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

Haerter, Jan O.

doi:10.1029/2018gl081817

Cited by 50 publications

(57 citation statements)

References 41 publications

(118 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…4b), a finding in line with collision effects of multiple cold pool gust fronts. 13,28 These interiour cells could further act to deepen and cool the combined cold pool driving the MCS expansion. Together, MCSs hence act to excite new convection both within and around the combined cold pool area.…”

Section: Discussionmentioning

confidence: 99%

Diurnal Self-Aggregation

Haerter¹,

Meyer²,

Nissen³

2020

Preprint

Self Cite

View full text Add to dashboard Cite

Convective self-aggregation is a modelling paradigm for thunderstorm organisation over a constant-temperature tropical sea surface. This setup can give rise to cloud clusters over timescales of weeks. In reality, sea surface temperatures do oscillate diurnally, affecting the atmospheric state. Over land, surface temperatures vary more strongly, and rain rate is significantly influenced. Here, we carry out a substantial suite of cloud-resolving numerical experiments, and find that even weak surface temperature oscillations enable qualitatively different dynamics to emerge: the spatial distribution of rainfall is only homogeneous during the first day. Already on the second day, the rain field is firmly structured. In later days, the clustering becomes stronger and alternates from day-to-day. We show that these features are robust to changes in resolution, domain size, and surface temperature, but can be removed by a reduction of the amplitude of oscillation, suggesting a transition to a clustered state. Maximal clustering occurs at a scale of l max ≈ 180 km, a scale we relate to the emergence of mesoscale convective systems. At l max rainfall is strongly enhanced and far exceeds the rainfall expected at random. We explain the transition to clustering using simple conceptual modelling. Our results may help clarify how continental extremes build up and how cloud clustering over the tropical ocean could emerge much faster than through conventional self-aggregation alone. Currently, general circulation models cannot simulate organised deep convection as these models describe convection as a collection of non-interacting convective cells. Yet, the increase in tropical rainfall was stated to predominantly stem from organised deep convection. 1 Similarly, in midlatitudes, the majority of flood-producing rainfall was attributed to mesoscale convective systems (MCSs), 2,3 that is, long-lived complexes of thunderstorms spanning ∼ 100 km in diameter. 4 In self-aggregation studies pronounced clustering occurs on the timescale of several weeks. 5-7 There, the radiative convective equilibrium scheme (RCE) 8,9 is usually employed, assuming spatially and temporally constant surface temperature (∼ 300 K). In self-aggregation, radiation feedbacks have emerged as the "smoking gun" for sustaining and increasing clustering. 5 Still, factors such as sea surface feedbacks, 10 domain size, geometry, and resolution, 11 as well as cold pool effects, 12,13 all contribute.Prescribing constant boundary conditions is an elegant model simplification, but not always realistic. Especially under weak surface wind conditions and strong insolation, sea surface temperature amplitudes were observed to be as large as two 14,15 to five kelvin 16 and suggested to modify atmospheric properties. 16 Such temperature variations may even affect the Madden-Julian Oscillation arXiv:2001.04740v1 [physics.ao-ph]

show abstract

Section: Discussionmentioning

confidence: 99%

Diurnal Self-Aggregation

Haerter¹,

Meyer²,

Nissen³

2020

Preprint

Self Cite

View full text Add to dashboard Cite

show abstract

“…Bell and Moore, 2000;Gupta et al, 1996;Kalinga and Gan, 2006). However, for convective rainfall, the change in rainfall spatial structure is expressed by generating high-intensity convective features (Haerter, 2019;Haerter et al, 2017) that are contributing higher volumes of rainfall to specific parts of the catchment in shorter durations, resulting in an enhancement of runoff and peak streamflow (e.g. Morin et al, 2006;Peleg et al, 2015;Yakir and Morin, 2011).…”

