The influence of the spatial distribution of leads and ice floes on the atmospheric boundary layer over fragmented sea ice

Abstract

The response of the atmospheric boundary layer (ABL) to subgrid-scale variations of sea ice properties and fracturing is poorly understood and not taken into account in mesoscale Numerical Weather Prediction (NWP) model parametrizations. In this paper we analyze three-dimensional air circulation within the ABL over fragmented sea ice. A series of idealized high-resolution simulations with the Weather Research and Forecasting (WRF) model is performed for several spatial distributions of ice floes and leads for two values of sea ice concentration (0.5 and 0.9) and several ambient wind speed profiles. The results show that the convective circulation within the ABL is sensitive to the subgrid-scale spatial distribution of sea ice. Considerable variability of several domain-averaged quantities – cloud liquid water content, surface turbulent heat flux (THF) – is found for different arrangements of floes. Moreover, the organized structure of air circulation leads to spatial covariance of variables characterizing the ABL. Based on the example of THF, it is demonstrated that this covariance may lead to substantial errors when THF values are estimated from area-averaged quantities, as it is done in mesoscale NWP models. This suggests the need for developing suitable parametrizations of ABL effects related to subgrid-scale sea ice features for these models.

Area-Averaged Surface Moisture Flux over Fragmented Sea Ice: Floe Size Distribution Effects and the Associated Convection Structure within the Atmospheric Boundary Layer

“…Other studies point out that not only the average lead width is important, but also their spatial arrangement and the shape of their size distribution [16]. According to modeling results [17,18], even very small open water areas might result in the development of small convective plumes which penetrate the ABL and modify its circulation and properties. The areas of updraft air motion can be recognized in the winter Arctic from the satellites due to the presence of steam fog.…”

“…It is a key parameter in describing the state of the marginal ice zone (MIZ) and the evolution of sea ice cover. The information about floe size is essential to determine the rate of lateral sea ice melt [34], to assess the sea ice dynamics and internal stresses [35], and to resolve sea ice interactions with oceanic [36] and atmospheric boundary layers [17]. The importance of FSD drives the effort to include its representation into sea ice and climate models.…”

“…Several aspects of the ABL response to sea ice heterogeneity have been studied thoroughly in the recent numerical study of Wenta and Herman [17]. They examined the influence of small-scale sea ice surface variability (different floe size distributions or different lead widths and orientation) on domain-averaged STHF and properties of the ABL (e.g., the total moisture content).…”

“…In this study, the influence of FSD on the ABL is studied further. The modeling results obtained by Wenta and Herman [17] are examined, with the focus on the convective structures within the ABL and the surface turbulent moisture heat flux (the second component of STHF, the sensible heat flux, will be analyzed in a subsequent study). The goal of the first part of the analysis (Section 3.1) is to explain how the spatial arrangement and size of convective structures depends on the floe size distribution, and to describe the properties of those structures found in the modeling results.…”

“…(Anticipating Section 3.1, it is important to stress here that convection cells in this case do not have characteristic, regular shapes typical for Rayleigh-Benard convection, and therefore they are referred to as convective structures rather than convective cells in the rest of this paper.) The convection analysis is also used to explain the variability of domain-averaged STHF and total cloud liquid water content found in Wenta and Herman [17].…”