Adiabatic Evolution of Cloud Droplet Spectral Width: A New Look at an Old Problem

Abstract

Spectral width of the cloud droplet spectrum is important for radiative properties and drizzle/rain development in warm ice‐free clouds. We use an adiabatic rising parcel model to study activation and diffusional growth of cloud droplets, focusing on the spectral width evolution, and contrasting clean and polluted environments. A comprehensive droplet growth equation is used that includes kinetic, solute, and surface tension effects. We show that those effects have an appreciable impact on the spectral width evolution above the cloud base. Without those effects, the droplet area standard deviation should not change once activation is completed. In contrast, simulation results show that the area standard deviation does increase with height, especially for weak updrafts and polluted environments. Implications of those results for cloud modeling, especially applying conventional bin microphysics, are discussed.

Is a more physical representation of aerosol chemistry needed for fog forecasting?

“…With constant droplet number concentration and a linear increase in LWC, the size of droplets should increase following the power law with an exponent of one‐third. A similar fact is revealed in the recent parcel model experiments with two aerosol concentrations reported by Grabowski and Pawlowska (2023).…”

“…(2019) have used low vertical velocity (0.1 m$$$ \cdotp $$$s$$$ {}^{-1} $$$) for fog simulation in the large‐eddy simulation model. In the parcel model, researchers typically use velocities ranging from 0.25 to 4 m$$$ \cdotp $$$s$$$ {}^{-1} $$$, as seen in the recent study by Grabowski and Pawlowska (2023).…”

“…It is worth noting that important studies by Boutle et al (2018) and Poku et al (2019) have used low vertical velocity (0.1 m⋅s −1 ) for fog simulation in the large-eddy simulation model. In the parcel model, researchers typically use velocities ranging from 0.25 to 4 m⋅s −1 , as seen in the recent study by Grabowski and Pawlowska (2023).…”

“…Boutle et al (2018) have shown that visibility is significantly affected by weak turbulence and the growth of aerosol particles at low supersaturation conditions. Aerosol number concentration and its hygroscopicity are important as they activate into droplet number concentration (Bhowmik et al, 2023;Chen et al, 2020;Grabowski & Pawlowska, 2023) and therefore affect fog (Boutle et al, 2018). It is also important to note that fog life cycles are influenced by aerosol-cloud interaction in the fog environment and planetary boundary-layer conditions (Shao et al, 2023), which is critical for fog formation.…”