skip to main content

Title: Regimes of Convective Self-Aggregation in Convection-Permitting Beta-Plane Simulations

The spontaneous self-aggregation (SA) of convection in idealized model experiments highlights the importance of interactions between tropical convection and the surrounding environment. The authors have shown that SA fundamentally changes with the background rotation in previousf-plane simulations, in terms of both the resulting forms of organized convection and the relative roles of the physical feedbacks driving them. This study considers the dependence of SA on rotation in one large domain on theβplane, introducing an additional layer of complexity. Simulations are performed with uniform thermal forcing and explicit convection. Focuses include statistical and structural analysis of the convective modes, process-oriented diagnostics of how they develop, and resulting mean states. Two regimes of SA emerge within the first 15 days, separated by a critical zone wherefis analogous to 10°–15° latitude. Organized convection at near-equatorial values offprimarily consists of convectively coupled Kelvin waves. Wind speed–surface enthalpy flux feedbacks are the dominant process driving moisture variability early on, then clear-sky shortwave radiative feedbacks are strongest in wave maintenance. In contrast, at higherf, numerous tropical cyclones develop and coexist, dominated by surface flux and longwave processes. Tropical cyclogenesis is most pronounced at intermediatef(analogous to 25°–40°), but are longer-lived at higherf. The resulting modes of SA at lowfdiffer between theseβ-plane simulations (convectively coupled waves) and priorf-plane simulations (weak tropical cyclones or nonrotating clusters). Otherwise, these results provide further evidence for the changing roles of radiative, surface flux, and advective processes in influencing SA asfchanges, as found in our previous study.

Significance Statement

In model simulations, convection often self-organizes due to interactions with its surrounding environment. These interactions are relevant in the real-world organization of rainfall and clouds, and may thus be useful to understand for improved prediction of tropical weather and climate. Previous work using a set of simple model experiments with constant Coriolis force showed that at different latitudes, different processes dominate, and different types of organized convection result. This study verifies that finding using a more complex and realistic model, where the Coriolis force varies within the domain to resemble different latitudes. Specifically, the convection here self-organizes into atmospheric waves (periodic disturbances) at low latitudes, and tropical cyclones at high latitudes.

more » « less
Award ID(s):
Author(s) / Creator(s):
Publisher / Repository:
American Meteorological Society
Date Published:
Journal Name:
Journal of the Atmospheric Sciences
Medium: X Size: p. 2187-2205
p. 2187-2205
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    High‐resolution modeling reveals a tendency for deep convection to spontaneously self‐aggregate from radiative‐convective equilibrium. Self‐aggregated convection takes different forms in nonrotating versus rotating environments, including tropical cyclones (TCs) in the latter. This suggests that self‐aggregation (SA), and the relative roles of the mechanisms that cause it, may undergo a gradual regime shift as the ambient rotation changes. We address this hypothesis using 31 cloud‐resolving model simulations onf‐planes corresponding to latitudes between 0.1° and 20°, spanning a range of weakly rotating environments largely unexplored in prior literature. Simulations are classified into three groups. The first (low‐f, 0.1°–5°) is characterized by the growth of several dry patches. Surface enthalpy flux feedbacks dominate in this initial growth phase, followed by radiative (primarily cloud longwave) effects. Eventually, convection takes the form of either a nonrotating band or a quasi‐circular cluster. In contrast, the 9°–20° (high‐f) group dries less rapidly in early stages, though enhanced surface flux effects form a moist anomaly that undergoes TC genesis. The TC then acts to dry the remainder of the domain. Finally, a set of 6°–8° (medium‐f) simulations fails to fully self‐aggregate, producing convection across most of the domain through the full 100‐day simulation. The combination of relatively weak diabatic feedbacks and a negative advective feedback prevents SA from completing in this group. The advective feedback becomes more negative with increasing rotation, but high‐fsimulations compensate by having sufficiently strong surface flux feedbacks to support TC genesis.

    more » « less
  2. Abstract

    In a modeled environment of rotating radiative‐convective equilibrium (RCE), convective self‐aggregation may take the form of spontaneous tropical cyclogenesis. We investigate the processes leading to tropical cyclogenesis in idealized simulations with a three‐dimensional cloud‐permitting model configured in rotating RCE, in which the background planetary vorticity is varied acrossf‐plane cases to represent a range of deep tropical and near‐equatorial environments. Convection is initialized randomly in an otherwise homogeneous environment, with no background wind, precursor disturbance, or other synoptic‐scale forcing. We examine the dynamic and thermodynamic evolution of cyclogenesis in these experiments and compare the physical mechanisms to current theories. All simulations with planetary vorticity corresponding to latitudes from 10°–20° generate intense tropical cyclones, with maximum wind speeds of 80 m s−1or above. Time to genesis varies widely, even within a five‐member ensemble of 20° simulations, indicating large stochastic variability. Shared across the 10°–20° group is the emergence of a midlevel vortex in the days leading to genesis, which has dynamic and thermodynamic implications on its environment that facilitate the spin‐up of a low‐level vortex. Tropical cyclogenesis is possible in this model at values of Coriolis parameter as low as that representative of 1°. In these experiments, convection self‐aggregates into a quasicircular cluster, which then begins to rotate and gradually strengthen into a tropical storm, aided by strong near‐surface inflow that is already established days prior. Other experiments at these lower Coriolis parameters instead self‐aggregate into a nonrotating elongated band and fail to undergo cyclogenesis over the 100‐day simulation.

