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  • 2010-2014  (7)
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  • 1
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    Use of the Parcel Buoyancy Minimum (Bmin) to Diagnose Simulated Thermodynamic Destabilization. Part I: Methodology and Case Studies of MCS Initiation Environments (2014)
    American Meteorological Society
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    Publication Date: 2014-03-01
    Description: A method based on parcel theory is developed to quantify mesoscale physical processes responsible for the removal of inhibition energy for convection initiation (CI). Convection-permitting simulations of three mesoscale convective systems (MCSs) initiating in differing environments are then used to demonstrate the method and gain insights on different ways that mesoscale thermodynamic destabilization can occur. Central to the method is a thermodynamic quantity Bmin, which is the buoyancy minimum experienced by an air parcel lifted from a specified height. For the cases studied, vertical profiles of Bmin using air parcels originating at different heights are qualitatively similar to corresponding profiles of convective inhibition (CIN). Though it provides less complete information than CIN, an advantage of using Bmin is that it does not require vertical integration, which simplifies budget calculations that enable attribution of the thermodynamic destabilization to specific physical processes. For a specified air parcel, Bmin budgets require knowledge of atmospheric forcing at only the parcel origination level and some approximate level where Bmin occurs. In a case of simulated daytime surface-based CI, destabilization in the planetary boundary layer (PBL) results from a combination of surface fluxes and upward motion above the PBL. Upward motion effects dominate the destabilizing effects of horizontal advections in two different simulated elevated CI cases, where the destabilizing layer occurs from 1 to 2.5 km AGL. In an elevated case with strong warm advection, changes to the parcel at its origination level dominate the reduction of negative buoyancy, whereas for a case lacking warm advection, adiabatic temperature changes to the environment near the location of Bmin dominate.
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    Thermodynamic Environments of Deep Convection in Atlantic Tropical Disturbances (2013)
    American Meteorological Society
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    Publication Date: 2013-07-01
    Description: Conditional composites of dropsondes deployed into eight tropical Atlantic weather systems during 2010 are analyzed. The samples are conditioned based on cloud-top temperature within 10 km of the dropsonde, the radius from the cyclonic circulation center of the disturbance, and the stage of system development toward tropical cyclogenesis. Statistical tests are performed to identify significant differences between composite profiles. Cold-cloud-region-composite profiles of virtual temperature deviations from a large-scale instantaneous average indicate enhanced static stability prior to genesis within 200 km of the center of circulation, with negative anomalies below 700 hPa and larger warm anomalies above 600 hPa. Moist static energy is enhanced in the middle troposphere in this composite mainly because of an increase in water vapor content. Prior to genesis the buoyancy of lifted parcels within 200 km of the circulation center is sharply reduced compared to the buoyancy of parcels farther from the center. These thermodynamic characteristics support the conceptual model of an altered mass flux profile prior to genesis that strongly favors convergence in the lower troposphere and rapid increase of circulation near the surface. It is also noted that the air–sea temperature difference is greatest in the inner core of the pregenesis composite, which suggests a means to preferentially initiate new convection in the inner core where the rotation is greatest.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 3
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    Observations of Temperature in the Upper Troposphere and Lower Stratosphere of Tropical Weather Disturbances (2014)
    American Meteorological Society
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    Publication Date: 2014-04-28
    Description: Microwave temperature profiler (MTP) data are analyzed to document temperature signatures in the upper troposphere and lower stratosphere that accompany Atlantic tropical weather disturbances. The MTP was deployed on the National Science Foundation–National Center for Atmospheric Research Gulfstream V (GV) aircraft during the Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) in August and September 2010. Temporal variations in cold-point temperature compared with infrared cloud-top temperature reveal that organized deep convection penetrated to near or beyond the cold point for each of the four disturbances that developed into a tropical cyclone. Relative to the lower-tropospheric circulation center, MTP and dropsonde data confirmed a stronger negative radial gradient of temperature in the upper troposphere (10–13 km) of developing disturbances prior to genesis compared with nondeveloping disturbances. The MTP data revealed a somewhat higher and shallower area of relative warmth near the center when compared with dropsonde data. MTP profiles through anvil cloud depicted cooling near 15 km and warming in the lower stratosphere near the time of maximum coverage of anvil clouds shortly after sunrise. Warming occurred through a deep layer of the upper troposphere toward local noon, presumably associated with radiative heating in cloud. The temperature signatures of anvil cloud above 10-km altitude contributed to the radial gradient of temperature because of the clustering of deep convection near the center of circulation. However, it is concluded that these signatures may be more a result of properties of convection than a direct distinguishing factor of genesis.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 4
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    Numerical Simulations of the Postsunrise Reorganization of a Nocturnal Mesoscale Convective System during 13 June IHOP_2002 (2011)
    American Meteorological Society
