Numerical studies of tropical convection

Idealized numerical model experiments are presented to investigate the convective generation of vertical vorticity in a tropical depression. The calculations are motivated by observations made during the recent PREDICT field experiment to study tropical cyclogenesis, and by a desire to understand the aggregation of vorticity debris produced by deep convection in models of tropical cyclogenesis to form a monopole vortex. One aim is to isolate and quantify the effects of low to mid level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. Another aim is to isolate the effects of a unidirectional boundary layer wind profile on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. A third aim is to isolate the effects of a vortex boundary-layer wind profile on tropical deep convection, focussing especially on the morphology of vertical vorticity that develops. The growing convective updraughts, that are initiated by a near surface thermal perturbation, amplify locally the ambient rotation at low levels by more than an order of magnitude and this vorticity persists long after the updraught has decayed, supporting the results of an earlier study. The results of calculations with dry air aloft do not support a common perception that the dry air produces stronger downdraughts. In calculations where the vertical wind shear changes sign at some level near the top of the boundary layer, as occurs in warm-cored disturbances such as tropical depressions or tropical cyclones, it was found that the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This feature is quite unlike the structure in a typical middle-latitude `supercell' storm. These results provide an essential first step to understanding the interaction between deep convective elements in a tropical depression or tropical cyclone. An increase in the magnitude of boundary-layer shear was found to have the dual effect of weakening the development of the initial thermal, which is detrimental to vertical vorticity production by stretching and tilting, while at the same time increasing the magnitude of horizontal vorticity that can be tilted. The results provide a basis for appraising a recent conjecture concerning the role of storm splitting in explaining the contraction of the eyewall in tropical cyclones

A unified view of tropical cyclogenesis and intensification

Quarterly Journal of the Royal Meteorological SocietyThe article of record as published may be found at http://dx.doi.org/10.1002/qj.2934Idealized high-resolution numerical simulations of tropical cyclogenesis are presented in a model that represents deep convection by a warm rain process only. Starting with an initially weak, cloud-free, axisymmetric warm-cored vortex (maximum wind speed 5 m s−1 at a radius of 100 km), rapid vortex intensification begins after a gestation period on the order of 2 days. From a three-dimensional perspective, the genesis process is similar to that in the rotating convection paradigm for vortex intensification starting with a much stronger initial vortex (Vmax = 15 m s−1). The patterns of deep convection and convectively amplified cyclonic relative vorticity are far from axisymmetric during the genesis period. Moreover, the organization of the cyclonic relative vorticity into a monopole structure occurs at relatively low wind speeds, before the maximum local wind speed has increased appreciably. Barotropic processes are shown to play an important role in helping to consolidate a single-signed vorticity monopole within a few hours near the intensification begin time. The rotating convection paradigm appears adequate to explain the basic genesis process within the weak initial vortex, providing strong support for a hypothesis of Montgomery and Smith that the genesis process is not fundamentally different from that of vortex intensification. In particular, genesis does not require a ‘trigger’ and does not depend on the prior existence of a mid-level vortex.Funded by Naval Postgraduate SchoolOffice of Naval Research GlobalNOAA HFIPNational Aeronautics and Space AdministrationDeutsche ForschungsgemeinschaftNational Science Foundatio

A numerical study of rotating convection during tropical cyclogenesis [seminar announcement}

An announcement of a seminar hosted by NPS Department of Meteorology, and presented by Gerard Kilroy of the University of Munich, LMU; Host, Professor Michael T. Montgomery.Idealized numerical model experiments are presented to investigate the convective generation of vertical vorticity in a tropical depression. The calculations are motivated by observations made during the recent PREDICT field experiment to study tropical cyclogenesis, and by a desire to understand the aggregation of vorticity debris produced by deep convection in models of tropical cyclogenesis to form a monopole vortex. One aim is to isolate and quantify the effects of low to mid level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. Another aim is to isolate the effects of a unidirectional boundary layer wind profile on storm structure, especially on vertical vorticity production. A third aim is to isolate the effects of a vortex boundary-layer wind profile on tropical deep convection, focusing especially on the morphology of vertical vorticity that develops. The growing convective updraughts, that are initiated by a near surface thermal perturbation, amplify locally the ambient rotation at low levels by more than an order of magnitude and this vorticity persists long after the updraught has decayed, supporting the results of an earlier study. The results of calculations with dry air aloft do not support a common perception that the dry air produces stronger downdraughts. In calculations where the vertical wind shear changes sign at some level near the top of the boundary layer, as occurs in warm-cored disturbances such as tropical depressions or tropical cyclones, it was found that the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This feature is quite unlike the structure in a typical middle-latitude `supercell' storm. These results provide an essential first step to understanding the interaction between deep convective elements in a tropical depression or tropical cyclone

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Why Do Model Tropical Cyclones Intensify More Rapidly at Low Latitudes?