    more » « less
  3. Abstract

    Westerly wind bursts (WWBs) are brief, anomalously westerly winds in the tropical Pacific that play a role in the dynamics of ENSO through their forcing of ocean Kelvin waves. They have been associated with atmospheric phenomena such as tropical cyclones, the MJO, and convectively coupled Rossby waves, yet their basic mechanism is not yet well understood. We study WWBs using an aquaplanet general circulation model, and find that eastward-propagating convective heating plays a key role in the generation of model WWBs, consistent with previous studies. Furthermore, wind-induced surface heat exchange (WISHE) acts on a short time scale of about two days to dramatically amplify the model WWB winds near the peak of the event. On the other hand, it is found that radiation feedbacks (i.e., changes in the net radiative anomalies accompanying westerly wind bursts) are not essential for the development of WWBs, and act as a weak negative feedback on WWBs and their associated convection. Similarly, sensible surface heat flux anomalies are not found to have an effect on the development of model WWBs.

    more » « less
  4. Abstract Westerly wind bursts (WWBs) are anomalous surface wind gusts that play an important role in ENSO dynamics. Previous studies have identified several mechanisms that may be involved in the dynamics of WWBs. In particular, many have examined the importance of atmospheric deep convection to WWBs, including convection due to tropical cyclones, equatorial waves, and the Madden Julian Oscillation. Still, the WWB mechanism is not yet fully understood. In this study, we investigate the location of atmospheric convection which leads to WWBs and the role of positive feedbacks involving surface evaporation. We find that disabling surface flux feedbacks a few days before a WWB peaks does not weaken the event, arguing against local surface flux feedbacks serving as a WWB growth mechanism on individual events. On the other hand, directly suppressing convection by inhibiting latent heat release or eliminating surface evaporation rapidly weakens a WWB. By selectively suppressing convection near or further away from the equator, we find that convection related to off-equatorial cyclonic vortices is most important to equatorial WWB winds, while on-equator convection is unimportant. Despite strong resemblance of WWB wind patterns to the Gill response to equatorial heating, our findings indicate that equatorial convection is not necessary for WWBs to develop. Our conclusions are consistent with the idea that tropical cyclones, generally occurring more than 5° away from the equator, may be responsible for the majority of WWBs. 
    more » « less
  5. Abstract

    Recent research has demonstrated a relationship between convectively coupled Kelvin waves (CCKWs) and tropical cyclogenesis, likely due to the influence of CCKWs on the large-scale environment. However, it remains unclear which environmental factors are most important and how they connect to TC genesis processes. Using a 39-yr database of African easterly waves (AEWs) to create composites of reanalysis and satellite data, it is shown that genesis may be facilitated by CCKW-driven modifications to convection and moisture. First, stand-alone composites of genesis demonstrate the significant role of environmental preconditioning and convective aggregation. A moist static energy variance budget indicates that convective aggregation during genesis is dominated by feedbacks between convection and longwave radiation. These processes begin over two days prior to genesis, supporting previous observational work. Shifting attention to CCKWs, up to 76% of developing AEWs encounter at least one CCKW in their lifetime. An increase in genesis events following convectively active CCKW phases is found, corroborating earlier studies. A decrease in genesis events following convectively suppressed phases is also identified. Using CCKW-centered composites, we show that the convectively active CCKW phases enhance convection and moisture content in the vicinity of AEWs prior to genesis. Furthermore, enhanced convective activity is the main discriminator between AEW–CCKW interactions that result in genesis versus those that do not. This analysis suggests that CCKWs may influence genesis through environmental preconditioning and radiative–convective feedbacks, among other factors. A secondary finding is that AEW attributes as far east as central Africa may be predictive of downstream genesis.

    Significance Statement

    The purpose of this work is to investigate how one type of atmospheric wave, known as convectively coupled Kelvin waves (CCKWs), impacts the formation (“genesis”) of tropical cyclones. Forecasting of genesis remains a significant challenge, so identifying how CCKWs influence this process could help improve forecasts and give communities greater lead times. Our results show that CCKWs could temporarily make genesis more likely by increasing atmospheric moisture content and convective activity. While not all CCKWs lead to genesis, those that do are associated with a particularly strong increase in convection. This provides a potential tool for forecasters monitoring CCKWs and TC genesis in real time and motivates follow-up work on this topic in numerical models.

    more » « less