Abstract
Abstract. A recent field campaign was conducted to measure the properties of thin, warm convective clouds forming under conditions of weak updrafts. During the campaign, short-lived clouds (on the order of minutes) with droplets' effective radius of 1–2 μm and low liquid water path (~ 500 mg m–2) were measured. These low values are puzzling, since in most studies an effective radius of 4 μm is reported to serve as the lower bound for clouds. A theoretical cloud model designed to resolve the droplet-activation process suggested conditions that favor the formation of such clouds. Here we show that these clouds, which mark the transition from haze to cloud, are highly sensitive to the magnitude of the initial perturbation that initiated them. We define these clouds as "transition-zone clouds". The existence of such clouds poses a key challenge for the analysis of atmospheric observations and models, since they "further smooth" the transition from dry aerosol through haze pockets to cumulus clouds.
Highlights
Extensively studied for decades, there is no clear definition of a cloud
Clouds are often defined by thresholds that depend on the measurement technique and application, such as the cloud optical depth (COD) threshold when using remote-sensing tools, or the liquid water content (LWC) threshold in cases of in situ measurements and cloud numerical models
We focused on a cloud regime that is usually overlooked
Summary
Clouds are often defined by thresholds that depend on the measurement technique and application, such as the cloud optical depth (COD) threshold when using remote-sensing tools, or the liquid water content (LWC) threshold in cases of in situ measurements and cloud numerical models. Such thresholds are not robust; they have been shown to misclassify the atmosphere and to introduce problems in analyses of both cloud and cloud-free regions (Koren et al, 2007; Charlson et al, 2007). Continental subarctic low-level clouds have been found to contain an average fraction of activation of 47 % (with respect to number size distribution), with variations between 9 and 86 % (Komppula et al, 2005)
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