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Abstract
Eulerian kinetic energy budgets for the synoptic-scale flow over North America were computed for two cases of cyclone development associated with severe prefrontal convection. Horizontal flux convergence constitutes the major energy source in both cases and assumes major importance in maintaining the strength of the upper tropospheric jet maxima. Generation of kinetic energy via cross-counter flow is, surprisingly, a persistent sink in one case and only a weak energy source for the cyclone in the second case. Cross-contour flow toward higher heights is generally found ahead of the upper level troughs, where the jet stream is moving through regions in which the contour gradient weakens downstream. Generation of kinetic energy is largely confined to the lower troposphere, reflecting frictional influence near the earth's surface. Dissipation of kinetic energy, computed as a residual, has local maxima both in the lower troposphere (nearly balancing the generation) and near the jet stream level. Subgrid-scale sources of kinetic energy are apparent in both cases and, at times, are particularly important in regions of widespread deep convection. These sources are associated with longitudinal shear in the polar jet stream and with the presence of deep convection. Convective region budgets show that the maximum rate of kinetic energy gain occurs during periods when the convection increases most rapidly both in intensity and in areal extent.
Abstract
Eulerian kinetic energy budgets for the synoptic-scale flow over North America were computed for two cases of cyclone development associated with severe prefrontal convection. Horizontal flux convergence constitutes the major energy source in both cases and assumes major importance in maintaining the strength of the upper tropospheric jet maxima. Generation of kinetic energy via cross-counter flow is, surprisingly, a persistent sink in one case and only a weak energy source for the cyclone in the second case. Cross-contour flow toward higher heights is generally found ahead of the upper level troughs, where the jet stream is moving through regions in which the contour gradient weakens downstream. Generation of kinetic energy is largely confined to the lower troposphere, reflecting frictional influence near the earth's surface. Dissipation of kinetic energy, computed as a residual, has local maxima both in the lower troposphere (nearly balancing the generation) and near the jet stream level. Subgrid-scale sources of kinetic energy are apparent in both cases and, at times, are particularly important in regions of widespread deep convection. These sources are associated with longitudinal shear in the polar jet stream and with the presence of deep convection. Convective region budgets show that the maximum rate of kinetic energy gain occurs during periods when the convection increases most rapidly both in intensity and in areal extent.
Abstract
Spatial fields of satellite-measured deep-layer temperatures are examined in the context of quasigeostrophic theory. It is found that midtropospheric geostrophic vorticity and quasigeostrophic vertical motions can be diagnosed from microwave temperature measurements of only two deep layers. The lower- (1000–400 hPa) and upper- (400–50 hPa) layer temperatures are estimated from limb-corrected TIROS-N Microwave Sounding Units (MSU) channel 2 and 3 data, spatial fields of which can be used to estimate the midtropospheric thermal wind and geostrophic vorticity fields. Together with Trenberth's simplification of the quasigeostrophic omega equation, these two quantities can be then used to estimate the geostrophic vorticity advection by the thermal wind, which is related to the quasigeostrophic vertical velocity in the midtroposphere.
Critical to the technique is the observation that geostrophic vorticity fields calculated from the channel 3 temperature features are very similar to those calculated from traditional, “bottom-up” integrated height fields from radiosonde data. This suggests a lack of cyclone-scale height features near the top of the channel 3 weighting function, making the channel 3 cyclone-scale “thickness” features approximately the same as height features near the bottom of the weighting function. Thus, the MSU data provide observational validation of the LID (level of insignificant dynamics) assumption of Hirshberg and Fritsch.
Abstract
Spatial fields of satellite-measured deep-layer temperatures are examined in the context of quasigeostrophic theory. It is found that midtropospheric geostrophic vorticity and quasigeostrophic vertical motions can be diagnosed from microwave temperature measurements of only two deep layers. The lower- (1000–400 hPa) and upper- (400–50 hPa) layer temperatures are estimated from limb-corrected TIROS-N Microwave Sounding Units (MSU) channel 2 and 3 data, spatial fields of which can be used to estimate the midtropospheric thermal wind and geostrophic vorticity fields. Together with Trenberth's simplification of the quasigeostrophic omega equation, these two quantities can be then used to estimate the geostrophic vorticity advection by the thermal wind, which is related to the quasigeostrophic vertical velocity in the midtroposphere.
Critical to the technique is the observation that geostrophic vorticity fields calculated from the channel 3 temperature features are very similar to those calculated from traditional, “bottom-up” integrated height fields from radiosonde data. This suggests a lack of cyclone-scale height features near the top of the channel 3 weighting function, making the channel 3 cyclone-scale “thickness” features approximately the same as height features near the bottom of the weighting function. Thus, the MSU data provide observational validation of the LID (level of insignificant dynamics) assumption of Hirshberg and Fritsch.
