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Use of the Parcel Buoyancy Minimum (Bmin) to Diagnose Simulated Thermodynamic Destabilization

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Use of the Parcel Buoyancy Minimum (Bmin) to Diagnose Simulated Thermodynamic Destabilization VOLUME 142 MONTHLY WEATHER REVIEW MARCH 2014 Use of the Parcel Buoyancy Minimum (Bmin) to Diagnose Simulated Thermodynamic Destabilization. Part I: Methodology and Case Studies of MCS Initiation Environments STANLEY B. TRIER,CHRISTOPHER A. DAVIS,DAVID A. AHIJEVYCH, AND KEVIN W. MANNING National Center for Atmospheric Research,* Boulder, Colorado (Manuscript received 29 August 2013, in final form 23 October 2013) ABSTRACT 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 thermody- namic 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. 1. Introduction phenomena are L ; 1–10 km. Horizontal convective rolls (HCRs) are one of the above examples that influence CI One of the most difficult aspects of deep, precipitating not only by the lifting they impart, but also because of the convection to quantify and predict is its initiation. This is thermodynamic contrasts within them (e.g., Mueller et al. because atmospheric processes of varying scales influence 1993; Weckwerth et al. 1996). The role of mesoscale (L ; both the thermodynamic vertical structure in which con- 100 km) environmental temperature/moisture advections vection occurs and how it is initiated. Recent research and vertical motions on organized convection is perhaps has emphasized the latter, partly because of the increasing even more widely recognized (e.g., Maddox 1983; Crook availability of radar, satellite, and high-resolution surface and Moncrieff 1988). However, because of our inability to data used to detect finescale flow features (e.g., drylines, sample these effects at appropriate resolution over me- sea-breeze fronts, remnant convective outflows, internal soscale regions, their influence on the thermodynamic gravity waves, and horizontal rolls) along which convec- vertical structure, crucial to CI, remains poorly quantified. tion initiation (CI) often occurs. The widths of these Though detailed observations are typically lacking, current research numerical models are capable of both * The National Center for Atmospheric Research is sponsored explicitly simulating deep, moist convection and rep- by the National Science Foundation. resenting the mesoscale environmental thermodynamic variations often associated with CI. In the current paper we develop a method, based on parcel theory, that allows Corresponding author address: StanleyB.Trier,National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO us to evaluate the influence of different physical processes 80307-3000. on the evolution of the mesoscale thermodynamic verti- E-mail: [email protected] cal structure. Case studies with a convection-permitting DOI: 10.1175/MWR-D-13-00272.1 Ó 2014 American Meteorological Society 945 946 MONTHLY WEATHER REVIEW VOLUME 142 numerical model are used to demonstrate the method and examine how these physical processes modify thermodynamic environments allowing the initiation of observed mesoscale convective systems (MCSs) in three different midlatitude warm-season meteorological situations. The widely used parcel theory of convection neglects vertical accelerations due to pressure forces, as well as buoyancy effects due to condensate loading and en- trainment of environmental air. Since these effects FIG. 1. CIN (gray shading) and minimum buoyancy Bmin (blue typically have a deleterious impact on updraft strength, line) for the most conditionally unstable 50-hPa averaged PBL parcel originating at the pressure p that is lifted to the pressure of the convective available potential energy, o minimum buoyancy pBmin . The vertical axis is log pressure (hPa) and ð the horizontal axis is skewed temperature. The thick green line is the pLFC 5 2 environmental dewpoint and the red line is the environmental CAPE R(Tvp Tve)d ln(p), (1) p temperature. The dashed aqua lines leaning to the right indicate LNB 2 constant mixing ratios (g kg 1) and the brown solid lines leaning to derived from parcel theory provides only an approxi- the left are dry adiabats indicating constant potential temperature mate upper bound on the strength of the convection. (8C). The nearly vertical green lines are pseudoadiabats indicating 8 The CAPE defined in (1) is the integrated