A Modeling Study on the Early Electrical Development of Tropical Convection: Continental and Oceanic (Monsoon) Storms

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
  • | 2 Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota
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Abstract

Numerical modeling studies of continental tropical and maritime tropical convection were conducted using the two-dimensional, nonhydrostatic, cloud electrification model developed at the South Dakota School of Mines and Technology. The model contains six classes of water (water vapor, cloud water, cloud ice, rain, snow, and graupel) and a full set of ion equations. All hydrometeors are permitted to exchange charge. Charge transfer between microphysical species is accomplished through a noninductive charging parameterization following Takahashi.

The goal of the numerical experiments was to examine the kinematic and microphysical differences that lead to marked differences in observed electrification between the break (continental) and monsoon (oceanic) convective regimes observed near Darwin, Australia. The break regime is associated with deep, intense convection that forms in high-CAPE (convective available potential energy) environments. Normally, copious amounts of lightning accompany break period convective events. Monsoon conditions are associated with heavy rain and relatively weak convection that forms in moderate to low-CAPE environments. Very little lightning activity is normally observed in the monsoon.

Three numerical simulations ranging from high- to low-CAPE conditions are presented. The results indicate that the electrification of the simulated storm critically depends on the juxtaposition of the level of charge reversal (LCR), which is in turn dependent on temperature and liquid water contents, and the particle interaction region, which is the level where ice particle collisions occur and thus where noninductive charging can take place. In the high-CAPE (break period) case, the LCR is located several kilometers below the interaction region, and strong in-cloud electric fields develop as a consequence. In the low- to moderate-CAPE (monsoon) cases, the LCR and interaction region are closely located in the vertical. As hydrometcors move across the LCR in both directions, the charge on their surfaces continually changes sign, thus preventing the development of a significant in-cloud electric field. It is further hypothesized that in conditions of zero to extremely low CAPE, the particle interaction region would he situated below the LCR, leading to the development of an inverted dipole (positive charge underlying negative charge), such as may occur in the stratiform regions of mesoscale convective systems.

Abstract

Numerical modeling studies of continental tropical and maritime tropical convection were conducted using the two-dimensional, nonhydrostatic, cloud electrification model developed at the South Dakota School of Mines and Technology. The model contains six classes of water (water vapor, cloud water, cloud ice, rain, snow, and graupel) and a full set of ion equations. All hydrometeors are permitted to exchange charge. Charge transfer between microphysical species is accomplished through a noninductive charging parameterization following Takahashi.

The goal of the numerical experiments was to examine the kinematic and microphysical differences that lead to marked differences in observed electrification between the break (continental) and monsoon (oceanic) convective regimes observed near Darwin, Australia. The break regime is associated with deep, intense convection that forms in high-CAPE (convective available potential energy) environments. Normally, copious amounts of lightning accompany break period convective events. Monsoon conditions are associated with heavy rain and relatively weak convection that forms in moderate to low-CAPE environments. Very little lightning activity is normally observed in the monsoon.

Three numerical simulations ranging from high- to low-CAPE conditions are presented. The results indicate that the electrification of the simulated storm critically depends on the juxtaposition of the level of charge reversal (LCR), which is in turn dependent on temperature and liquid water contents, and the particle interaction region, which is the level where ice particle collisions occur and thus where noninductive charging can take place. In the high-CAPE (break period) case, the LCR is located several kilometers below the interaction region, and strong in-cloud electric fields develop as a consequence. In the low- to moderate-CAPE (monsoon) cases, the LCR and interaction region are closely located in the vertical. As hydrometcors move across the LCR in both directions, the charge on their surfaces continually changes sign, thus preventing the development of a significant in-cloud electric field. It is further hypothesized that in conditions of zero to extremely low CAPE, the particle interaction region would he situated below the LCR, leading to the development of an inverted dipole (positive charge underlying negative charge), such as may occur in the stratiform regions of mesoscale convective systems.

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