A barrier to vertical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements

Abstract

Abstract. We use ozonesondes launched from Samoa (14øS) during the Pacific Exploratory Mission (PEM) Tropics A to show that O3 mixing ratios usually start increasing toward stratospheric values near 14 km. This is well below the tropical tropopause (as defined either in terms of lapse rate or cold point), which usually occurs between 16 and 17 km. We argue that the main reason for this discrepancy in height between the chemopause and tropopause is that there is very little convective detrainment of ozone-depleted marine boundary layer air above 14 km. We conjecture that the top of the Hadley circulation occurs at roughly 14 km, that convective penetration above this altitude is rare, and that air that is injected above this height subsequently participates in a slow vertical ascent into the stratosphere. The observed dependence of ozone on potential temperature in the transitional zone between the 14-km chemopause and the tropical tropopause is consistent with what would be expected from this hypothesis given calculated clear-sky heating rates and typical in situ ozone production rates in this region. An observed anticorrelation between ozone and equivalent potential temperature below 14 km is consistent with what would be expected from an overturning Hadley circulation, with some transport of high O3/low 0• air from midlatitudes. We also argue that the positive correlations between 03 and N20 in the transitional zone obtained during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft) (ASHOE/MAESA) campaign support the notion that air in this region does have trace elements of stratospheric air (as conjectured previously), so that some of the ozone in the transitional zone does originate from the stratosphere rather than being entirely produced in situ.

Tropical tropospheric ozone column retrieval for GOME-2

“…These cloud levels are more than 100 hPa below the tropopause, which is located around 100 hPa in the tropics. This is in agreement with other studies that suggest that most convective cloud tops do not extend up to the tropopause, but to the bottom of the tropical transition layer (Folkins et al, 1999;Fueglistaler et al, 2009), several kilometres below the tropopause. Furthermore, the effective (also called optical centroid) cloud pressure derived from VIS and NIR (oxygen A-band) satellite measurements using the Lambertian cloud model lies well below the physical cloud top pressure (Joiner et al, 2012;Ziemke et al, 2009a).…”

Changes in surface ozone levels due to convective downdrafts over the Bay of Bengal

“…In agreement with typical q E values in the tropical MBL [Folkins et al, 1999], the present observations show values of $354 -357 K outside the downdraft regime for these two days of measurements (Figure 4).…”

“…Surface sensible and latent heat fluxes are considered the main sources of q E in the atmosphere [Betts et al, 1992], while the primary sink is radiative cooling of the troposphere. Consequently, the tropical lower troposphere is characterized by a decrease in q E and an increase in O 3 with height [Folkins et al, 1999;Zachariasse et al, 2000;Betts et al, 2002]. Independent meteorological parameters such as temperature and pressure may not be conserved in downdrafts, while q E is near conservative.…”

Seasonal variability of middle‐latitude ozone in the lowermost stratosphere derived from probability distribution functions

“…This exchange is related to Rossby wave mixing, which is ultimately correlated with monsoonal circulations. Further, Folkins et al [1999] among others, have identified in the tropics a region, below the tropopause and above 14 km, which contains air with stratospheric ozone, suggesting a mixed region between the troposphere and the stratosphere at low latitudes. In total, current observations point to a need to reconsider the traditional concepts used to describe the mechanisms of stratosphere-troposphere exchange.…”