buoyancy, constant wet-bulb potential temperature ( C). B 5 R(Tvp 2 Tve)/p, from the level of free convection (LFC) to the level of neutral buoyancy (LNB) arising For a given Tve vertical profile, CAPE depends only from differences between the parcel (Tvp) and envi- on the air parcel’s moist static energy, H 5 cpT 1 Lyq 1 ronmental (Tve) virtual temperatures. gz, where c is a specific heat at a constant pressure, Ly is Although the CAPE provides an estimate of how p the latent heat of vaporization, and g is gravity. This strong convection can become, it offers little guidance 2 implies dq (g kg 1) ’ 2.5dT (K) for equivalent moist on whether it will actually occur. Indeed, expansive areas p p static energy perturbations to the parcel. The contribu- of positive CAPE often occur during the midlatitude tions of parcel temperature and moisture perturbations warm season (e.g., Dai et al. 1999), but areas experi- to CIN changes are more complicated and, unlike for encing deep convection at any given time are much CAPE, do not depend uniquely on moist static energy smaller. Given positive CAPE, the magnitude of the changes. In contrast to CAPE, which is completely speci- convective inhibition, fied by initial parcel pressure, environmental virtual tem- ð p perature, and parcel vertical displacements along a moist 5 o 2 adiabat where H is constant, contributions to CIN also CIN R(Tvp Tve)d ln(p), (2) pLFC depend on unsaturated lifting beneath the parcel’s lifted condensation level (LCL). Thus, for a fixed increment of which is the vertically integrated negative buoyancy parcel dH, the relative importance of dTp and dqp to between the parcel origination level po and its LFC, is CIN changes depends on both the parcel’s relative hu- a better indicator of whether deep convection is likely to midity at its source level and the environmental static occur. Convective inhibition (CIN; Colby 1984) is illus- stability (Crook 1996). trated schematically by the shaded region in Fig. 1 for Previous diagnostic studies have used CIN and related a planetary boundary layer (PBL) air parcel. thermodynamic quantities to assess physical factors The minimum vertical velocity of an air parcel, wmin, influencing CI. For example, Ziegler and Rasmussen required to overcome the CIN may be estimated by (1998) used soundings modified by a simple numerical substituting the form of the vertical momentum equation transport model to illustrate the importance of bound- derived from parcel theory, dwp/dt 5 wp(dwp/dz) 5 B,into ary layer circulations in reducing CIN to values sup- (2) and taking the square root, which yields 2(2CIN)0.5. portive of CI. Carlson et al. (1980, 1983) developed the Thus, even for a seemingly small magnitude of CIN 5 lid strength index (LSI) to illustrate the roles of differ- 21 21 250 J kg , a strong updraft of wmin 5 10 m s is required ential horizontal advection and transverse vertical cir- to initiate deep convection. The infrequent occurrence of culations on focusing CI. Unlike CIN, the LSI is not an vertical velocities of this magnitude outside of preexisting integral quantity and considers average parcel and en- convection indicates the importance of physical processes vironmental properties within two separate layers. acting over longer time scales to eliminate or substantially In the current study we analyze physical factors influ- reduce the CIN. encing Bmin 5 min(Tvp 2 Tve), which is the minimum MARCH 2014 T R I E R E T A L . 947 buoyancy (in temperature units) of a vertically displaced illustrates the association of CI with local and regional air parcel. The blue line in Fig. 1 illustrates Bmin for a evolutions in Bmin, and compares these Bmin evolutions 50-hPa-deep surface-based parcel. This thermodynamic with evolutions of more widely used thermodynamic parameter is related to the CIN in a manner analogous variables such as CIN and CAPE. Our budget method- to how the minimum lifted index (LI) is related to the ology used to estimate the causes of Bmin changes in the CAPE, and has been used successfully to determine thermodynamic destabilization time intervals prior to CI, the susceptibility to deep convection in modeling studies and results from this analysis for the three different ranging from the examination of the effects of land– simulated CI cases, are presented in sections 4 and 5, atmosphere exchange strength on deep convection (Trier respectively.
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