A Sensitivity Study of Arctic Air‐Mass Transformation Using Large Eddy Simulation. Issue 6 (18th March 2020)

• Record Type: Journal Article
• Authors: Dimitrelos, Antonios; Ekman, Annica M.L.; Caballero, Rodrigo; Savre, Julien
• Is Part Of: Journal of geophysical research. Volume 125:Issue 6(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-03-18
• Subjects: Mixed‐phase cloud formation -- arctic air mass transformation -- supercooled liquid layer at cloud top -- entrainment at cloud top -- cloud lifetime -- cloud radiative properties; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2019JD031738
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Arctic air mass transformation is linked to the evolution of low‐level mixed‐phase clouds. These clouds can alter the structure of the boundary layer and modify the surface energy budget. In this study, we use three‐dimensional large eddy simulation and a bulk sea ice model to examine the lifecycle of clouds formed during wintertime advection of moist and warm air over sea ice, following a Lagrangian perspective. We investigate the stages of cloud formation, evolution, and decay. The results show that radiative cooling at the surface gives rise to fog formation which subsequently rises and transforms into a mixed‐phase cloud. In our baseline simulation, the cloud persists for about 5 days and increases the surface temperature by on average 17 °C . Sensitivity tests show that the lifetime of the cloud is sensitive to changes in the vapor supply at cloud top. This flux is mainly impacted by changes in the divergence rate; an imposed convergence decreases the lifetime to 2 days while an imposed large‐scale divergence increases the lifetime to more than 6 days. The largest difference in cloud radiative properties is found in the experiment with increased ice crystal number concentrations. In this case, the lifetime of the cloud is similar compared to baseline but the amount of liquid water is clearly depleted throughout the whole cloud sequence and the surface temperature is on average 6 °C cooler. The cloud condensation nuclei concentration has a weaker effect on the radiative properties and lifetime of the cloud. Plain Language Summary: Arctic air mass transformation is a process in which an air mass originating over the open ocean enters the high Arctic and cools. Low‐altitude clouds form and are often very persistent. They can exist for several days and warm the surface by emitting infrared radiation towards the surface. In this study, we have investigated the effect of the cloud on the surface energy budget by conducting large eddy simulations. In the model code we have incorporated a module that considers the thermodynamics of the sea ice surface. Knowing the sensitivity of these clouds to different parameters and physical processes will make us capable of predicting the cloud lifetime and radiative properties, and thus the induced warming effect on the sea ice surface. We have found that an increased ice crystal number concentration leads to a tenuous form of the cloud that only weakly warms the surface. An imposed large‐scale ascent or descent affects the cloud lifetime by more than a day. Increasing the number of cloud condensation nuclei enhances the warming effect of the cloud. Key Points: Mixed‐phase cloud formation during Arctic air mass transformation exhibits four different stages: fog, drizzle, transition and decay The existence of a supercooled liquid layer at cloud top is the main driver of cloud sustenance. Entrainment at cloud top is a source of moisture Subsidence and ascent have the largest impact on cloud lifetime while changes in ice crystal concentration have the largest impact on cloud radiative properties