Conclusions

Many new satellite-based data sets are now available to allow us to explore tropical cyclones with unprecedented temporal and spatial scales. These tropical systems have very strong interactions with the surface, but in this study we focused on the effects of hurricanes at the higher altitudes of the TTL. With lightning activity being associated with strong updrafts and therefore deeper convection, we explored the impact that lightning frequency had on TTL water vapor within hurricanes.

Our analysis was limited to five hurricanes in the Tropical Americas region in 2005 when both lightning and MLS water vapor data were available. We found weak, but statistically significant correlations (within measurement uncertainty) between lightning and MLS water vapor. Hydration at the 215 hPa level was positively correlated with lightning frequency. At 215 hPa, water vapor was also positively correlated with water vapor at 147 hPa. At 147 hPa, however, water vapor was negatively correlated with water vapor at 100. In other words, an increase in lightning frequency favors hydration at the 215 hPa level, which in turn favors hydration aloft at the 147 hPa level. However, when this hydration occurs below, the 100 hPa level experiences dehydration instead. While physically plausible, the strength of this mechanism by which the upper troposphere is hydrated and the 100 hPa level is dehydrated as a result of increasing lightning frequency within a hurricane should be further explored with a larger data set.

From the climate perspective, it is necessary to investigate the fate of the added moisture to the TTL by hurricanes. Are these air masses returning to the troposphere or are they being transported irreversibly into the stratosphere? With a forecast of increasing sea surface temperatures and strengthening of hurricanes in the Atlantic basin in particular (Kossin et al. 2007), is lightning frequency going to increase and affect the chemical and radiative properties of the TTL via transport of boundary layer air and production of ozone and NOx, for example? Many questions and many uncertainties remain. However, addition of new measurements such as space-borne radars and lidars flying on the CloudSat and Calipso satellites, and lightning instruments proposed to fly on geostationary satellites should give us more insights into the structure, evolution, and impact of these powerful tropical systems.

Acknowledgements This research was supported by an appointment to the NASA Postdoctoral Program at Marshall Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. LLDN data provided by the NASA Lightning Imaging Sensor (LIS) instrument team and the LIS data center via the Global Hydrology Resource Center (GHRC) located at the Global Hydrology and Climate Center (GHCC), Huntsville, Alabama through a license agreement with Global Atmospherics, Inc (GAI). The data available from the GHRC are restricted to LIS science team collaborators and to NASA EOS and TRMM investigators.

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