The article of record as published may be found at http://dx.doi.org/10.1175/JAS-D-14-0044.1The authors examine the problem of why model tropical cyclones intensify more rapidly at low latitudes. The answer to this question touches on practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes the conventional spin-up mechanism, as articulated in classical and recent work, together with a boundary layer feedback mechanism linking the strength of the boundary layer inflow to that of the diabatic forcing of the meridional overturning circulation. The specific role of the frictional boundary layer in regulating the dependence of the intensification rate on latitude is discussed. It is shown that, even if the tangential wind profile at the top of the boundary layer is held fixed, a simple, steady boundary layer model produces stronger low-level inflow and stronger, more confined ascent out of the boundary layer as the latitude is decreased, similar to the behavior found in a timedependent, three-dimensional numerical model. In an azimuthally averaged view of the problem, the most prominent quantitative differences between the time-dependent simulations at 108 and 308N are the stronger boundary layer inflow and the stronger ascent of air exiting the boundary layer, together with the much larger diabatic heating rate and its radial gradient above the boundary layer at the lower latitude. These differences, in conjunction with the convectively induced convergence of absolute angular momentum, greatly surpass the effects of rotational stiffness (inertial stability) and evaporative-wind feedback that have been proposed in some prior explanations

Tropical low formation and intensification over land as seen in ECMWF analyses

Quarterly Journal of the Royal Meteorological SocietyThe article of record as published may be found at http://dx.doi.org/10.1002/qj.2963Case studies of the formation and intensification over land of three tropical lows in northern Australia are described. The case studies are based on European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The aim is to investigate the generality of recent results concerning the dynamics and thermodynamics of tropical lows. Consistent with these results, it is found that the processes of low formation and intensification are the same over land as over the ocean. An important element of the intensification process is the need for bursts of deep convection to persist near the circulation centre, which, in turn, requires that convective instability be maintained by surface moisture fluxes. The moist monsoonal environment locally surrounding the storm provides a shield against the adverse effects of dry-air intrusion from the Australian continent.Funded by Naval Postgraduate SchoolNSFOffice of Naval Research GlobalNOAA HFIPNational Aeronautics and Space AdministrationDeutsche Forschungsgemeinschaf

The role of heating and cooling associated with ice processes on tropical cyclogenesis and intensification: Tropical Cyclogenesis

The article of record as published may be found at http://dx.doi.org/10.1002/qj.3187A recent idealized numerical study of tropical cyclogenesis and subsequent intensification using warm-rain-only microphysics is extended to examine the modifications brought about by a representation of ice processes. It is found that the time taken to reach cyclogenesis is more than twice that in the equivalent warm-rain-only simulation. The subtle reasons for the difference in the length of the gestation period are discussed. A mid-level vortex forms during the early gestation period when ice processes are present, but not when warm-rain- only processes are present. Axisymmetric balance calculations show that the spin-up of this mid-level vortex is related to the different spatial distribution of diabatic heating rate in the presence of ice, which leads to a system-scale radial influx of absolute vorticity in the middle troposphere. The tropical-cyclone vortex that forms in the simulation with ice is similar to that in the warm-rain-only simulation, with the strengthening frictional boundary layer exerting a progressively important role in focusing inner-core deep convection. This vortex develops in situ on a much smaller scale than the mid-level vortex and there is no evidence that it is a result of the mid-level vortex being somehow carried downwards, as has been suggested previously by some researchers. Some implications of the results in relation to previous theories of tropical cyclogenesis are discussed.GK and RKS acknowledge support for tropical cyclone research from the German Research Council (Deutsche Forschungsge- meinschaft) under grants SM30/23-3 and SM30/23-4 and the Office of Naval Research Global under Grant No. N62909- 15-1-N021. MTM acknowledges the support of NSF grant AGS-1313948, NOAA HFIP grant N0017315WR00048, NASA grant NNG11PK021 and the US Naval Postgraduate School.GK and RKS acknowledge support for tropical cyclone research from the German Research Council (Deutsche Forschungsge- meinschaft) under grants SM30/23-3 and SM30/23-4 and the Office of Naval Research Global under Grant No. N62909- 15-1-N021. MTM acknowledges the support of NSF grant AGS-1313948, NOAA HFIP grant N0017315WR00048, NASA grant NNG11PK021 and the US Naval Postgraduate School

