Simulations of a typical midlatitude squall line were used to investigate a mechanism for discrete propagation, defined as convective initiation ahead of an existing squall line leading to a faster propagation speed for the storm complex. Radar imagery often shows new cells appearing in advance of squall lines, suggesting a causal relationship and prompting the search for an “action-at-a-distance” mechanism to explain the phenomenon. In the simulations presented, the identified mechanism involves gravity waves of both low and high frequency generated in response to the latent heating, which subsequently propagate out ahead of the storm. The net result of the low-frequency response, combined with surface fluxes and radiative processes, was a cooler and more moist lower troposphere, establishing a shallow cloud deck extending ahead of the storm. High-frequency gravity waves, excited in response to fluctuations in convective activity in the main storm, were subsequently ducted by the storm’s own upper-tropospheric forward anvil outflow. These waves helped positively buoyant cumulus clouds to occasionally form in the deck. A fraction of these clouds persisted long enough to merge with the main line, invigorating the parent storm. Discrete propagation occurred when clouds developed into deep convection prior to merger, weakening the parent storm. The ducting conditions, as diagnosed with the Scorer parameter, are shown to be sensitive to vertical wind shear and radiation, but not to the microphysical parameterization or simulation geometry.

In earlier work, a three-dimensional cloud model was used to simulate the interaction between the sea-breeze front (SBF) and front-parallel horizontal convective rolls (HCRs), resulting in the SBF systematically encountering roll updrafts and downdrafts as it progressed inland. Interestingly, deep convection was spawned above an HCR updraft ahead of the SBF as the front approached, well before the inevitable front–roll merger. Ostensibly, both the sea-breeze and roll circulations were required for deep convection to be present in this case at all because convection was entirely absent when either phenomenon was removed. Further analysis reveals why both circulations were necessary yet not sufficient for the excitation of deep convection in this case. The sea-breeze circulation (SBC) made its upstream (inland) environment more favorable for convection by bringing about persistent if gentle lifting over an extended region stretching well ahead of the SBF. This persistent ascent established a moist and cool tongue of air, manifested by a visible and/or subvisible cloud feature termed the cloud shelf emanating ahead of the front. Though this lifting moistened and destabilized the environment, the roll’s direct and indirect effects on this moist tongue were also required. The former consisted of a moisture plume lofted by the roll updraft, and the latter consisted of obstacle effect gravity waves generated as the roll drafts penetrated through the top of the boundary layer, into the SBC-associated offshore flow farther aloft. These provided the missing spark, which led to rapid growth of cumulus above the roll updraft, drawing first from air located above the boundary layer. Once established, deep convection above the roll updraft modulated cloudiness above the approaching SBF, at first suppressing it but subsequently assuring its reestablishment and eventual growth into deep convection, again prior to the front–roll merger. This resulted from the influence of gravity waves excited owing to heating and cooling within the roll cloud.

Analyses of tropical cyclones created in an idealized environment reveal how and why cloud microphysical assumptions can influence storm motion, including speed and direction. It is well known that in the absence of a mean flow, a leading factor in storm propagation is the establishment of “beta gyres” owing to planetary vorticity advection by the storm’s circulation. Previous research demonstrated that tangential winds well beyond the core influence storm motion by helping to determine the gyres’ orientation and intensity. Microphysical assumptions, especially involving average particle fall speeds, can strongly influence the winds at outer radius. More specifically, microphysics modulates the radial distribution of column-average virtual temperature, which largely determines the radial surface pressure gradient and therefore the winds because they tend to be in gradient balance beyond the core. Microphysics schemes can differ markedly with respect to average fall speed, depending on the complexity of the scheme and how interactions among condensation types are handled. Average fall speed controls the outward movement of particles produced in the eyewall into the anvil, where they can influence the environment through cloud–radiative interactions and phase changes. With the assistance of some special sensitivity tests, the influence of microphysics and fall speed on radial temperature gradients, leading to different outer wind strengths and tracks, is shown. Among other things, this work demonstrates that the treatment of outer rainbands in operational models can potentially influence how simulated storms move, thus affecting position forecasts.

The authors study herein the convective cell life cycle and the cell generation process in mature, multicellular squall-line storms possessing well-developed subcloud cold pools using two- and three-dimensional models. The multicellular storm establishes new cells on its forward side, in the vicinity of the forced updraft formed at the pool boundary, that first intensify and then decay as they travel rearward within the storm’s upward sloping front-to-rear airflow. The principal effort is expended on the two-dimensional case owing to the strong similarity in basic behavior seen in the two geometries. The cell life cycle is examined in several complementary fashions. The cells are shown to be convectively active entities that induce local circulations that alternately enhance and suppress the forced updraft, modulating the influx of the potentially warm inflow. This transient circulation also drives the episodic mixing of stable air into the inflow that results in the cell’s ultimate dissipation. ... Abstract The authors study herein the convective cell life cycle and the cell generation process in mature, multicellular squall-line storms possessing well-developed subcloud cold pools using two- and three-dimensional models. The multicellular storm establishes new cells on its forward side, in the vicinity of the forced updraft formed at the pool boundary, that first intensify and then decay as they travel rearward within the storm’s upward sloping front-to-rear airflow. The principal effort is expended on the two-dimensional case owing to the strong similarity in basic behavior seen in the two geometries. The cell life cycle is examined in several complementary fashions. The cells are shown to be convectively active entities that induce local circulations that alternately enhance and suppress the forced updraft, modulating the influx of the potentially warm inflow. This transient circulation also drives the episodic mixing of stable air into the inflow that results in the cell’s ultimate dissipation. ...

This paper examines the effect of topographically phase-locked convection on the motion of typhoons across the island of Taiwan. Data for 84 typhoons that reached Taiwan’s eastern coast from 1960 to 2010 are analyzed, with motions compared to the long-term average overland translation speed. For 61 continuous-track typhoons among all cases, 77% of the slow-moving tropical cyclones (TCs) made landfall on the northern end of Taiwan’s eastern coast, while 60% of the fast storms had southeastern coastal landfalls. This geographic asymmetry with respect to typhoon translation speeds widened after landfall, as the slow movers typically decelerated during the overland period, whereas the faster TCs sped up. In particular, the average overland duration was 16 h for the slow class, compared to only 3 h for the fast-moving typhoons. The combination of slower translation with longer duration for the northern class of TCs led to large rainfall on the southwestern slope of the island’s Central Mountain Range. Weather Research and Forecasting model experiments are used to study the effect of convection on storm motion over a mountainous island resembling Taiwan. The authors find that the topographically phase-locked convection acts to slow down (speed up) the northern (southern) landfalling typhoons. The model results also suggest that a positive feedback mechanism exists for the slow storms, in which the convective heating pattern forced by topography acts to reduce the TC motion, leading to even more prolonged precipitation and heating, yielding further speed reductions.