Testing the influence of small crystals on ice size spectra using Doppler lidar observations

TL;DR

This study uses Doppler lidar to analyze ice crystal velocities in stratiform clouds over 17 months, revealing temperature-dependent velocity distributions and suggesting small crystals are less prevalent than in-situ measurements indicate, impacting climate models.

Contribution

It provides the first detailed analysis of ice crystal velocities using Doppler lidar and challenges the inclusion of small crystals in climate model parametrizations based on in-situ data.

Findings

Velocity increases with temperature from 0.2m/s at -40C to 0.6m/s at -10C

Small crystals are less abundant than in-situ measurements suggest

Shattering on probes likely causes overestimation of small crystals in in-situ data

Abstract

Measurements of the vertical velocity of ice crystals using a 1.5micron Doppler lidar are described. The statistics from a continuous sample of stratiform ice clouds over 17 months are analysed: the distribution of velocity varies strongly with temperature, with average Doppler velocities of 0.2m/s at -40C increasing to 0.6m/s at -10C presumably due to particle growth and broadening of the size spectrum. We examine the likely influence of small crystals less than 60 microns by forward modelling their effect on the area-weighted fall speed, and comparing the results to the lidar observations. The comparison strongly suggests that the concentration of these small crystals in most ice clouds is much lower than measured in-situ by cloud droplet probes. The discrepancy is attributed to shattering of large crystals on the probe inlet, and we argue that these numerous small particles should not be included in numerical weather and climate model parametrizations.