Vertical Velocity Deduced from Ice Change in Deep Convection

Vertical velocity inference is essential to advance our knowledge of deep convection. The Convective Core Observations through Microwave Derivatives in the Tropics (C2OMODO) mission aims to retrieve vertical velocity from ice-sensitive measurements in the microwaves taken at two successive short time intervals. The relation between vertical velocity and ice change is examined here for two large-eddy simulations run with the Meso-NH model, one over the sea and the other over land. A linear relationship is found between the vertical velocity and the ice change in convective cores. This relationship is valid in the upper part of the convective cores, above the altitude of vertical velocity maximum, and depends on the inverse of the vertical ice gradient. Knowing the altitude and value of the vertical velocity maximum, as well as the altitude of the top of the convective core, allows us to deduce the complete vertical velocity profile. Significant ice changes are also found in anvils. As they are mainly due to horizontal ice advection, they must be clearly differentiated from those due to vertical ice advection found in convective cores. The relationship between vertical velocity and ice change is highly sensitive to time delay so that the slope between w and the ice change increases by a factor of 3 between 1 and 5 min and the magnitude in ice change decreases by a factor of 5. A delay less than 2 min between successive measurements of ice is therefore recommended to deduce vertical velocity in deep convection. Significance Statement Vertical velocity is the key variable characterizing upward transport in convective storms. Rarely estimated from observations, it can be highly uncertain in numerical models. This study shows how vertical velocity is related with ice change in cloud tops and how this relationship varies with time delay. It suggests that ice change should be measured in less than 2 min from space, so that vertical velocity can be inferred by time-delayed tandem microwave observations. Future work will investigate the value of additional radar measurements for estimating the full vertical velocity profile.