Shallow Convection and Precipitation Over the Southern Ocean: A Case Study During the CAPRICORN 2016 Field Campaign

Abstract

Marine boundary layer clouds and precipitation observed in a sustained period of open mesoscale cellular convection (MCC) over the Southern Ocean (SO) are investigated using Clouds, Aerosols, Precipitation, Radiation, and atmospherIc Composition Over the southeRn oceaN 2016 observations, Himawari‐8 products, and numerical simulations. The shallow convection was characterized by the presence of supercooled liquid water and mixed‐phase clouds in the sub‐freezing temperature range, consistent with earlier in‐situ observations where ice multiplication is found to be active in producing large quantities of ice in open MCC clouds. Ice‐phase precipitation was observed to melt below cloud base with evidence of cold pools produced in a decoupled boundary layer. Convection‐permitting simulations using the weather research and forecasting model were able to reproduce many of the surface meteorological features and their evolution. However, the evolution of the boundary layer height and the degree of decoupling were poorly simulated, along with the absence of cold pools. The observed cloud morphology and microphysical characteristics were also not well reproduced in the control simulation with the Thompson microphysics scheme, where too much supercooled water was simulated in a too homogenous cloud field. Sensitivity experiments with modified microphysical parameters led to a higher production of glaciated clouds and precipitation. Sensitivity experiments with different boundary layer schemes and vertical resolution, however, showed a smaller impact. A bias of ∼4°C in the initial boundary conditions of the sea surface temperature is discussed. This study highlights the challenge of representing the complex physical processes that underpin the cloud, precipitation, and boundary layer characteristics of the open MCC over the SO.

Southern Ocean Precipitation Characteristics Observed From CloudSat and Ground Instrumentation During the Macquarie Island Cloud & Radiation Experiment (MICRE): April 2016 to March 2017

“…Sounding data from Macquarie Island frequently show a weakly decoupled boundary layer with a moist subcloud “buffer layer” (Hande et al., 2012), whose existence we speculate has much to do with stabilization by evaporation of precipitation. Melting of frozen/mixed precipitation may also impact boundary layer dynamics by providing instantaneous cooling in below‐cloud downdrafts and forming cold pools at the decoupled surface (Lang et al., 2021).…”

A Characterization of Clouds and Precipitation Over the Southern Ocean From Synoptic to Micro Scales During the CAPRICORN Field Campaigns

“…Using remote‐sensing and in‐situ observations, recent studies have refined our understanding of the bulk statistics of cloud occurrence and phase partitioning (Mace & Protat, 2018a), properties of non‐precipitating liquid‐phase low‐level clouds (Mace & Protat, 2018b; Mace et al., 2021; Y. Wang et al., 2021), as well as cloud properties in the cold sector of extratropical cyclones (D’Alessandro et al., 2021; Y. Wang et al., 2020; Zaremba et al., 2020). Case studies have also been carried out to examine special phenomena such as an atmospheric river (Finlon et al., 2020), mesoscale cellular convection (Lang et al., 2021), and convective generating cells near the cloud tops (Alexander et al., 2021; Y. Wang et al., 2020). These new observations have also enabled the evaluation of model simulations across a range of spatial and temporal scales (Atlas et al., 2020; Gettelman et al., 2020; Protat et al., 2017; Zhou et al., 2020).…”

“…Wang et al, 2020;Zaremba et al, 2020). Case studies have also been carried out to examine special phenomena such as an atmospheric river (Finlon et al, 2020), mesoscale cellular convection (Lang et al, 2021), and convective generating cells near the cloud tops (Alexander et al, 2021;Y. Wang et al, 2020).…”

Southern Ocean precipitation: Toward a process‐level understanding

“…Adams (2009) was the first to note this strong positive trend in this record, finding a 35% increase in precipitation from 1970 to 2008, an increase of 0.25 mm day −1 decade −1 . More recent records show the continuation of this trend (Jovanovic et al, 2012; Lang et al, 2021) over the next decade. We note here that the observed trend at Macquarie Island (1979–2021) is roughly four times greater than that produced by ERA5 (Figure 1) at the corresponding grid.…”

“…At higher latitudes, sub‐Antarctic mesoscale polar cyclones (Rasmussen & Turner, 2003) are widely present, but their ability to generate precipitation remains poorly understood. At lower latitudes, the balance between the shallow convection of open and closed mesoscale cellular convection (MCC) may be a defining feature for the precipitation rate (Ahn et al, 2017; Lang et al, 2021). And while these mechanisms are readily found across the North Pacific and North Atlantic, the pristine state of the SO, being relatively free of terrestrial and anthropogenic aerosols, provides a unique microphysical environment for the development of precipitation.…”