Abstract. Mid-tropospheric mesoscale convective vortices have been
often observed to precede tropical cyclogenesis. Moreover, recent cloud-resolving numerical modelling studies that are initialized with a weak
cyclonic mid-tropospheric vortex sometimes show a considerable
intensification of the mid-level circulation prior to the development of the
strong cyclonic surface winds that characterize tropical cyclogenesis. The
objective of this two-part study is to determine the processes that lead to
the development of a prominent mid-level vortex during a simulation of the
transformation of a tropical disturbance into a tropical depression, in
particular the role of diabatic heating and cooling. For simplicity
simulations are initialized from a quiescent environment. In this first
part, results of the numerical simulation are described and the response to
stratiform components of the diabatic forcing is investigated. In the second
part, the contribution of diabatic heating in convective cells to the
development of the mid-level vortex is examined. Results show that after a period of intense convective activity, merging of
anvils from numerous cells creates an expansive stratiform ice region in the
upper troposphere, and at its base a mid-level inflow starts to develop.
Subsequently conservation of angular momentum leads to strengthening of the
mid-level circulation. A 12 h period of mid-level vortex
intensification is examined during which the mid-level tangential winds
become stronger than those at the surface. The main method employed to
determine the role of diabatic forcing in causing the mid-level inflow is to
diagnose it from the full physics simulation and then impose it in a
simulation with hydrometeors removed and the microphysics scheme turned off.
Removal of hydrometeors is achieved primarily through artificially
increasing their fall speeds 3 h prior to the 12 h period.
This results in a state that is in approximate gradient wind balance, with
only a weak secondary circulation. Then, estimates of various components of
the diabatic forcing are imposed as source terms in the thermodynamic
equation in order to examine the circulations that they independently
induce. Sublimation cooling at the base of the stratiform ice region is
shown to be the main factor responsible for causing the strong mid-level
vortex to develop, with smaller contributions from stratiform heating aloft
and low-level melting and evaporation. This contrasts with the findings of
previous studies of mid-latitude vortices that indicate sublimation plays a
relatively minor role. An unanticipated result is that the central cool
region that develops near the melting level is to a large degree due to
compensating adiabatic ascent in response to descent driven by diabatic
cooling adjacent to the central region, rather than in situ diabatic
cooling. The mid-level inflow estimated from stratiform processes is notably
weaker than for the full physics simulation, suggesting a moderate
contribution from diabatic forcing in convective cells.
Evidence for the role of the diabatic heating in synoptic scale processes: a case study example