Thermal analysis of the deformed samples has indicated a loss of structural OH at a temperature which is well below the classical dehydroxylation temperature of the undeformed Ca-rich montmorillonite. This is a consequence of the disordered and defective structure (produced by the mechanical treatment), which requires a lower energy to induce the dehydroxylation of montmorillonite. Also the formation of mullite produced by heating montmorillonite at high temperature is affected by the mechanical treatment. The deformed samples yielded mullite with a faster kinetics at a lower temperature compared to their original counterparts (1155 instead of 1270 °C).

Study of structural properties of ion treated and mechanically deformed commercial bentonite

This study deals with the investigation of the structural changes of Ca-montmorillonite present in a commercial bentonite produced by mechanical deformation via high-energy ball milling or by ionised argon interaction, both in controlled thermodynamic environment. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier-transform infra-red (FTIR), X-ray powder diffraction (XRPD), particle size laser diffraction (PSLD) and swelling index (SI) were used for the characterization of the samples. (i) Ball milling. We have observed that high-energy ball milling produced a progressive reduction of the d (001) lattice spacing of the montmorillonite from 1.50 to 1.30 nm as a function of time (at a constant cycle frequency). Simultaneously, a severe decrease and broadening of the XRD spectrum peaks were also detected, indicating a reduction of crystallinity and an increase of microstrain. Analysis by laser-diffraction indicates a bimodal distribution of the particle size, and a progressive increase of the number of particles with size in the range 0.5–1 μm and greater than 20 μm, for the material milled up to 20 h. However, by using increased spatial resolution, sub-micrometric rounded particles were observed after 20 h of milling by SEM analysis, and the presence of nanometric particles by TEM. FTIR and DSC clearly showed a structural destabilization both in the interlayer and in the intralayer and a progressive loss of interlayer water. The swelling index drastically reduced as a function of milling time. Finally, as expected, the more pronounced transformations were observed for milling in medium vacuum of 0.13 Pa (10 −3 Torr), than for in inert gas or air atmospheres because of the more dehydrating environment. (ii) Argon-ion treatment. X-ray diffraction analysis showed that the mean d (001) montmorillonite lattice spacing shifted from 1.50 to 1.35 nm, whilst the peak intensity decreased and broadened, with a decrease of the crystallite size in the 001 direction. SEM showed an increased presence of sub-micrometric rounded particles at the surface of the treated samples than for the untreated ones. DSC indicated a decrease in water content after the gas treatment. Laser diffraction showed that the ion treatment increased the number of submicron-sized particles with a concomitant reduction of the number of particles with bigger sizes, leading to an increase of the volumetric surface. These very preliminary results suggest that ionised argon treatment performed in a controlled environment can be exploited to reduce the amount of interlayer water molecules in montmorillonite, to modify its surface characteristics and particle size distribution in a way to increase the specific surface without particle agglomeration or compaction. Applications to other clay minerals (for example, talc and kaolinite, etc.) are envisaged and in progress. Hence, both mechanical deformation and interactions of particular ionised gases can induce significative changes of the physical and chemical properties of bentonite and other clay minerals and further systematic studies are in progress in order to evaluate the potentiality for low-cost industrial applications.

Tropical cyclone environments over the northeastern Pacific, including mid-level dry intrusion cases

This study uses a 1°×1° lat/long dataset, extracted from ECMWF re-analyses for the 15-year period 1979–1993 (ERA-15), to diagnose the synoptic-scale kinematic, thermodynamic and moisture environments in the vicinity of named tropical cyclones (TCs) in the eastern North Pacific. Based on the NCDC best track dataset, TCs are partitioned into one of three categories: weak (W), strong (S) or intensifying (I). In total, 63 TCs are examined: 8 Ws and 20 Is at point A (maximum intensification) and 11 Ws, 13 Ss and 11 Is at point B (maximum frequency). Composite maps are compiled for all five groups, and six individual case studies are examined, four for extreme TC cases and two for cases involving dry air intrusions.

Hailstorms in Northern Greece: synoptic patterns and thermodynamic environment

¶The synoptic scale atmospheric circulation prevailing during hail days in Northern Greece is examined for a period of 26 years (1976–2001) during the warm season. The initial objective of this classification scheme is to serve as a predictive tool for hail forecasting and permit a specification of greater or lesser hail risk, within the forecast activities of the Greek National Hail Suppression Program. The atmospheric circulation was classified into seven synoptic types and their frequency distribution was examined on an annual and monthly basis. Southwesterly flow (SW) and Shortwave trough (SWT) appear to be the most conducive synoptic types for hailstorm activity. Furthermore, the thermodynamic environment favoring the hailstorm development and evolution was examined using selected instability indices. The frequency distribution of the instability indices for a period of 11 years (1991–2001) in association with the synoptic types demonstrated their performance as a forecasting aid for thunderstorm activity to occur in a given synoptic environment.

