The lapse rate and the cold point tropopause in the Asian Summer Monsoon anticyclone

Tropospheric and stratospheric airmasses are separated by the tropopause. Here we investigate the lapse rate tropopause and the cold point tropopause in the Asian summer monsoon anticyclone (ASMA) based on high-altitude airborne measurements in summer 2017. We find that the lapse rate tropopause, and not the cold point, constitutes a good estimate of the upper boundary of the well mixed tropospheric air for many species. There is slow, diabatic, upward transport in the vicinity of the lapse rate tropopause and above. The cold point is located on average about 1 km above the lapse rate tropopause and is about 3 K colder (pressure lower by about 12 hPa). The cold point is in particular important for water vapour. Above the cold point in the ASMA molar water vapour mixing ratios (including hydration patches) range between ∼3 and 10 ppm. In the observations, no indication of substantial dehydration above the cold point was found. Ozone mixing ratios increase substantially with altitude; between the lapse rate and the cold point tropopause molar ozone mixing ratios are in the range of 50–200 ppb. For strong convection (flight on 10 August 2017) there is substantial dehydration at the cold point tropopause (indicated by high values of total water, ice particle occurrence, and strong supersaturation). Above the cold point, under such conditions, neither ice particle occurrence, nor enhanced molar mixing ratios of water vapour (above about 6 ppm) are observed.