The polar region cloud system, one of the key components governing climate change, is extremely sensitive to sea ice distribution, lower-tropospheric stability, and atmospheric circulation. There have been numerous studies on clouds over Antarctica, the Southern Ocean (SO), and the complete high-latitude region of the Southern Hemisphere. This article looks at Sato, K. and Inoue, J.'s research paper published in Geophysical Research Letters.
Image Credit: papa studio/Shutterstock.com
The total cloud cover over the SO tends to be greater than that over Antarctica, even poleward of 60 °S, and the most frequent type is low-level cloud (base height of <2.0 km). In autumn-winter, the formation of low-level supercooled liquid water clouds declines with an increase in sea ice concentration (SIC). By contrast, in summer, there is no difference in low-level cloud extent between open water and sea ice areas.
This phenomenon indicates that sea ice variability does not affect cloud properties. Moreover, the frequency of occurrence of low-level clouds below 2 km over the SO is underestimated by general circulation modeling and reanalysis data.
Hence, this study investigated the fraction of ice cloud and temperature in the troposphere over the SO and Antarctica using the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, or CALIPSO, data. The study also offers insights into the link between lower-tropospheric ice-cloud fraction over the SO and phytoplankton numbers.
The CALIPSO satellite equipped with a Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument measured backscatter profiles with high vertical resolution. The measurement details can be used to predict cloud properties. The data from this satellite was used by the JAXA EarthCARE program to develop a cloud data set with a horizontal resolution of 240 m.
There are seven types of cloud particles—supercooled water, warm water, randomly oriented ice crystals (3D ice), horizontally oriented plates (2D plate), the blend of 3D ice and 2D plate, unknown1 (possibly ice crystals with horizontally oriented plates that have weak specular reflection), and unknown2 (randomly oriented ice crystals or liquid droplets).
The Clouds and the Earth’s Radiant Energy System-Energy Balanced and Filled data was used to evaluate downward shortwave and longwave radiation. The presence of phytoplanktons over the SO has been analyzed using MODIS-Aqua satellite monthly mean chlorophyll-α concentration data.
When the chlorophyll-α concentration is relatively low (high) over sea ice areas, errors in the water-leaving radiance can lead to overestimation (underestimation) of daily chlorophyll-α concentration for some pixels. This problem was solved by averaging monthly data.
Data obtained from the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) datasets were used to distinguish open water and sea ice areas. SST over sea ice areas was calculated using SST–SIC statistical relationships.
Monthly mean ERA5 data from the European Centre for Medium-Range Weather Forecasts were used for atmospheric parameters (10 m wind speed, 2 m air temperature, and sensible heat flux). The 10-ensemble mean data was used to reduce parameter uncertainty in the ERA5 data.
Figures 1a–1d illustrate the seasonal distribution of the fraction of ice cloud (Fice) below 8 km over Antarctica and the SO. From Figure 1d, it is clear that Fice is highest in winter over Antarctica and the SO, while the magnitude and distribution of Fice in spring and autumn are similar (see Figures 1a and 1c). As shown in Figure 1b, in summer, Fice is smaller than in winter due to warmer temperatures.
Figure 1. Fraction of ice cloud relative to total cloud fraction (Fice: %) in 2006–2015: (a) Spring (SON), (b) Summer (DJF), (c) Autumn (MAM), and (d) Winter (JJA). Seasonal ranges in (e–h) are the same as in (a–d) but depict chlorophyll data (mg/m2). Image Credit: Sato & Inoue, 2021
Figures 2a and 2d show that high-level Fice does not change seasonally over the SO and Antarctica as all liquid cloud droplets tend to freeze under extremely cold conditions in the upper troposphere. By contrast, clear seasonal change in Fice can be seen in low- and mid-level layers (see Figures 2b and 2c). Figures 2e and 2f illustrate that in winter, cold conditions cause the highest Fice over the SO and Antarctica at these levels.