Thermodynamic environments in proteins: fundamental determinants of fold specificity.

To investigate the relationship between an amino acid sequence and its corresponding protein fold, a database of thermodynamic stability information was assembled as a function of residue type from 81 nonhomologous proteins. This information was obtained using the COREX algorithm, which computes an ensemble-based description of the native state of proteins. Dissection of the COREX stability constant into its fundamental energetic components resulted in 12 thermodynamic environments describing the tertiary architecture of protein folds. Because of the observation that residue types partitioned unequally between these environments, it was hypothesized that thermodynamic environments contained energetic information that connected sequence to fold. To test the significance of this hypothesis, the thermodynamic stability information was incorporated into a three-dimensional-to-one-dimensional scoring matrix, and simple fold recognition experiments were performed in a manner such that information about the fold target was never included in the scoring. For 60 out of 81 fold targets, the correct sequence for the target scored in the top 5% of 3858 decoy sequences, with Z-scores ranging from 1.76 to 12.23. Furthermore, a scoring matrix assembled from the residues of 40 nonhomologous all-alpha proteins was used to thread sequences against 12 nonhomologous all-beta protein targets. In 10 of 12 cases, sequences known to adopt the native all-beta structure scored in the top 5% of 3858 decoy sequences, with Z-scores ranging from 1.99 to 7.94. These results indicate that energetic information encoded by thermodynamic environments represents a fundamental property of proteins that underlies classifications based on secondary structure.

A unified description of localization for application to large‐scale tectonics

Localized regions of deformation such as faults and shear zones are ubiquitous in the Earth's lithosphere. However, we lack a simple unified framework of localization that is independent of the mechanism or scale of localization. We address this issue by introducing the effective stress exponent,ne, a parameter that describes how a material responds to a local perturbation of an internal variable being tested for localization. The value ofneis based on micromechanics. A localizing regime has a negativene, indicating a weakening behavior, and localization is stronger for more negative 1/ne. We present expressions for the effective stress exponent associated with several mechanisms that trigger localization at large scale: brittle failure with loss of cohesion, elastoplasticity, rate‐ and state‐dependent friction, shear heating, and grain‐size feedback in ductile rocks. In most cases, localization does not arise solely from the relation between stress and deformation but instead requires a positive feedback between the rheology and internal variables. Brittle mechanisms (failure and friction) are generally described byneof the order of −100. Shear heating requires an already localized forcing, which could be provided by a brittle fault at shallower levels of the lithosphere. Grain size reduction, combined with a transition from dislocation to diffusion creep, leads to localization only if the grain size departs significantly from its equilibrium value, because either large‐scale flow moves rocks through different thermodynamic environments or new grains are nucleated. When shear heating or grain‐size feedback produce localization, 1/necan be extremely negative and can control lithospheric‐scale localization.

The Spatial, Temporal, and Thermodynamic Characteristics of Southern-Atlantic United States Tornado Events

Tornadic storm environments are not well understood in regions beyond the Great Plains. Severe weather forecasting index values are derived for 129 Southern-Atlantic (SA) Coast (South Carolina, North Carolina, and Virginia) and 98 southern Great Plains (Texas, Oklahoma, and Kansas) tornado days through spatial and temporal interpolations of the proximity balloon soundings. Analyses of these data suggest that the two regions are associated with significantly different environments. The SA storm environments would be considered benign by Great Plains standards. In fact, 95% of the SA tornado events fall below the threshold for severe thunderstorms for the Severe Weather Threat (SWEAT) index. Therefore, the universal use of index threshold values by forecasters is inappropriate. Cluster and density analysis of the spatial data found that tornado events in the SA region are clustered in three general regions. One of the regions is likely influenced by the existence of a Sea-Breeze front or the Piedmont trough. However, given the benign thermodynamic environment, mesoscale augmentation of shear and instability is likely in the other two clustered regions as well.

Airflow Configurations of Warm Season Southerly Low-Level Wind Maxima in the Great Plains. Part I: Spatial and Temporal Characteristics and Relationship to Convection

Abstract A synoptic climatological approach was used to investigate 1) the airflow characteristics of southerly low-level wind maxima in the Great Plains, 2) the typical thermodynamic environments in which low-level wind maxima form, 3) the convergence fields accompanying the wind maxima, and 4) the associated patterns of cloud-to-ground lightning activity. A total of 260 low-level jet events that occurred at either 0000 or 1200 UTC during the warm seasons (Apr–Sep) of 1991–92 were employed in the analysis. The spatial configuration of southerly low-level jets was shown to be considerably more complex than previously portrayed. A subjective classification of the jet events—based on streamline curvature, latitudinal extent, and the orientation of confluence and deformation zones—resulted in 12 distinct configuration types. Only 37% of the events were broadly classified as anticyclonically curving jets, even though the majority of previous case studies focused on this type of wind maximum. Another unanticip...

