Assessment of Atmospheric Reanalyses With Independent Observations in the Weddell Sea, the Antarctic

Abstract

Surface layer and upper‐air in situ observations from two research vessel cruises and an ice station in the Weddell Sea from 1992 and 1996 are used to validate four current atmospheric reanalysis products: ERA‐Interim, CFSR, JRA‐55, and MERRA‐2. Three of the observation data sets were not available for assimilation, providing a rare opportunity to validate the reanalyses in the otherwise datasparse region of the Antarctic against independent data. All four reanalyses produce 2 m temperatures warmer than the observations, and the biases vary from +2.0 K in CFSR to +2.8 K in MERRA‐2. All four reanalyses are generally too warm also higher up in the atmospheric boundary layer (ABL), with biases up to +1.4 K (ERA‐Interim). Cloud fractions are relatively poorly reproduced by the reanalyses, MERRA‐2 and JRA‐55 having the strongest positive and negative biases of about +30 % and −17 %, respectively. Skill scores of the error statistics reveal that ERA‐Interim compares generally the most favorably against both the surface layer and the upper‐air observations. CFSR compares the second best and JRA‐55 and MERRA‐2 have the least favorable scores. The ABL warm bias is consistent with previous evaluation studies in high latitudes, where more recent observations have been applied. As the amount of observations has varied depending on the decade, season, and region, the consistency of the warm bias suggests a need to improve the modeling systems, including data assimilation as well as ABL and surface parameterizations.

Influence of ocean tides and ice shelves on ocean–ice interactions and dense shelf water formation in the D’Urville Sea, Antarctica

“…One could expect to obtain different ocean-ice shelf interactions and impact on polynyas if the stratification is entirely eroded each winter. One potential cause resides in the atmospheric forcing resolution: despite the relatively high resolution of the ERA5 dataset (31 km), it is probably not enough to catch the detailed topography which leads to the exceptional intensity of katabatic winds in Adélie Land (as do most of the global atmospheric reanalysis (Jones et al, 2016;Jonassen et al, 2019). This can have the effect of underestimating the actual sea ice production in coastal polynyas.…”

The Performance of the ERA‐Interim and ERA5 Atmospheric Reanalyses Over Weddell Sea Pack Ice

“…Jonassen et al. (2019) used observations made at the drifting camp Ice Station Weddell during 1992 to validate the ECMWF ERA‐Interim reanalysis and found a mean warm bias in air temperature of around 2.5°C, with the largest biases again found at the lowest temperatures. Biases in downwelling shortwave and longwave radiation in the reanalysis were found to be insignificant.…”

Mechanisms driving the asymmetric seasonal cycle of Antarctic Sea Ice in the CESM Large Ensemble

“…2c), but with the lack of observations of sea level pressure in the Southern Ocean, it is difficult to assess whether this is due to an overestimation in the model or an underestimation in the reanalyses. Given the Antarctic sea-ice zone is almost entirely devoid of conventional meteorological observations (Jonassen and others, 2019), assessments of the performance of the reanalysis datasets in the Southern Ocean are primarily based on data collected at weather stations on the continent. The bias in reanalysis pressure fields from these studies is 2–3 Pa (Bracegirdle and Marshall, 2012; Bromwich and others, 2013), which is similar to the difference in the SAO index between CESM-LENS and ERA5.…”