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

Abstract. 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, in the ASMA, the lapse rate tropopause, and not the cold point, constitutes a good estimate of the upper boundary of the well mixed tropospheric air mass. On average the cold point is located about 1 km above the lapse rate tropopause and is about about 3 K colder with the pressure lower by about 12 hPa. Above the cold point tropopause molar water vapour mixing ratios range between ∼3 and 10 ppm. Molar ozone mixing ratios increase substantially with altitude; between the lapse rate tropopause and the cold point tropopause molar ozone mixing ratios are in the range of 50–200 ppb. There is slow, diabatic, upward transport in the vicinity of the lapse rate tropopause and above. Hydration patches above the cold point tropopause were observed with water vapour mixing ratios of ≲ 10 ppm. No indication of substantial dehydration above the cold point tropopause in the ASMA was found in the observations. 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 tropopause, under such conditions, neither ice particle occurrence, nor enhanced molar mixing ratios of water vapour (above ≲ 6 ppm) are observed.