Statistical And Modeling Studies Of Summer Explosive Cyclones Over The Southern Ocean

Explosive cyclones, referred to as"meteorological bombs", are usually accompanied with the rapid reduction of cyclone central pressure and extreme intensity enhancement. It is defined as the cyclone whose central pressure normalized at 60°N deceases more than 24 hPa within 24 hours (i.e., deepening rate > 1 Bergeron). During the past decades, most studies aiming to explore the mechanisms that trigger the rapid reduction of central pressure and to document the characteristics of explosive cyclones focused on the cyclones over the Northern Hemisphere. However, explosive cyclones over the South Hemisphere still remained poorly known due to the extreme weather conditions over the vast oceans. This paper firstly analyzed the characteristics of cyclones and explosive cyclones over the Southern Oceans during four summers (December, January, February) from 2004 to 2008 by using the Final Analysis (FNL) data provided by National Centers for Environmental Prediction (NCEP). Then, two quite different cyclones: one is a strong intensification (ST) cyclone (defined as central deepening rate greater than 1.8 Bergeron) over the Southern Indian Ocean on 25 February 2008, the other one is a weak intensification (WE) cyclone (defined as central deepening rate between 1.0 and 1.3 Bergeron) over the Southern Pacific Ocean on 10 December 2007, were simulated by using high-resolution of WRF (Weather Research & Forecast) model. Based upon modeling results, the detailed tempral-spatial structures and development mechanisms of these two explosive cyclones were comprehensively examined by using all available observational data in that area and data assimilation techniques.Statistical results indicated that January was the favorable month for the formation of summer explosive cyclones over the Southern Ocean. Most explosive cyclones formed in the mid-latitude with relatively more generating in the south of Atlantic Ocean and the south of the Indian Ocean. The maximum deepening rate of explosive cyclones predominantly occurred within the latitudinal belt 45°S-60°S, while the locations of minimum center pressure were usually found within the high-latitude zone 60°S-70°S. About 56% of the summer explosive cyclones were found to be weak intensification ones and they moved east-southeastwards with rare exceptions moving northeastwards. The life-time of explosive cyclones over the Southern Ocean was about 3~5 days and their horizontal scales were close to 3000 km. Composite analyses of those explosive cyclones with deepening rate greater than 1.3 Bergeron revealed that upper jet stream and baroclinicity were two key factors for the development of explosive cyclones.For the case of 2008, the ST cyclone initially formed around 06 UTC 25 February over the Southern Indian Ocean, and disappeared around 00 UTC 29 February 2008, lasting about 90 hours with intense explosive development in the next 24 hours. Its deepening rate of central pressure reached to 2.9 Bergeron around 12 UTC 25 February 2008, which was the maximum deepening rate in our four-year analyses. There are distinct differences in the structure of cyclone before and after its explosive development, revealed by all available observation data and numerical simulation results of the high-resolution WRF model. Specifically, vorticity, potential vorticity, and pressure vertical velocity all rapidly increased and reached to their maximum values when the cyclone was explosively developing. After that, the ST cyclone entered into its mature phase. During the explosive development, there was an upper-level trough occurring at 500 hPa and strong baroclinicity belts around the cyclone center at 250 hPa, 500 hPa and 850 hPa, respectively. The cyclone initially formed about 700 km in front of the upper-level trough with baroclinic energy as a key factor supporting its formation and development. Horizontal advection of negative vorticity through the large-scale upper-level trough and the intensified baroclinicity due to the interaction of cold and warm advections before and after the upper-level trough provided a favorable environment for the cyclone to develop and maintain. Moreover, convergence and warm advections at 250 hPa around the outlet of upper jet stream, and the strong warm advection and water vapor passage provided by the north-east jet stream at 850 hPa were believed to contribute to the cyclone's initial explosive development. Later, at upper-level high vorticity center started to move above the center of the cyclone during the mid-phase of the explosive development, which further accelerated the cyclone's development. Sensitivity studies of numerical modeling indicated that the release of latent heat was an important mechanism affecting cyclone's explosive development. On the other hand, there was no direct tight relationship between sea surface temperature (SST) and cyclone's development found in this study, suugesting that cyclone can explosively develop within a wide SST range.The second explosive cyclone (WE) initially formed around 18 UTC 10 December 2007 belongs to a weak category with the maximum deepening rate of 1.2 Bergeron. It lasted about 114 hours with two intermittent weak explosive developments. There was a jet stream at 850 hPa close to the cyclone center during the processes of explosive developments. Northerly prevailed and reached to its maximum during the first weak explosive development period. The WE cyclone entered into its mature phase after the jet stream at 850 hPa rapidly caught with the lower cyclone center. Similar to the ST cyclone, release of latent heat was still an important mechanism affecting the WE cyclone's explosive development. Analyses from the FNL data and simulation results indicated that high potential vorticity energy propagating downward from the tropopause, and the warm and moisturized airflow energized the cyclone for its first explosive development, while the second explosive development of the cyclone was largely induced by the downward propagation of momentum from the upper-level jet stream. Numerical sensitivity studies of various modeling schemes, as well as the data assimilation experiment used by WRF data assimilation system and COSMIC GPS RO (Constellation Observing System for Meteorology Ionosphere and Climate Global Positioning System radio occultation) data showed that WRF data assimilation system was able to simulate the WE cyclone's first explosive development and its moving track more reasonably than other modeling.By comparison, both the ST cyclone and the WE cyclone had explosive development lasting for a relatively long time period. Both of them formed in front of the upper-level trough at 500 hPa and were maintained by the release of latent heat, the existence of jet streams in different vertical levels and the downward propagation of momentum from the upper-level jet stream. Nevertheless, the ST cyclone was found strong in developing intensity, short in life-cycle, and large in horizontal scale. It formed in the westerly of the Southern Indian Ocean, which is known as a frequent birthplace for explosive cyclones due to the strong but favorable year-round jet stream in middle and low vertical levels. In contrast, the WE cyclone formed over the Ross Sea, where few explosive cyclones were found previously due to the impediments caused by the existence of massive sea ice. Wind speed in jet stream at 250 hPa in the ST cyclone was about 20 m/s higher than the weak one. There was also a clear downward propagating trend of the wind speed from upper-level in strong explosive cyclone, which was not observed in the weak one. Considering the generating mechanism, the explosive development of ST cyclone was initiated by the downward propagating of momentum from the upper-level jet stream, while the WE cyclone was predominantly initiated by the downward propagating of high vorticity energy from the tropopause.