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Convective Initiation in an Idealized Cloud Model Using an Updraft Nudging Technique

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Convective Initiation in an Idealized Cloud Model Using an Updraft Nudging Technique NOVEMBER 2012 N A Y L O R A N D G I L M O R E 3699 Convective Initiation in an Idealized Cloud Model Using an Updraft Nudging Technique JASON NAYLOR AND MATTHEW S. GILMORE Department of Atmospheric Sciences, University of North Dakota, Grand Forks, North Dakota (Manuscript received 5 June 2012, in final form 7 August 2012) ABSTRACT Previous cloud modeling studies have noted difficulty in producing strong, sustained deep convection in environments with convective inhibition and/or midlevel dryness when the thermal bubble technique is used to initiate convection. This difficulty is also demonstrated herein, using 113 supercell proximity soundings— most of which contain capping inversions and some amount of convective inhibition. Instead, by using an updraft nudging initiation technique, substantially more supercells result and for a longer period. Addi- tionally, the number of supercell-producing cases is maximized when updraft nudging is applied for only the first 15 min of cloud time near the top of the boundary layer instead of longer/shorter periods or when nudging is applied near the surface. 1. Introduction used soundings for idealized supercells initiated using the bubble technique have capping inversions that Because the initial environment in most idealized have either been removed, were not resolved by the supercell simulations is devoid of horizontal gradients, vertical model grid spacing, or were absent by design. convective development must be initiated artificially. These are the 20 May 1977 Del City, Oklahoma, By far the most common method is the thermal per- sounding (e.g., Klemp et al. 1981; Grasso and Cotton turbation (or warm bubble) technique (Klemp and 1995; Gilmore and Wicker 1998; Adlerman et al. 1999; Wilhelmson 1978). With this method, a spheroid of Adlerman and Droegemeier 2005) and the Weisman positive potential temperature perturbation is inserted and Klemp analytical sounding (e.g., Weisman and in the center of the domain at the initial time. In the Klemp 1982, 1984; Brooks and Wilhelmson 1993; Brooks appropriate environment, the positively buoyant air in et al. 1994; Wicker and Wilhelmson 1995; Richardson this spheroid will rise—creating convergence and addi- et al. 2007). tional vertical motion in its wake. Over time, a strong Ziegler et al. (1997) state that these two soundings are convective updraft develops. Although the warm bubble similar to the narrow convective initiation regions ob- technique is widely used, it is not without drawbacks. served along drylines but are not representative of the The vast majority of environments observed near mature storm environment. For instance, in the well- mature supercells have some amount of convective in- studied 22 May 1981 Binger, Oklahoma, supercell, the hibition (CIN; e.g., Thompson et al. 2003; Davies 2004). storm survived and was not tornadic until it moved However, several idealized simulation studies have re- into an area with larger values of convective inhibi- ported difficulty in using the warm bubble method to tion (Ziegler et al. 2010). Mun˜ oz (1994) emulated such initiate convection in environments containing capping environmental changes within an idealized cloud inversions (e.g., Chen and Orville 1980; Wicker et al. model by initializing the warm bubble in the uncapped 1997; Elmore et al. 2002; Letkewicz and Parker 2011) or sounding and progressively nudging a capping in- lacking deep moisture (e.g., McCaul and Cohen 2004). version into the model for a maturing supercell storm. Perhaps it is not surprising that the two most commonly Sustained convection would not initiate using only the capped environment. Corresponding author address: Jason Naylor, NorthWest Re- An alternative to using a warm bubble to initiate search Associates, 3380 Mitchell Ln., Boulder, CO 80301. convection is to apply a convergent wind field. Tripoli E-mail: [email protected] and Cotton (1980) and Loftus et al. (2008) both used DOI: 10.1175/MWR-D-12-00163.1 Ó 2012 American Meteorological Society Unauthenticated | Downloaded 09/30/21 06:27 AM UTC 3700 MONTHLY WEATHER REVIEW VOLUME 140 FIG. 1. Box and whisker plots of (a) mixed layer CAPE, (b) magnitude of mixed layer CIN, (c) CAPE/CIN ratio, and (d) 2–5-km average relative humidity for the supercell-producing (SUP) and nonsupercell plus NULL producing simulations (NON 1 NULL) using warm bubble convective initiation. CAPE and CIN are calculated using a 500-m- thick parcel to represent the surface and virtual temperature. The whiskers represent 2.5 times the standard deviation from the mean. a sustained initiation technique to study convective This study has three main purposes: 1) to demonstrate development by nudging the model winds with a near- that a sustained updraft nudging initiation technique surface convergence field, but this was tested using is substantially more effective than the instantaneous environments without capping inversions. The hori- warm bubble technique at producing supercells in hori- zontal convergence produces a positive pressure zontally homogeneous environments with capping in- anomaly, which drives an upward-directed pertur- versions; 2) determine which updraft nudging settings bation pressure force and resulting updraft to help produce the strongest, longest-lived supercells; and parcels reach their level of free convection (LFC). 