Shear-induced electrical changes in the base of thin layer-cloud

TL;DR

This study observes how shear-induced electrical changes at the cloud top dynamically influence the charge distribution and physical properties of thin layer clouds, revealing a new mechanism of charge modification within clouds.

Contribution

It demonstrates, for the first time, that shear-induced electrical changes at cloud tops can dynamically modify charge distribution within layer clouds.

Findings

Surface electric potential gradient changes precede cloud base fluctuations by 2 minutes.

Shear at cloud top correlates with fluctuations in cloud base height and optical thickness.

Charge redistribution within clouds can influence local electric fields and discharge phenomena.

Abstract

Charging of upper and lower horizontal boundaries of extensive layer clouds results from current flow in the global electric circuit. Layer-cloud charge accumulation has previously been considered a solely electrostatic phenomenon, but it does not occur in isolation from meteorological processes, which can transport charge. Thin layer clouds provide special circumstances for investigating this dynamical charge transport, as disruption at the cloud-top may reach the cloud base, observable from the surface. Here, a thin (~300 m) persistent layer-cloud with base at 300 m and strong wind shear at cloud-top was observed to generate strongly correlated fluctuations in cloud base height, optical thickness and surface electric Potential Gradient (PG) beneath. PG changes are identified to precede the cloud base fluctuations by 2 minutes, consistent with shear-induced cloud-top electrical changes followed by cloud base changes. These observations demonstrate, for the first time, dynamically driven modification of charge within a layer-cloud. Even in weakly charged layer-clouds, redistribution of charge will modify local electric fields within the cloud and the collisional behaviour of interacting charged cloud droplets. Local field intensification may also explain previously observed electrostatic discharges in warm clouds.