Figure 2. Monthly average fraction of ice cloud relative to total cloud (Fice: %) over Antarctica (red: poleward of 60°S) and the Southern Ocean (blue: poleward of 50°S) averaged over 0°–360°E: (a) High-level (>6 km), (b) Mid-level (2–6 km), (c) Low-level (<2 km) in 2006–2015. Shading indicates amplitude between monthly minimum and maximum encountered during 2006–2015. (d–f) same as (a–c) but for temperature (°C) when ice cloud was present. ). Image Credit: Sato & Inoue, 2021
As shown in Figure 3, the SO was divided into three regions (Atlantic, Indian, and Pacific sectors; see also Figure 1a) to investigate the zonally averaged, low-level-cloud Fice over Antarctica and the SO (poleward of 50°S) at various temperatures and intervals of 2.5 °C as a function of latitude during summer and winter.
In winter, the highest Fice is exhibited by the low-level cloud over all regions at extremely cold temperatures (see Figures 3a–3c). On the contrary, in summer, ice cloud over all regions forms at temperatures greater than −35 °C (see Figures 3d–3f).
Figure 3. Dependence on latitude and cloud temperature of averaged low-level ice-cloud fraction relative to total cloud (Fice: %): (a) Indian, (b) Pacific, and (c) Atlantic sectors of the Southern Ocean in austral winter 2006–2015. Contours (1,000, 5,000, 10,000, 50,000, 100,000, and 200,000) show numbers of Nice and Nwater. (d–f) Same as (a–c) but during austral summer. ). Image Credit: Sato & Inoue, 2021
As shown in Figures 4f–4h, in summer at higher temperatures (−7.5 °C to 0 °C), low-level cloud over near-coastal Antarctic regions of all SO sectors exhibits a higher Fice than that over the ocean. Furthermore, the concentration of chlorophyll-α in high-latitude areas with sea ice is higher than in open water (see Figure 4i).
The high AOD close to coastal Antarctic regions under low ASE index conditions over the Atlantic and Pacific sectors indicates the transportation of aerosols from lower latitudes. Hence, marine bioaerosols released from high-latitude oceans likely add to the higher low-level cloud Fice at higher temperatures over coastal Antarctic regions (see Figures 4g and 4h).
Figure 4. Dependence of averaged low-level ice-cloud fraction relative to total cloud (Fice) at higher temperatures (−17.5 °C to −10 °C) as a function of latitude and ASE index (shading: %) with aerosol optical depth for all aerosols (contours) averaged over the (a) Indian, (b) Pacific, and (c) Atlantic sectors of the Southern Ocean in austral winter 2006–2015. Values are correlation coefficients between monthly mean Fice at warm temperatures and ASE index for each region. (d) Chlorophyll concentration (mg/m2) and (e) Sea ice concentration (SIC: %) as function of latitude averaged over the Indian (blue), Pacific (red), and Atlantic sectors (green) of the Southern Ocean. Values are correlation coefficients between monthly mean Fice at higher temperatures and each parameter for each region. (f–j) Same as (a–e) but for Fice at higher temperatures (−7.5 °C to 0 °C) in austral summer. Image Credit: Sato & Inoue, 2021
Seasonal changes of Fice in low- and mid-level layers were observed from wintertime maxima and summertime minima. The Fice of low-level clouds over the three SO sectors and Antarctica is highest in winter. By contrast, Fice is comparatively large at temperatures varying from −17.5 °C to −10 °C under high ASE index conditions, specifically over near-coastal Antarctic regions.
Changes to total tropospheric Fice impact surface longwave/shortwave radiation budgets as the optical ice cloud depth is thinner than that of water cloud. Distributions of ice crystal and water cloud droplet size have an effect on outgoing solar radiation, but it is regarded that a comparatively small total tropospheric water cloud content is the main cause of reduction in solar radiation reflection over these regions.
During summer, the number of bioaerosols causing increased ice-cloud formation is increased by a rise in the incoming shortwave radiation at the surface through positive ice cloud-bioaerosol feedback. An extended ice-free ocean that features higher wave conditions is also propitious for increased sea spray production, promoting marine aerosol emission into the atmosphere in all seasons.
Sato, K & Inoue, J (2021) Seasonal Change in Satellite-Retrieved Lower-Tropospheric Ice-Cloud Fraction Over the Southern Ocean. Geophysical Research Letters, 48(23), p. e2021GL095295. Available Online: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL095295.
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