Exchange between the upper tropical troposphere and the lower stratosphere studied with aircraft observations

Abstract

Exchange between the upper tropical troposphere and the lower stratosphere is considered by examining WB57F and ER‐2 aircraft observations of water, ozone, wind, and temperature in the potential temperature range 360 < θ < 420 K. These processes are examined in part by using the technique of unified scale invariance on the airborne data, as has been done previously for the lower stratospheric polar vortex. Scale invariance is found, on scales from a few hundred meters to the maximum flown, 2700 km (25 great circle degrees). The results apply both to vertical exchange at the tropical tropopause and to isentropic exchange at the subtropical jet stream. All scales participate in the maintenance of the mean state, with substantial contributions from relatively infrequent but intense events in the long tails of the probability distributions. Past data are examined and found to fit this general framework. A unique mapping of tropical tropopause temperature to the total hydrogen content of the middleworld and overworld should not be expected; the head of the “tape recorder” is at 50–60 hPa rather than 90–100 hPa. The tropical tropopause is observed at potential temperatures θT greater than the maximum moist static surface values θW, such that θT − θW varies between 10 K in fall and up to 40 K in spring. The meridional gradient of θT is directed from the subtropical jet stream to the inner tropics, with θT declining by approximately 10 K from near 30°N to near 10°N in the vicinity of 95°W. The maintenance of these θT values is discussed. Total water (measured as the sum of vapor and vaporized ice) and ozone, major absorbers of solar radiation and emitters/absorbers of terrestrial infrared radiation, show scale invariance in the upper tropical troposphere. The implications of this result for the notion of a conservative cascade of energy via fluid dynamics from the largest to the smallest scales are discussed. The scaling exponents Hz for total water and ozone in the upper tropical troposphere are not the value, 5/9, expected for a passive scalar, probably indicating the presence of sources and/or sinks operating faster than mixing.