3) demonstrate that a sustained forcing technique is Alternatively, one may nudge an updraft within the most effective when elevated off of the surface. boundary layer (e.g., Ziegler et al. 2010). This is similar to the convergent wind technique, except that the horizontal wind field responds to the updraft instead of 2. Methodology vice versa. However, there is some question as to the a. Model setup time period and altitude that the low-level updraft nudging should be applied to overcome a typical cap- Simulations with 1-km horizontal grid spacing were ping inversion. Too low, and the air may slow or even performed using version 14 of the Bryan cloud model stop its vertical motion, causing the air to diverge (Bryan and Fritsch 2002). The model setup follows horizontally beneath the capping inversion. Too high, Naylor et al. (2012), which includes a single moment, and the updraft may not be able to draw in air from bulk ice microphysics parameterization (Gilmore et al. below the capping inversion. 2004) with default parameters; a model grid that moves Unauthenticated | Downloaded 09/30/21 06:27 AM UTC NOVEMBER 2012 N A Y L O R A N D G I L M O R E 3701 TABLE 1. Number of supercell-producing simulations and number of simulations that produce a supercell that exceeds specified durations for the various UN configurations. The boldface entries indicate the maximum for each column. The number in parentheses in the last column represents the longest possible supercell duration after UN is shut off for each 120-min simulation. None of the UN configurations produced NULL cases. Total No. of Supercell Supercell Supercell duration supercells .60 min .75 min of simulation UN5min 91 52 43 0 (115 min) UN10min 99 61 52 33 (110 min) UN15min 102 61 53 33 (105 min) UN20min 102 56 49 35 (100 min) UN25min 98 48 42 31 (95 min) FIG. 2. Average supercell duration (black solid line) and average UN30min 98 51 44 36 (90 min) updraft helicity (gray dashed line) for all 113 cases as a function of UN35min 97 50 43 37 (85 min) updraft nudging duration. For comparison, the average supercell UN40min 97 48 42 40 (80 min) duration for the bubble technique is 4625 s and average updraft 2 UN45min 94 47 39 39 (75 min) helicity is 472 m2 s 2 (not plotted). with the 0–6-km mean wind; and a simulation run time is not applied once the vertical velocity exceeds wmax. 21 21 of 2 h. Each idealized simulation, using a horizontally Here, a 5 0.5 s and wmax 5 10 m s . Nudging starts homogeneous environment, was initialized with 1 of at t 5 0 and lasts a specified duration. The durations 113 Rapid Update Cycle-2 (RUC-2) supercell proximity tested herein varied from 5 to 45 min at 5-min soundings from Thompson et al. (2003, 2007). increments—a total of nine UN tests for each sounding environment. b. Convective initiation c. Supercell detection Supercell simulations were initiated using two methods. First used was the traditional warm bubble method Supercell presence is determined based on threshold 2 (hereafter BUB) which is defined by a spheroid with values of 2–5-km updraft helicity (UH . 180 m2 s 2 10-km horizontal radius and 1.5-km vertical radius for 20 min; following Naylor et al. 2012). This method centered at z 5 1.5 km with a 4-K maximum potential is used to determine both supercell duration and su- perturbation. Brooks (1992) and McCaul and Cohen percell intensity (time average of domain maximum (2004) have shown that a 4-K perturbation can produce updraft helicity) based on model output available sustained convection in a wider range of environments every 60 s during the simulation. Naylor et al. (2012) (i.e., smaller CAPE and moisture content) than a 2-K showed that the UH technique has a small (roughly perturbation. 5%) false alarm rate from successive nonsupercell The second initiation method utilized updraft nudg- mesocyclones that jointly exceed the 20-min temporal ing (UN). A spheroid with the same dimensions and criteria. location as the warm bubble is used here except that the updraft at a particular time and grid point (wt)isde- termined by 3. Results and discussion 8 < p a. Simulations with the warm bubble technique w cos2 b ,if0# b # 1 w 5 max 2 , (1) Of the 113 simulations completed with BUB, only 35 mag : b . 0, if 1 (31%) had supercells detected at least once during the simulation (SUP).
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