Section: Physical Interpretation Of the Hydro-morphological Responsesmentioning

confidence: 99%

Temperature effects on the spatial structure of heavy rainfall modify catchment hydro-morphological response

et al. 2020

View full text Add to dashboard Cite

Abstract. Heavy rainfall is expected to intensify with increasing temperatures, which will likely affect rainfall spatial characteristics. The spatial variability of rainfall can affect streamflow and sediment transport volumes and peaks. Yet, the effect of climate change on the small-scale spatial structure of heavy rainfall and subsequent impacts on hydrology and geomorphology remain largely unexplored. In this study, the sensitivity of the hydro-morphological response to heavy rainfall at the small-scale resolution of minutes and hundreds of metres was investigated. A numerical experiment was conducted in which synthetic rainfall fields representing heavy rainfall events of two types, stratiform and convective, were simulated using a space-time rainfall generator model. The rainfall fields were modified to follow different spatial rainfall scenarios associated with increasing temperatures and used as inputs into a landscape evolution model. The experiment was conducted over a complex topography, a medium-sized (477 km2) Alpine catchment in central Switzerland. It was found that the responses of the streamflow and sediment yields are highly sensitive to changes in total rainfall volume and to a lesser extent to changes in local peak rainfall intensities. The results highlight that the morphological components are more sensitive to changes in rainfall spatial structure in comparison to the hydrological components. The hydro-morphological features were found to respond more to convective rainfall than stratiform rainfall because of localized runoff and erosion production. It is further shown that assuming heavy rainfall to intensify with increasing temperatures without introducing changes in the rainfall spatial structure might lead to overestimation of future climate impacts on basin hydro-morphology.

show abstract

“…Visual inspection of the convergence field and the liquid water path field (not shown) suggests, as expected from previous studies (e.g., Schlemmer & Hohenegger, ; Tompkins, ), a strong correlation between convection and low‐level convergence, in particular we find indications of convection mostly on top of either cold pool centers (i.e., at the location of strong divergence) or cold pool edges (i.e., on top of the surrounding convergence lines). Note that the triggering of deep convection by cold pools has, in fact, been argued to be a possible driver for self‐aggregation (Haerter, ). Similar to what we find here, their simple model of cold pool interaction suggests that the surface in the convecting region is completely populated by cold pools.…”

Section: Super‐cold‐poolmentioning

confidence: 99%

Convection On the Edge

Windmiller

Hohenegger

2019

J Adv Model Earth Syst

View full text Add to dashboard Cite

Deep convection over tropical oceans often appears intensified at the edge of convectively active regions, both in idealized studies and in observations. This edge intensification of convection is studied in detail here, using the steady state of a radiative-convective equilibrium study, marked by a single convective cluster with deep convection intensified at the edge of this cluster. The cause for edge intensification and its dependence on the cluster area is investigated by comparing the spatial distribution of deep convection to different variables known to be important for convection. Analysis of the simulation suggests that the edge is marked by an increased probability for the triggering of convection rather than by stronger updrafts. In particular, while the edge of the moist region is not thermodynamically more favorable, we find strong surface convergence and therefore dynamical lifting at this edge. The surface convergence is shown to result from two opposing flows. On the one hand, there is, as expected from previous radiative-convective equilibrium simulations, a low-level inflow directed toward the moist region. On the other hand, there is a positive density anomaly at the surface which is the result of continuously forming cold pools within the convectively active region, creating a super-cold-pool. As the velocity of the low-level inflow approximately matches the potential propagation speed of the super-cold-pool boundary, these opposing flows explain the presence of strong convergence at the edge of this region. Whether the resulting lifting induces the formation of deep convection is shown to depend on the large-scale instability. Key Points:• Precipitation tends to intensify at the edge of the convectively active region if the region is large enough • Edge intensification in a RCE simulation with a single convective cluster cannot be explained by spatial variations of CAPE or moisture • Edge intensification in this case is linked to the formation of a super-cold-pool in the convective cluster balanced by a low-level inflow

show abstract

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

Cited by 50 publications

References 41 publications

Diurnal Self-Aggregation

Diurnal Self-Aggregation

Temperature effects on the spatial structure of heavy rainfall modify catchment hydro-morphological response

Convection On the Edge

Contact Info

Product

Resources

About