On Cold Pool Collisions in Tropical Boundary Layers

Geophysical Research Letters

2019

Convective self‐aggregation is when thunderstorm clouds cluster over a constant temperature surface in radiative convective equilibrium. Self‐aggregation was implicated in the Madden‐Julian Oscillation and hurricanes. Yet, numerical simulations succeed or fail at producing self‐aggregation, depending on modeling choices. Common explanations for self‐aggregation invoke radiative effects, acting to concentrate moisture in a subdomain. Interaction between cold pools, caused by rain evaporation, drives reorganization of boundary layer moisture and triggers new updrafts. We propose a simple model for aggregation by cold pool interaction, assuming a local number density ρ(r) of precipitation cells, and that interaction scales quadratically with ρ(r). Our model mimics global energy constraints by limiting further cell production when many cells are present. The phase diagram shows a continuous phase transition between a continuum and an aggregated state. Strong cold pool‐cold pool interaction gives a uniform convective phase, while weak interaction yields few and independent cells. Segregation results for intermediate interaction strength.

Section: Modelmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Convective Self‐Aggregation As a Cold Pool‐Driven Critical Phenomenon

Geophysical Research Letters

2019

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

Meyer

2020

Preprint

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.

On the sensitivity of convective cold pools to mesh resolution

Fiévet

Meyer

2022

Preprint

It is well-recognized that triggering of convective cells through cold pools is key to the organization of convection. Yet, numerous studies have found that both the characterization and parameterization of these effects in numerical models is cumbersomein part due to the lack of numerical convergence, $\Delta x\rightarrow 0$, achieved in typical cloud-resolving simulators. Through a comprehensive numerical convergence study we systematically approach the $\Delta x \rightarrow 0$ limit in a set of idealized large-eddy simulations capturing key cold pool processes: free propagation, frontal collision and incident merging of gust fronts. We characterize at which $\Delta x$ convergence is achieved for physically relevant quantities, namely accumulated upwards water mass fluxes, leading front vortical rates, tropospheric moistening and group velocity. The understanding gained from this analysis lays the groundwork to develop robust subgrid models for CP dynamics able to sustain their growth and combat artificial numerical dissipation and dispersion.