skip to main content


Title: Control of Convection in High‐Resolution Simulations of Tropical Cyclogenesis
Abstract

Three idealized high‐resolution simulations of tropical storm formation from a weak vortex are analyzed. The three simulations include a case using warm rain microphysics, a similar case in which surface friction is omitted, and a case in which ice microphysics is used. The goal is to understand the mechanisms controlling the intensity and distribution of convection in the formation process in each of these cases. Simulations of convection in weak temperature gradient convective models show that a combination of low to middle tropospheric moist convective instability, the saturation fraction or column relative humidity, and the surface moist entropy flux explain a high percentage of the variance in precipitation and lower tropospheric vertical mass flux. Tropical cyclones differ from other convective environments in that intense frictional convergence occurs in the boundary layer. Adding a measure of convective inhibition to account for this process enables the lower tropospheric mass flux to be predicted even in the core regions of the simulated tropical cyclones. These results are pertinent to the development of more accurate convective parameterizations for large‐scale models.

 
more » « less
NSF-PAR ID:
10460637
Author(s) / Creator(s):
 ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Advances in Modeling Earth Systems
Volume:
11
Issue:
6
ISSN:
1942-2466
Page Range / eLocation ID:
p. 1582-1599
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Data from recent field programs studying deep convection may be useful in constraining cumulus parameterizations. To this end, gridded dropsonde analyses are made using data from the OTREC (Organization of Tropical East Pacific Convection) and PREDICT (PreDepression Investigation of Cloud‐Systems in the Tropics) projects to characterize the mesoscale properties of tropical oceanic convection in terms of selected thermodynamic parameters computable from the explicit grids of large‐scale models. In particular, saturation fraction, lower tropospheric moist convective instability, and convective inhibition appear to govern column‐integrated moisture convergence, while sea surface temperature is related to the top‐heaviness of mass flux profiles and the integrated entropy divergence. Local (as opposed to global) surface heat and moisture fluxes and convective available potential energy correlate weakly with these quantities. Recommendations to improve cumulus parameterizations are enumerated.

     
    more » « less
  2. Abstract

    Idealized numerical simulations of weak tropical cyclones (e.g., tropical depressions and tropical storms) in sheared environments indicate that vortex tilt reduction and convective symmetrization are key structural changes that can precede intensification. Through a series of ensembles of idealized numerical simulations, this study demonstrates that including radiation in the simulations affects the timing and variability of those structural changes. The underlying reason for those effects is a background thermodynamic profile with reduced energy available to fuel strong downdrafts; such a profile leads to weaker lower-tropospheric ventilation, greater azimuthal coverage of clouds and precipitation, and smaller vortex tilt with radiation. Consequently, the simulations with radiation allow for earlier intensification at stronger shear magnitudes than without radiation. An unexpected finding from this work is a reduction of both vortex tilt and intensity variability with radiation in environments with 5 m s−1 deep-layer shear. This reduction stems from reduced variability in nonlinear feedbacks between lower-tropospheric ventilation, cold pools, convection, and vortex tilt. Sensitivity experiments confirm the relationship between those processes and suggest that microphysical processes (e.g., rain evaporation) are major sources of uncertainty in the representation of weak, sheared tropical cyclones in numerical weather prediction models.

     
    more » « less
  3. Abstract

    A cloud-resolving model is used to examine the intensification of tilted tropical cyclones from depression to hurricane strength over relatively cool and warm oceans under idealized conditions where environmental vertical wind shear has become minimal. Variation of the SST does not substantially change the time-averaged relationship between tilt and the radial length scale of the inner core, or between tilt and the azimuthal distribution of precipitation during the hurricane formation period (HFP). By contrast, for systems having similar structural parameters, the HFP lengthens superlinearly in association with a decline of the precipitation rate as the SST decreases from 30° to 26°C. In many simulations, hurricane formation progresses from a phase of slow or neutral intensification to fast spinup. The transition to fast spinup occurs after the magnitudes of tilt and convective asymmetry drop below certain SST-dependent levels following an alignment process explained in an earlier paper. For reasons examined herein, the alignment coincides with enhancements of lower–middle-tropospheric relative humidity and lower-tropospheric CAPE inward of the radius of maximum surface wind speedrm. Such moist-thermodynamic modifications appear to facilitate initiation of the faster mode of intensification, which involves contraction ofrmand the characteristic radius of deep convection. The mean transitional values of the tilt magnitude and lower–middle-tropospheric relative humidity for SSTs of 28°–30°C are respectively higher and lower than their counterparts at 26°C. Greater magnitudes of the surface enthalpy flux and core deep-layer CAPE found at the higher SSTs plausibly compensate for less complete alignment and core humidification at the transition time.

     
    more » « less
  4. 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
  5. Abstract

    Orographically‐locked diurnal convection involves interactions between local circulation and the thermodynamic environment of convection. Here, the relationships of convective updraft structures over orographic precipitation hotspots and their upstream environment in the TaiwanVVM large‐eddy simulations are analyzed for the occurrence of the orographic locking features. Strong convective updraft columns within heavily precipitating, organized systems exhibit a mass flux profile gradually increasing with height through a deep lower‐tropospheric inflow layer. Enhanced convective development is associated with higher upstream moist static energy (MSE) transport through this deep‐inflow layer via local circulation, augmenting the rain rate by 36% in precipitation hotspots. The simulations provide practical guidance for targeted observations within the most common deep‐inflow path. Preliminary field measurements support the presence of high MSE transport within the deep‐inflow layer when organized convection occurs at the hotspot. Orographically‐locked convection facilitate both modeling and field campaign design to examine the general properties of active deep convection.

     
    more » « less