Tracking the Gust Fronts of Convective Cold Pools

Abstract

It is increasingly acknowledged that cold pools can influence the initiation of new convective cells. Yet the full complexity of convective organization through cold pool interaction is poorly understood. This lack of understanding may partially be due to the intricacy of the dynamical pattern formed by precipitation cells and their cold pools. Additionally, how exactly cold pools interact is insufficiently known. To better understand this dynamics, we develop a tracking algorithm for cold pool gust fronts. Rather than tracking thermodynamic anomalies, which do not generally coincide with the gust front boundaries, our approach tracks the dynamical cold pool outflow. Our algorithm first determines the locus of the precipitation event. Second, relative to this origin and for each azimuthal bin, the steepest gradient in the near‐surface horizontal radial velocity vr is employed to determine the respective locus of the cold pool gust front edge. Steepest vr gradients imply largest updraft velocities, hence strongest dynamical triggering. Results are compared to a previous algorithm based on the steepest gradient in temperature—highlighting the benefit of the method described here in determining dynamically active gust front regions. Applying the method to a range of numerical experiments, the algorithm successfully tracks an ensemble of cold pools. A linear relation emerges between the peak rain intensity of a given event and maximal vr for its associated cold pool gust front—a relation found to be nearly independent of the specific sensitivity experiment.

Mechanical Forcing of Convection by Cold Pools: Collisions and Energy Scaling

“…The profiles of temperature and velocity ( Fig. 3a-c) compare well to previous numerical studies, both in idealised (Droegemeier & Wilhelmson, 1987;Romps & Jeevanjee, 2016) and more comprehensive setups (Drager & van den Heever, 2017;Fournier & Haerter, 2019). The main difference to the latter study is, that the CP head there is less detached from the CP interior due to the more continuous forcing from rain events with a finite life time.…”

Particle‐Based Tracking of Cold Pool Gust Fronts

“…Thus, the area of CPs identified by temperature‐based methods tends to reduce during later stages of the CP lifetime (Feng et al, ). This retreating feature stands in contradiction to the dynamics of CP gust fronts, which often continue to spread by inertia, even if the temperature anomaly is reduced (Fournier & Haerter, ; Grant & van den Heever, ). Thus, if we are interested in the mechanical triggering of convective triggering, it is useful to define the CP as the area confined by the band of strong horizontal convergence near the surface.…”

“…Triggering of subsequent convection, however, often occurs near the narrow convergence bands (Figures c and d), which need not coincide with any thermodynamic measure. Indeed, when CPs age, their initial temperature depression gradually equilibrates (Grant & van den Heever, ; Szoeke et al, ; Torri & Kuang, ) and virtual potential temperature may even increase at the gust front (Fournier & Haerter, ; Tompkins, ). Thus, the area of CPs identified by temperature‐based methods tends to reduce during later stages of the CP lifetime (Feng et al, ).…”

“…Thus, if we are interested in the mechanical triggering of convective triggering, it is useful to define the CP as the area confined by the band of strong horizontal convergence near the surface. CP gust fronts have previously been identified based on the wind field, using a radial gradient‐based method (Fournier & Haerter, ). In the following, we refer to the region of the convergence peak as the CP edge .…”

Evaluating the Detection of Mesoscale Outflow Boundaries Using Scatterometer Winds at Different Spatial Resolutions

“…A tracking algorithm for gust fronts of convective cold pools was developed by Fournier and Haerter [28], which was based on the gradient of the radial velocity. The algorithm can be described in three phases: first a rain cell tracking is performed, followed by the calculation of the radial velocity and its gradient.…”