The accurate forecast of changes in the climate system requires understanding of processes that control the chemical composition and the radiative balance of the atmosphere. One of such processes is troposphere-to-stratosphere transport (TST). Exchange between these two regions of the atmosphere occurs predominantly in the tropics and requires crossing of the cold-point tropopause. In the extratropics, the tropopause acts a material surface separating tropospheric from stratospheric air, whereas in the tropics this separation occurs gradually within a layer. This layer is referred to as the Tropical Tropopause Layer (TTL), where air transitions from the convectively-dominated troposphere into the radiatively-dominated stratosphere. This layer is physically bound from below by the level of neutral buoyancy or height of main convective outflow (located at ^350 K, or ^150 hPa, or ~14 km) and from above by the cold-point tropopause (located at ^380 K, ^100 hPa, ^17 km) (Gettelman and Forster 2002).
The most powerful and naturally encountered greenhouse gas in the atmosphere is water vapor. This gas plays a critical role in the radiative balance of the TTL, which contains the transition from the net radiative cooling region where air sinks to the net radiative heating region where air rises. The net radiative cooling is dominated by longwave cooling due to water vapor, while the net radiative heating is dominated by shortwave heating due to ozone (Gettelman et al. 2004). Water vapor, however, offsets both the longwave and the shortwave heating by ozone. In addition to locally impacting heating and cooling rates, model runs and observations have shown that water vapor at the bottom of the TTL has a positive climate feedback on surface temperatures (i.e., warmer surface temperatures drive deeper convective systems that increase the concentration of water vapor at the bottom of the TTL via deep convection, which in return serves to further increase surface warming) (Minschwaner et al. 2006; Minschwaner and Dessler 2004).
The thermodynamic and convective properties of the TTL regulate the amount of water vapor that eventually reaches into the tropical stratosphere. Mechanisms ranging from freeze-drying by the cold-point tropical tropopause (Brewer 1949) to freeze-drying by horizontal advection through cold pools during slow diabatic ascent in the TTL (Holton and Gettelman 2001) to overshooting convection (Danielsen 1993) to mixing of dry and moist air in the TTL (Sherwood and Dessler 2001) to cloud microphysics (Jensen et al. 2001; Jensen and Pfister 2004) have been proposed to control TTL water vapor. Once in the stratosphere, water vapor plays a critical role in stratospheric ozone chemistry by providing OH radicals that can directly destroy ozone molecules and by providing one of the ingredients needed in the formation of polar stratospheric clouds, where activation of ozone-destroying chlorine radicals is initialized. Besides stratospheric chemistry, the concentration of water vapor can affect stratospheric temperatures (Forster and Shine 1999) and changes in the stratosphere can be reflected in tropospheric circulation (Shindell et al. 1999). Given the broad and significant impact that water vapor has on the chemical composition and radiative balance of the lower atmosphere, and in particular the TTL, it is essential to have an accurate understanding of the processes that regulate its concentration.
One of the mechanisms that affect water vapor concentrations in the TTL is deep convection. This relation has been confirmed using lightning activity, as a proxy for deep convection, and the NCEP/NCAR reanalysis water vapor product at 300 hPa (Price and Asfur 2006).
Within deep convective cloud systems, lightning activity is an electrical manifestation of thermodynamic and mechanical work performed by vertical air motion varying non-linearly with the updraft speed (Baker et al. 1999). The basis for this sensitivity lies in the supply of condensate to drive mixed phase cloud microphysics (Petersen and Rutledge 2001). Satellite observations have shown that lightning is more likely to occur over land where mixed phase microphysics and the strength of convective updrafts are typically more pronounced than over oceans (Christian et al. 2003). Conversely, updraft velocities in oceanic convection are often too small to support the production of robust mixed phase processes and lightning.
Hurricanes are a type of oceanic convection whose main driving force is horizontal advection. These storms are accompanied by fairly weak vertical updrafts, even in the eyewall region where updrafts are the strongest. This condition results in limited charge separation and hence lightning activity (Saunders 1993). Several studies have reported on the scarcity of lightning activity in hurricanes (Black and Hallett 1999; Molinari et al. 1994). Lightning activity, however, is not always absent or insignificant in hurricanes. An examination of different hurricanes using continuous ground-base observations of lightning revealed the presence of significant lightning outbreaks in the eyewall region coincident in time with storm intensification (Molinari et al. 1998). So far, many uncertainties still remain in our understanding of the driving mechanism(s) for lightning generation in hurricanes and the cause(s) for storm-to-storm variability.
The goal of this study is to investigate the effect that hurricanes, which are large-scale and longer-lived oceanic convective systems, have on TTL water vapor. A recent study by Ray and Rosenlof (2007) showed that hurricanes serve to moisten an area ^1500 km2 around the center of the storm at altitudes between 300 and 150 hPa. Our study seeks to explore the relation between lightning frequency, which is typically associated with strong updrafts and thus deeper convection, and TTL water vapor. We address the question; can we see an increase in TTL moisture when lightning frequency increases?
Hurricanes are not only of interest to our study because of their spatial and temporal scales. Theory, model, and observations predict an increase in hurricane intensity as a result of warmer sea surface temperatures due to global warming (Emanuel 1987; Knutson and Tuleya 2004; Kossin et al. 2007). Therefore, a forecast of warmer environmental conditions might make hurricanes a significant source of water vapor to the TTL in the years to come.
We focus our study on hurricanes that developed and/or evolved in the Tropical Americas region. In addition to being a hurricane-active region and a region predicted to see an increase in hurricane intensity based on trends observed over the last 23 years (Kossin et al. 2007), this region has the following additional characteristics during the summertime: (i) it becomes the second most dominant source of air to the TTL as revealed by trajectory calculations (Fueglistaler et al. 2004), (ii) it exhibits the largest seasonal increase in percent contribution to stratospheric moisture (Fueglistaler et al. 2004), and (iii) it has the warmest tropical tropopause temperatures that can sustain saturation mixing ratios ranging from 7 to 13 parts per million by volume (ppmv) compared to a global average in the tropics of 6.6 ppmv and Western Tropical Pacific ranges of 3 to 5 ppmv (Pittman 2005).
This paper is structured as follows. We introduce the data and methodology in Section 2. We discuss our findings in Section 3. We present our conclusions in Section 4.
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