The role of boundary-layer friction on tropical cyclogenesis and subsequent intensification: Tropical Cyclogenesis and Subsequent Intensification

The article of record as published may be found at http://dx.doi.org/10.1002/qj.3104A recent idealized, high-resolution, numerical model simulation of tropical cyclogenesis is compared with a simulation in which the surface drag is set to zero. It is shown that, while spin-up occurs in both simulations, the vortex in the one without surface drag takes over twice as long to reach its intensification begin time. When surface friction is not included, the inner core size of the simulated vortex is considerably larger and the subsequent vortex intensity is significantly weaker than in the case with friction. In the absence of surface drag, the convection eventually develops without any systematic organization and lies often outside the radius of azimuthally averaged maximum tangential winds. The results underscore the crucial role of friction in organizing deep convection in the inner core of the nascent vortex and raise the possibility that the timing of tropical cyclogenesis in numerical models may have an important dependence on the boundary-layer parametrization scheme used in the model.GK and RKS acknowledge financial support for tropical cyclone research from the German Research Council (Deutsche Forschungsgemeinschaft) under grant numbers SM30/23-3 and SM30/23-4 and the Office of Naval Research Global under grant no. N62909-15-1-N021. MTM acknowledges the support of NSF grant AGS-1313948, NOAA HFIP grant N0017315WR00048, NASA grant NNG11PK021 and the US Naval Postgraduate School.GK and RKS acknowledge financial support for tropical cyclone research from the German Research Council (Deutsche Forschungsgemeinschaft) under grant numbers SM30/23-3 and SM30/23-4 and the Office of Naval Research Global under grant no. N62909-15-1-N021. MTM acknowledges the support of NSF grant AGS-1313948, NOAA HFIP grant N0017315WR00048, NASA grant NNG11PK021 and the US Naval Postgraduate School

A case-study of a monsoon low that formed over the sea and intensified over land as seen in ECMWF analyses: Monsoon Low Intensification over Land in ECMWF Analyses

A case study is presented of a tropical low that formed near Darwin, Australia, during the monsoon and subsequently intensified over land. The study is based on European Centre for Medium Range Weather Forecast (ECMWF) analyses. Interpretations of the formation over the sea are given in terms of vorticity dynamics. The thermodynamic support for the intensification and maintenance of the low over land is investigated also. The analyses indicate that the intensification of the low depends on repeated bursts of deep convection occurring near the centre of the circulation that promote the further concentration of vorticity near the centre. This concentration of vorticity increases the local circulation about the centre, which amounts to increasing the local tangential wind speed and, through approximate gradient wind balance above the boundary layer, to a lowering of the central pressure. It is found that the horizontal transport of moisture into a mesoscale column centred on the low is approximately equal to the moisture lost by precipitation so that total precipitable water levels are not rapidly depleted over land. While the contribution to the overall moisture budget by surface fluxes is comparatively small, these fluxes are necessary to maintain conditionally unstable conditions near the vortex centre so that deep convective bursts can continue to occur there, even when the system is located far inland.GK and RKS acknowledges funding for tropical cyclone research from the German Research Council (Deutsche Forschungsgemeinschaft) under Grant no SM30/23-4 and the Office of Naval Research Global under Grant No. N62909-15-1-N021. MTM acknowledges the support of NSF grant AGS-1313948, NOAA HFIP grant N0017315WR00048, NASA grant NNG11PK021 and the U.S. Naval Postgraduate School.GK and RKS acknowledges funding for tropical cyclone research from the German Research Council (Deutsche Forschungsgemeinschaft) under Grant no SM30/23-4 and the Office of Naval Research Global under Grant No. N62909-15-1-N021. MTM acknowledges the support of NSF grant AGS-1313948, NOAA HFIP grant N0017315WR00048, NASA grant NNG11PK021 and the U.S. Naval Postgraduate School