Aircraft-Measured High Vapor Supersaturation in Deep Marine Tropical Convective Clouds Supports the Aerosol Convective Invigoration Hypothesis

A critical link in the aerosol convective invigoration hypothesis is the occurrence of high water vapor supersaturation (S) in clouds with moderate updrafts (W > 5 m s^-1) and small cloud drop concentrations (N_d) that allow fast drop coalescence that reduces the available water surface area for condensation. The in-cloud observations of high supersaturation exceeding 10% were documented for the first time in tropical deep convective clouds that were measured in the Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP²Ex) during August-October 2019. The supersaturation was obtained based on machine-learning based correction to the difference between the aircraft-measured temperature and dew point with root mean square error of 0.38 K, which translates to 2.7%. The calculation took into account the cloud and liquid water content during the last 4 seconds, airspeed and pressure. The calculation also rejected faster changes then could be captured by the response time of the temperature and dew point probes. The results lend observational support to the process of water phase aerosol convective invigoration hypothesis.

Figure captions:

A: The dependence of supersaturation on updraft and cloud drop concentrations. The calculated S is the inferred contribution of the combined effects of N_d and updraft (W). The legend shows the updraft intervals and the number of points in each interval. The numbers on the red line show the number of data points in each N_d bin at W>5 m s^-1. The data are for clouds warmer than -5 C. Note that large S occur only with W> 5 m/s and N_d < 50 cm^-3.

B: The calculated supersaturation (S) as a function of temperature, for points with updraft > 5 m s^-1. The colors of points indicate the corresponding cloud number concentration (N_d), and the size indicate cloud effect radius (r_e). The black line is the averaged N_d as a function of temperature, and the numbers on the black line show the number of data points. Note that S increases with height (lower temperature), and extends to the supercooled regions of the cloud. The increase in S with height is associated with the decrease of N_d with height due to the increased coalescence with height and the respective loss of surface area available for condensation.