Mechanical forcing of convection by cold pools: collisions and energy scaling.

Abstract

Forced mechanical lifting through cold pool gust fronts can trigger new convection and, as previous work highlights, is enhanced when cold pools collide. However, as shown by conceptual models, the organization of the convective cloud field emerging from two versus three colliding cold pools differs strongly. In idealized dry large-eddy simulations we therefore compare collisions between two and three cold pools. The triggering likelihood is quantified in terms of the cumulative vertical mass flux of boundary layer air and the instantaneous updraft strength, generated at the cold pool gust fronts. We find that cold pool expansion can be well described by initial potential energy alone. Cold pool expansion monotonically slows but shows an abrupt transition between an axisymmetric and a broken-symmetric state mirrored by a sudden drop in expansion speed. We characterize these two dynamic regimes by two distinct power law exponents and explain the transition by the onset of "lobe-and-cleft" instabilities at the cold pool head. Two-cold pool collisions produce the strongest instantaneous updrafts in the lower boundary layer, which we expect to be important in environments with strong convective inhibition. Three-cold pool collisions generate weaker but deeper updrafts and the strongest cumulative mass flux and are thus predicted to induce the largest midlevel moistening, which has been identified as a precursor for the transition from shallow to deep convection. Combined, our findings may help decipher the role of cold pools in spatially organizing convection and precipitation. Plain Language SummaryThe arrival of a convective thunderstorm is often announced by strong and cold wind gusts that can be felt by an observer at the surface. These gust fronts constitute the outer edge of cold pools, which are formed underneath clouds when part of the rain reevaporates before reaching the surface, thereby cooling the air. These cold pools have received increasing attention due to their contribution in the generation of new convective rain events, thereby affecting the spatial pattern of the cloud field. In this study we use a high-resolution numerical model to study the life cycle of single cold pools and their collision with other cold pools. We assume that the likelihood that a cold pool causes a new rain event depends on (i) the vertical velocity of the wind gusts produced at its gust front and where it collides and (ii) how much moisture it can transport upward to a height where the water condenses and forms clouds. We show that both these factors are strongly increased where two or more cold pools collide, highlighting the importance of the representation of cold pool collisions in climate models to achieve a more realistic representation of clouds and rain.

Cold pools over the Netherlands: A statistical study from tower and radar observations

“…A scatter plot of the wind peak against the relative temperature drop for the 189 CPs detected in the ten summers 2010–2019 (Figure 7) indeed suggests increasing gust front speed for larger temperature drops. Viewing as a proxy for the CP's total kinetic energy density and as a representation of the CP's potential energy density, this indicates a monotonic relation between the kinetic and potential energy of the CP (Meyer and Haerter, 2020). We note that the median value of is and that of is ms.…”

“…Beyond these macrophysical conditions, microphysical parameters, crucially the drop-size distribution, influence the rain evaporation (Seifert, 2008). CP characteristics are often studied using numerical simulations that now approach the fine scales needed to resolve some CP properties, that is, horizontal grid resolutions of substantially less than 1 km (Drager and van den Heever, 2017;Fournier and Haerter, 2019;Cafaro and Rooney, 2018;Drager et al, 2020;Meyer and Haerter, 2020). Observational work is, however, indispensable as a means of comparison and validation for numerical studies.…”

Shear‐Convection Interactions and Orientation of Tropical Squall Lines

“…To quantify this from our composite figures, we compute the cold pool potential energy in J kg −1 (following Rotunno et al., 1988; Meyer and Haerter, 2020), defined as $$${E}_{p}={\int }_{cp}B(x,z)dxdz$$$ where cp denotes the domain inside the cold pool, and B ( x , z ) the composite value of buoyancy in (x,z) cross section around the location of maximum precipitation (see Figure 4a, 4b, and 4c). We define the cold pool upper boundary as the b = −0.02 K buoyancy contour (following Tompkins, 2001b), black contour Figure 4 top panels), and integrate (Equation 1) from the surface to this upper boundary.…”

C‐POOL: A scheme for modelling convective cold pools in the Met Office Unified Model

“…Alongside microphysical and meteorological controls on downdraught generation, numerous fluid processes such as jet/plume impingement, wind effects, and the dynamics of cold-pool spread and collisions are relevant to the formation and evolution of downdraughts and cold pools (e.g., Lundgren et al, 1992;Parker, 1996;Rooney and Linden, 2012;Cafaro and Rooney, 2018;Kruger, 2019;Williams, 2020). The possible dynamical and thermodynamical pathways by which cold pools may feed back on the convective cycle must also be considered (Tompkins and Craig, 1998;Tompkins, 2001;Cohen and Craig, 2004;Khairoutdinov and Randall, 2006;Böing et al, 2012;Torri et al, 2015;Hirt et al, 2020;Meyer and Haerter, 2020). In the last decade Grandpeix and Lafore (2010) and Rio et al (2013) have developed a parametrization which aims to represent cold-pool activity at the gridbox level in a numerical weather prediction (NWP) model.…”