Biases in the thermodynamic structure over the Southern Ocean in ERA5 and their radiative implications

Abstract

The thermodynamic structure of the lower troposphere in the 37 standard levels ERA5 reanalysis has been evaluated against 2,186 high‐resolution upper air soundings collected over the Southern Ocean (SO). The reanalysis, which incorporated these soundings, was found to be skilled in depicting the general synoptic meteorology and thermodynamic structure as defined by the cluster analysis of Truong et al. (2020) Journal of Geophysical Research: Atmospheres, 125, e2020JD033214. Using dew‐point depression as a proxy for cloud, however, we found a significant reduction in the number of inferred cloud layers, which is inherited from a bias in the specific humidity in the ERA5 reanalysis, most notably over the high latitudes of the SO, where a multilayer cloud structure is frequently observed. The reanalysis was also found to have thinner inferred cloud geometric layer and shallower cloud top heights. Further analysis showed that the reanalysis displays a greater percentage of soundings having no inversion with this bias being more pronounced at high latitudes that tends to be associated with the colder sea surface temperature. While the statistics of the main inversion height are largely consistent, the average inversion strength in the ERA5 reanalysis is found to be weaker than the observations. We anticipate the 137‐level ERA5 reanalysis simulation yields a smoothed vertical structure, from which the 37 standard levels ERA5 reanalysis is linearly interpolated. An examination of the sensitivity of the radiative transfer to cloud macrophysics suggests that the correct representation of thin multiple cloud layers can help reduce the amount of downward shortwave surface radiation over the SO.

Cloud Top Thermodynamic Phase from Synergistic Lidar-Radar Cloud Products from Polar Orbiting Satellites: Implications for Observations from Geostationary Satellites

“…The highest cloudiness values are found over the Southern Ocean and high northern latitudes over the ocean in both resolutions with cloudiness values up to 97%. In fact, the Southern Ocean and the Antarctic seas have the largest cloud fractions as already demonstrated in previous studies using other synergistic A-Train products [37,[65][66][67][68][69][70]. In the tropics near the equator, cloudiness values are also enhanced compared to the average value on the Meteosat disk because of frequent convection.…”

Southern Ocean precipitation: Toward a process‐level understanding

“…Our understanding of precipitation processes at the high latitudes of the SO is, arguably, the most challenged of any place on Earth. The region is masked by heavy cloud cover year-round, often in a multilayer structure unmatched over the North Pacific and North Atlantic (Mace et al, 2009;Truong et al, 2022), challenging many of the underlying assumptions employed in precipitation algorithms. We have virtually no ship-based observations across this region in the austral winter season (JJA), and those from the summer season are heavily biased as the ships commonly aim to F I G U R E 7 Same as Figure 6 for January 1, 2018 avoid encountering high-latitude mesocyclones.…”

Characteristics and Variability of Precipitation Across Different Sectors of an Extra‐Tropical Cyclone: A Case Study Over the High‐Latitudes of the Southern Ocean

“…While numerous precipitation products have been evaluated against surface observations from Macquarie Island (Lang et al., 2020; Tansey et al., 2022; Wang et al., 2015), this site is located at 54.5°S, amid the SO storm track rather than at high latitudes. Both satellite observations (Mace et al., 2009) and recent field observations (Montoya Duque et al., 2022; Truong et al., 2020, 2022) have identified the unique nature of the thermodynamics and cloud structure across the lower free troposphere and boundary layer at these high latitudes. Mace et al.…”

“…While numerous precipitation products have been evaluated against surface observations from Macquarie Island (Lang et al, 2020;Tansey et al, 2022;Wang et al, 2015), this site is located at 54.5°S, amid the SO storm track rather than at high latitudes. Both satellite observations (Mace et al, 2009) and recent field observations (Montoya Duque et al, 2022;Truong et al, 2020Truong et al, , 2022 have identified the unique nature of the thermodynamics and cloud structure across the lower free troposphere and boundary layer at these high latitudes. Mace et al (2009) used active remote sensing from the A-train satellites to note that this region has the highest frequency of low and mid-level clouds across the globe, even in comparison with comparable latitudes of the North Pacific and North Atlantic.…”