Examination of Derecho Environments Using Proximity Soundings

Observed upper air soundings that occurred within 2 h and 167 km of derechos were collected and analyzed to document atmospheric stability and wind shear conditions associated with long-lived convective windstorms. Sixty-seven derechos, accompanied by 113 proximity soundings, were identified during the years 1983–93. Owing to the large variability of the synoptic-scale environments associated with derechos, each derecho was further divided into categories based on the strength of synoptic-scale forcing associated with each event. Derechos are shown to develop and persist in a wide range of shear and instability conditions. Although this range of shear and instability narrows when derechos are grouped by synoptic-scale forcing strength, considerable variation of values remains, primarily with the shear. These results suggest that ambient shear and instability values alone are not sufficient to distinguish derecho environments from those associated with nonsevere mesoscale convective system (MCS) environments. Though the ground-relative (ambient) shear values vary, more consistency is found in the system-relative winds. It is found that midlevel system-relative winds are consistently weak, while low-level system-relative inflow is strong. This is especially valid for events associated with weak synoptic-scale forcing. Fast forward propagation is associated with weak system-relative midlevel winds, which may in turn be associated with outflow-dominated storms having strong cold pools at the ground. In addition, a comparison between weak forcing derecho and nonderecho MCSs suggests that it is the strength of the mean flow, and its possible effects on speed of movement, that enhance the potential for sustained severe wind gusts at the surface, given similar thermodynamic environments. These results indicate that there is a larger range of shear and instability environments associated with derechos than has been suggested by some observational studies and numerical cloud simulations. In addition, little correlation is evident in operational observations between events with strong cold pools and those with correspondingly strong low-level (0–2/0–3 km) ambient shear. Owing to the large variance of convective available potential energy (CAPE)/shear values found in this observational study, forecasters should be aware of the potential for derecho formation within environments with weaker shear than suggested by numerical simulations. This is especially true in the absence of a deep, progressive midtropospheric trough and an associated surface cold front. When synoptic-scale forcing is strong, derechos can develop and persist within environments with almost no CAPE evident on nearby upper air soundings.

Thermodynamic propensities of amino acids in the native state ensemble: implications for fold recognition.

An amino acid sequence, in the context of the solvent environment, contains all of the thermodynamic information necessary to encode a three-dimensional protein structure. To investigate the relationship between an amino acid sequence and its corresponding protein fold, a database of thermodynamic stability information was assembled that spanned 2951 residues from 44 nonhomologous proteins. This information was obtained using the COREX algorithm, which computes an ensemble-based description of the native state of a protein. It was observed that amino acid types partitioned unequally into high, medium, and low thermodynamic stability environments. Furthermore, these distributions were reproducible and were significantly different than those expected from random partitioning. To assess the structural importance of the distributions, simple fold-recognition experiments were performed based on a 3D-1D scoring matrix containing only COREX residue stability information. This procedure was able to recover amino acid sequences corresponding to correct target structures more effectively than scoring matrices derived from randomized data. High-scoring sequences were often aligned correctly with their corresponding target profiles, suggesting that calculated thermodynamic stability profiles have the potential to encode sequence information. As a control, identical fold-recognition experiments were performed on the same database of proteins using DSSP secondary structure information in the scoring matrix, instead of COREX residue stability information. The comparable performance of both approaches suggested that COREX residue stability information and secondary structure information could be of equivalent utility in more sophisticated fold-recognition techniques. The results of this work are a consequence of the idea that amino acid sequences fold not into single, rigidly stable structures but rather into thermodynamic ensembles best represented by a time-averaged structure.

Long-Lived Mesoconvective Vortices and Their Environment. Part I: Observations from the Central United States during the 1998 Warm Season

Abstract Observations from the modernized United States National Weather Service (NWS) data network are used to assess the frequency and general characteristics of midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems (MCSs). Results from the 1998 convective season (15 May–15 September) over the central United States suggest that long-lived MCVs, which persist after the dissipation of the initiating MCS, are more common than previously documented. These MCVs occur in weaker ambient vertical shear (both in the lower troposphere and through a nominal vortex layer) than MCSs from which no detectable MCVs are spawned. An important aspect of MCVs is that they may focus subsequent convective development within long-lived discontinuous heavy precipitation episodes. Subsequent deep convection is observed in the vicinity of MCVs in slightly greater than 1/2 of the MCV cases. This subsequent convection occurs in thermodynamic environments of moderate-to-large convective available potent...