This paper investigates the influence of tropical cyclones on water vapor concentrations in the upper atmosphere above these storms. We use independent data sets of tropical storm intensity, water vapor and lightning activity to investigate this relationship. Water vapor in the upper troposphere is a key greenhouse gas, with direct impacts on surface temperatures. Both the amount and altitude of water vapor impact the radiative balance and the greenhouse effect of the atmosphere. The water vapor enters the upper troposphere through deep convective storms, often associated with lightning activity. The intensity of the lightning activity represents the intensity of the convection in these storms, and hence the amount of water vapor transported aloft. In this paper, we investigate the role of tropical cyclones on the contribution of water vapor to the upper atmosphere moistening. Tropical cyclones are the largest most intense storms on Earth and can last for up to two weeks at a time. There is also evidence that the intensity of tropical cyclones is increasing, and will continue to increase, due to global warming. In this study we find that the maximum moistening of the upper atmosphere occurs at the 200 hPa level (~12 km altitude), with a lag of 1–2 days after the maximum sustained winds in the tropical cyclone. While the water vapor peaks after the maximum of the storm intensity, the lightning activity peaks before the maximum intensity of the storms, as shown previously. We show here that the absolute amount of water vapor in the upper troposphere above tropical storms increases linearly with the intensity of the storms. For every 10 hPa decrease in the minimum pressure of tropical storms, the specific humidity increases around 0.2 g/kg at the 200 hPa level.
<p>Tropical cyclones have been observed in recent years to be increasing in intensity due to global warming, and projections for the future are for further shifts to stronger tropical cyclones, while the changes in the number of storms is less certain in the future.&#160; These storms have been shown to exhibit strong lightning activity in the eyewall and rainbands, and some studies (Price et al., 2009) showed that the lightning activity peaks before the maximum intensity of the tropical cyclones.&#160; Now we have investigated the impact of these tropical storms on the upper tropospheric water vapor (UTWV) content.&#160; Using the ERA5 reanalysis product from the ECMWF center, together with lightning data from the ENTLN network, we show that the lightning activity in tropical cyclones is closely linked to the increase in UTWV above these storms.&#160; We find the maximum enhancement in UTWV occurs between the 100-300 mb pressure levels, with a lag of 0-2 days after the peak of the storm intensity (measured by the maximum sustained winds in the eyewall).&#160; The lightning activity peaks before the storm reaches its maximum intensity, as found in previous studies.&#160; The interest in UTWV concentrations is due to the strong positive feedback that exists between the amounts of UTWV and surface global warming.&#160; Water Vapor is a strong greenhouse gas which is most efficient in trapping in longwave radiation emitted from the Earth in the upper troposphere.&#160; Small changes in UTWV over time can result in strong surface warming.&#160; If tropical cyclones increase in intensity in the future, this will likely result in increases in UTWV, reducing the natural cooling ability of the Earth.&#160; Lightning may be a useful tool to monitor these changes.</p>
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