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The role of the ocean in convective burst initiation implications for tropical cyclone intensification /

by Hennon, Paula Ann

Abstract (Summary)
The upper ocean significantly influences tropical cyclone structure and intensity. These effects, however, are not well understood mostly due to a lack of oceanic and atmospheric boundary layer observations within the inner-core region. This study relates ocean-atmosphere energy exchange processes to mid-to-upper tropospheric latent heating using mesoscale inner-core convective burst events. A global survey of convective burst events in tropical cyclones from the year 1999 -- 2001 was constructed. This study shows that 80% of tropical cyclones have at least one convective burst event and that convective burst events usually occur during the intensification phase of the storm life cycle. Convective bursts are usually accompanied by a moderate (5-15 kt) wind speed increase, although some have little or no wind speed change during the burst itself. However, a period of intensification often follows a convective burst event within 18 to 24 hours. To determine atmospheric and oceanic variables useful in identifying conditions typical of convective burst activity, an ensemble of discriminant analyses were performed. The first procedure tested solely atmospheric variables; the second tested the oceanic variables by themselves, and finally, a combined procedure attempted to distinguish convective burst events using both the atmospheric and oceanic variables. Four main atmospheric conditions characterize convective burst existence when compared to periods with no convective burst: 1) increased precipitable water at 200 km and 500 km, 2) increased 150 mb divergence at 600 km, 3) 2-1/2 times more convective instability in the large-scale environment, 4) 1-1/2 to 2 times more 850 mb moisture divergence at 200 km and 600 km. The main characteristic differences in ocean conditions during convective burst events are: 1) the mean climatological SST is 1.25°C greater 2) the "hurricane heat content" is double 3) less inner-core ocean cooling occurs. The combined analysis suggests that the moist static energy provided by the warm ocean is more influential on convective burst occurrence than simply having “enough” available atmospheric moisture. A multivariate Lagrangian time series of the inner-core SST, the inner-core-wake SST, the ahead-of-storm SST, and measures of spatial variability of these variables for 30 ii tropical cyclones was constructed using an objectively interpolated SST tropical cyclone coldwake climatology. Latent and sensible heat flux estimates and a measure of upper-ocean energy utilization were calculated for the inner-core ( < .5° radius) and the near-core (.5° - 1° radius). This study found that tropical cyclones generally utilize only about 8% of the total enthalpy flux available from the ocean/atmosphere boundary layer. Storms with convective bursts utilize more energy from the ocean (11%) than storms with no convective burst (2%). Sea-air fluxes are greatly enhanced (doubled) during convective burst time periods. These along-track ocean-atmosphere analyses were compared to vertical profiles of atmospheric latent heating calculated using a combined active and passive TRMM PR and TMI retrieval algorithm. Results show strong positive space and time correlations between ocean-air fluxes and mid-upper tropospheric latent heating. Additionally, the 30 storms analyzed were categorized by the presence or absence of convective burst events during the storm lifecycle. Composite atmospheric latent heating profiles constructed for each group show a two-fold release in energy for the storms with convective burst events compared to storms with no convective burst event. Finally, seven case studies are presented which attempt to resolve The upscale energy cascade of the tropical cyclone with a convective burst event from the ocean through the troposphere. For these case studies, the TRMM vertical profiles of latent heating are compared with AMSU temperature anomalies in an attempt to link the enhanced tropospheric latent heat release with a developing inner-eye warm core anomaly. The findings of this study support the following scenario linking convective bursts to tropical cyclone intensification: A very warm ocean with a deep mixed layer is the base of the energy supply for the intensifying tropical cyclone. The undilute convection of the mesoscale convective burst mines the enhanced boundary layer gaining almost twice the total enthalpy flux of a storm with no convective burst. This twofold energy utilization is apparent in the midupper troposphere as TRMM vertical profiles of latent energy release show 2 to 2-1/2 times the magnitude of profiles of non-convective burst time periods. Finally, in most cases and after a 12 to 24 hour lag, an enhanced warm core anomaly appears in AMSU analyses. This lag time is consistent with the convective timescale necessary for adiabatic warming through subsidence along the inner edge of the tropical cyclone eyewall. iii
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Advisor:

School:The Ohio State University

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:cyclones hurricanes convection meteorology boundary layer ocean atmosphere interaction enthalpy tropics

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