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Precipitation and Kinematic Structure of an Oceanic Mesoscale Convective System. Part I: Momentum Transport and Generation

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
  • | 2 NOAA/NSSL/Mesoscale Research Division, Boulder, Colorado
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Abstract

This is the second paper of a two-part series documenting the structure of and momentum transport by a subtropical mesoscale convective system near Taiwan, using Doppler radar data and in situ data from the NOAA P-3. Part I defines the basic system structure and evolution. In Part II, the momentum transport by the system is estimated and related to system structure, and the momentum budget for a portion of the embedded convective band is evaluated.

Profiles of the vertical flux of horizontal momentum are constructed from in situ data, Doppler radar data, and both combined, in a coordinate system with u normal to the line and positive eastward, since the low-level air is feeding the line from the east. Differences in the fluxes from the two sources appear to be mainly due to an underestimation of the mean vertical velocity from the Doppler radar data. The discrepancy results partially from the concentration of convergence in the boundary layer—precisely where the Doppler cannot adequately sample the convergence—and partially from Doppler problems above 5 km. However, the momentum-flux profile generated from both data sources has features consistent with the structure of the line: p̄uw is negative at lower levels, consistent with the westward tilt of most updrafts at those levels, and positive at upper levels, consistent with the updrafts' eastward tilt. This positive flux is countergradient and not consistent with previous observations, but is suggested in numerical simulations of systems in an environment similar to that for this system, with relatively low convective available potential energy(CAPE), high relative humidity aloft, and positive u shear through the depth of the system. The simulated systems have relatively weak updrafts and gust fronts, also matching this case. The flux p̄vw is downgradient above ∼5 km and countergradient below, but is consistent with the average positive vertical velocity carrying southerlies (V̄>0) upward.

The momentum budget reveals some behavior that differs from that of earlier systems such as that studied by Lafore et al. For example, above 7 km the momentum transport and pressure gradient reinforce to produce substantial acceleration of air exiting the band at high levels toward the front (east), although the vertical transport contributes only a small amount to the observed acceleration. The u positive acceleration at higher levels, being larger than the Doppler estimates of dŪ/dt at lower levels, increases the overall u shear within the convective band. Estimation of the vertical momentum-flux divergence and pressure-gradient term at low levels from the in situ data supports this results. In previously observed tropical systems, u shear was increased by convective bands only when the u shear was negative. At midlevels, the vertical transport of line-parallel wind (v) by the line acts to increase and slightly elevate the southerly jet maximum in the environmental wind profile usually seen in this region. As in previously documented systems, dV̄/dz decreases with time within the band.

Abstract

This is the second paper of a two-part series documenting the structure of and momentum transport by a subtropical mesoscale convective system near Taiwan, using Doppler radar data and in situ data from the NOAA P-3. Part I defines the basic system structure and evolution. In Part II, the momentum transport by the system is estimated and related to system structure, and the momentum budget for a portion of the embedded convective band is evaluated.

Profiles of the vertical flux of horizontal momentum are constructed from in situ data, Doppler radar data, and both combined, in a coordinate system with u normal to the line and positive eastward, since the low-level air is feeding the line from the east. Differences in the fluxes from the two sources appear to be mainly due to an underestimation of the mean vertical velocity from the Doppler radar data. The discrepancy results partially from the concentration of convergence in the boundary layer—precisely where the Doppler cannot adequately sample the convergence—and partially from Doppler problems above 5 km. However, the momentum-flux profile generated from both data sources has features consistent with the structure of the line: p̄uw is negative at lower levels, consistent with the westward tilt of most updrafts at those levels, and positive at upper levels, consistent with the updrafts' eastward tilt. This positive flux is countergradient and not consistent with previous observations, but is suggested in numerical simulations of systems in an environment similar to that for this system, with relatively low convective available potential energy(CAPE), high relative humidity aloft, and positive u shear through the depth of the system. The simulated systems have relatively weak updrafts and gust fronts, also matching this case. The flux p̄vw is downgradient above ∼5 km and countergradient below, but is consistent with the average positive vertical velocity carrying southerlies (V̄>0) upward.

The momentum budget reveals some behavior that differs from that of earlier systems such as that studied by Lafore et al. For example, above 7 km the momentum transport and pressure gradient reinforce to produce substantial acceleration of air exiting the band at high levels toward the front (east), although the vertical transport contributes only a small amount to the observed acceleration. The u positive acceleration at higher levels, being larger than the Doppler estimates of dŪ/dt at lower levels, increases the overall u shear within the convective band. Estimation of the vertical momentum-flux divergence and pressure-gradient term at low levels from the in situ data supports this results. In previously observed tropical systems, u shear was increased by convective bands only when the u shear was negative. At midlevels, the vertical transport of line-parallel wind (v) by the line acts to increase and slightly elevate the southerly jet maximum in the environmental wind profile usually seen in this region. As in previously documented systems, dV̄/dz decreases with time within the band.

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