    Details
    Publication Date: 2011-12-01
    Description: A convection-permitting numerical model is used to simulate the postsunrise reorganization of a nocturnal mesoscale convective system (MCS) observed over western and central Oklahoma on 13 June 2002 during the International H2O (IHOP_2002) Field Experiment. The MCS reorganization consists of a transition from northwest–southeast-oriented convective rainbands near sunrise to a single northeast–southwest (NE–SW)-oriented convective rainband with trailing stratiform precipitation later in the morning. Results indicate the importance of environmental preconditioning on MCS reorganization. In particular, the development of the NE–SW rainband that redefines the MCS organization is facilitated by a similarly oriented zone of antecedent mesoscale upward motion, which increases the depth of large water vapor mixing ratios. This allows convective updrafts to be fed primarily by moist and conditionally unstable air from 1 to 2.5 km AGL in the NE–SW-oriented rainband, which lacks a surface cold pool during its incipient postsunrise stage. The MCS develops a strong surface cold pool from latent cooling–induced downdrafts by midmorning and evolves into an upshear-tilted squall-type system. These downdrafts and the resulting cold pool are not necessary for the overall reorganization and maintenance of the MCS in this environment where earlier mesoscale ascent has occurred. However, the latent cooling from downdrafts does influence the MCS strength, vertical structure, and horizontal motion by early in the postsunrise stage. In contrast, surface heating of the preconvective environment has little effect on the strength and structural characteristics of the MCS until midday, by which time the convection has become primarily surface based.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 5
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    Mesoscale Structural Evolution of Three Tropical Weather Systems Observed during PREDICT (2012)
    American Meteorological Society
    Details
    Publication Date: 2012-03-30
    Description: Three well-observed Atlantic tropical weather systems that occurred during the 2010 hurricane season are analyzed. One case was former Tropical Storm Gaston that failed to redevelop into a tropical cyclone; the other two cases were developing storms Karl and Matthew. Geostationary satellite, multisensor-derived precipitation, and dropsondes from the National Science Foundation (NSF)–NCAR Gulfstream V (GV), NASA DC-8, and the NOAA Gulfstream IV (G-IV) and WP-3D Orion (P-3) aircraft are analyzed in a system-following frame to quantify the mesoscale dynamics of these systems. Gaston featured extensive dry air surrounding an initially moist core. Vertical shear forced a misalignment of midtropospheric and lower-tropospheric circulation centers. This misalignment allowed dry air to intrude above the lower-tropospheric center and severely limited the area influenced by deep moist convection, thus providing little chance of maintaining or rebuilding the vortex in sheared flow. By contrast, Karl and Matthew developed in a moister environment overall, with moisture increasing with time in the middle and upper troposphere. Deep moist convection was quasi-diurnal prior to genesis. For Karl, deep convection was initially organized away from the lower-tropospheric circulation center, creating a misalignment of the vortex. The vortex gradually realigned over several days and genesis followed this realignment within roughly one day. Matthew experienced weaker shear, was vertically aligned through most of its early evolution, and developed more rapidly than Karl. The evolutions of the three cases are interpreted in the context of recent theories of tropical cyclone formation.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 6
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    Use of the Parcel Buoyancy Minimum (Bmin) to Diagnose Simulated Thermodynamic Destabilization. Part II: Composite Analysis of Mature MCS Environments (2014)
    American Meteorological Society
    Details
    Publication Date: 2014-03-01
    Description: Herein, the parcel buoyancy minimum (Bmin) defined in Part I of this two-part paper is used to examine physical processes influencing thermodynamic destabilization in environments of mature simulated mesoscale convective systems (MCSs). These convection-permitting simulations consist of twelve 24-h forecasts during two 6-day periods characterized by two different commonly occurring warm-season weather regimes that support MCSs over the central United States. A composite analysis of 22 MCS environments is performed where cases are stratified into surface-based (SB), elevated squall (ES), and elevated nonsquall (ENS) categories. A gradual reduction of lower-tropospheric Bmin to values indicative of small convection inhibition, occurring over horizontal scales 〉100 km from the MCS leading edge, is a common aspect of each category. These negative buoyancy decreases are most pronounced for the ES and ENS environments, in which convective available potential energy (CAPE) is greatest for air parcels originating above the surface. The implication is that the vertical structure of the mesoscale environment plays a key role in the evolution and sustenance of convection long after convection initiation and internal MCS circulations develop, particularly in elevated systems. Budgets of Bmin forcing are computed for the nocturnally maturing ES and ENS composites. Though warm advection occurs through the entire 1.5-km-deep layer comprising the vertical intersection of the largest environmental CAPE and smallest environmental Bmin magnitude, the net effect of terms involving vertical motion dominate the destabilization in both composites. These effects include humidity increases in air parcels due to vertical moisture advection and the adiabatic cooling of the environment above.
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 7
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    Verifying Forecasts Spatially (2010)
    American Meteorological Society
    Details
    Publication Date: 2010-10-01
    Print ISSN: 0003-0007
    Electronic ISSN: 1520-0477
    Topics: Geography , Physics
    Published by American Meteorological Society
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