Ice and Supercooled Liquid Water Distributions over the Southern Ocean based on In Situ Observations and Climate Model Simulations

Abstract The cloud structure associated with two frontal passages over the Southern Ocean and Tasmania is investigated. The first event, during August 2006, is characterized by large quantities of supercooled liquid water and little ice. The second case, during October 2007, is more mixed phase. The Weather Research and Forecasting model (WRFV2.2.1) is evaluated using remote sensed and in situ observations within the post frontal air mass. The Thompson microphysics module is used to describe in-cloud processes, where ice is initiated using the Cooper parameterization at temperatures lower than −8°C or at ice supersaturations greater than 8%. The evaluated cases are then used to numerically investigate the prevalence of supercooled and mixed-phase clouds over Tasmania and the ocean to the west. The simulations produce marine stratocumulus-like clouds with maximum heights of between 3 and 5 km. These are capped by weak temperature and strong moisture inversions. When the inversion is at temperatures warmer than −10°C, WRF produces widespread supercooled cloud fields with little glaciation. This is consistent with the limited in situ observations. When the inversion is at higher altitudes, allowing cooler cloud tops, glaciated (and to a lesser extent mixed phase) clouds are more common. The simulations are further explored to evaluate any orographic signature within the cloud structure over Tasmania. No consistent signature is found between the two cases.

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The importance of mixed-phase clouds for climate sensitivity in the global aerosol-climate model ECHAM6-HAM2

10.5194/acp-2018-97 ◽

2018 ◽

Cited By ~ 1

Author(s):

Ulrike Lohmann ◽

David Neubauer

Keyword(s):

Southern Ocean ◽

Climate Sensitivity ◽

Liquid Water ◽

Climate Model ◽

Global Climate Model ◽

Supercooled Liquid ◽

Mixed Phase ◽

Radiation Budget ◽

Shortwave Radiation ◽

Mixed Phase Clouds

Abstract. Clouds are important in the climate system because of their large influence on the radiation budget. On the one hand, they scatter solar radiation and with that cool the climate. On the other hand, they absorb and re-emit terrestrial radiation, which causes a warming. How clouds change in a warmer climate is one of the largest uncertainties for the equilibrium climate sensitivity (ECS). While a large spread in the cloud feedback arises from low-level clouds, it was recently shown that also mixed-phase clouds are important for ECS. If mixed-phase clouds in the current climate contain too few supercooled cloud droplets, too much ice will change to liquid water in a warmer climate. As shown by Tan et al. (2016), this overestimates the negative cloud phase feedback and underestimates ECS in the CAM global climate model (GCM). Here we are using the newest version of the ECHAM6-HAM2 GCM to investigate the importance of mixed-phase clouds for ECS. Although we also considerably underestimate the fraction of supercooled liquid water globally in the reference version of ECHAM6-HAM2 GCM, we do not obtain increases in ECS in simulations with more supercooled liquid water in the present-day climate, contrary to the findings by Tan et al. (2016). We hypothesize that it is not the global supercooled liquid water fraction that matters, but only how well low- and mid-level mixed-phase clouds with cloud top temperatures in the mixed-phase temperature range between 0 and −35 ºC are simulated. These occur most frequent in mid-latitudes, in particular over the Southern Ocean where they determine the amount of absorbed shortwave radiation. In ECHAM6-HAM2 the amount of absorbed shortwave radiation over the Southern Ocean is only overestimated if all clouds below 0 ºC consist exclusively of ice and only in this simulation is ECS is significantly smaller than in all other simulations. Hence, the negative cloud phase feedback seems to be important only if the optically thin low- and mid-level mid-latitude clouds have the wrong phase (ice instead of liquid water) in the absence of overlying clouds. In all other simulations, changes in cloud feedbacks associated with cloud amount and cloud top pressure, dominate.

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Impact of meltwater on high-latitude early Last Interglacial climate

Climate of the Past ◽

10.5194/cp-12-1919-2016 ◽

2016 ◽

Vol 12 (9) ◽

pp. 1919-1932 ◽

Cited By ~ 14

Author(s):

Emma J. Stone ◽

Emilie Capron ◽

Daniel J. Lunt ◽

Antony J. Payne ◽

Joy S. Singarayer ◽

...

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Climate Model ◽

Model Data ◽

Last Interglacial ◽

Model Simulations ◽

The North ◽

Freshwater Forcing ◽

The North Atlantic ◽

Climate Model Simulations

Abstract. Recent data compilations of the early Last Interglacial period have indicated a bipolar temperature response at 130 ka, with colder-than-present temperatures in the North Atlantic and warmer-than-present temperatures in the Southern Ocean and over Antarctica. However, climate model simulations of this period have been unable to reproduce this response, when only orbital and greenhouse gas forcings are considered in a climate model framework. Using a full-complexity general circulation model we perform climate model simulations representative of 130 ka conditions which include a magnitude of freshwater forcing derived from data at this time. We show that this meltwater from the remnant Northern Hemisphere ice sheets during the glacial–interglacial transition produces a modelled climate response similar to the observed colder-than-present temperatures in the North Atlantic at 130 ka and also results in warmer-than-present temperatures in the Southern Ocean via the bipolar seesaw mechanism. Further simulations in which the West Antarctic Ice Sheet is also removed lead to warming in East Antarctica and the Southern Ocean but do not appreciably improve the model–data comparison. This integrated model–data approach provides evidence that Northern Hemisphere freshwater forcing is an important player in the evolution of early Last Interglacial climate.

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Strong control of Southern Ocean cloud reflectivity by ice-nucleating particles

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1721627115 ◽

2018 ◽

Vol 115 (11) ◽

pp. 2687-2692 ◽

Cited By ~ 49

Author(s):

Jesús Vergara-Temprado ◽

Annette K. Miltenberger ◽

Kalli Furtado ◽

Daniel P. Grosvenor ◽

Ben J. Shipway ◽

...

Keyword(s):

Southern Ocean ◽

Climate Models ◽

Climate Model ◽

Cloud Droplet ◽

Radiative Properties ◽

Model Simulations ◽

Cloud Resolving Model ◽

Climate Model Simulations ◽

Mixed Phase Clouds ◽

Strong Control

Large biases in climate model simulations of cloud radiative properties over the Southern Ocean cause large errors in modeled sea surface temperatures, atmospheric circulation, and climate sensitivity. Here, we combine cloud-resolving model simulations with estimates of the concentration of ice-nucleating particles in this region to show that our simulated Southern Ocean clouds reflect far more radiation than predicted by global models, in agreement with satellite observations. Specifically, we show that the clouds that are most sensitive to the concentration of ice-nucleating particles are low-level mixed-phase clouds in the cold sectors of extratropical cyclones, which have previously been identified as a main contributor to the Southern Ocean radiation bias. The very low ice-nucleating particle concentrations that prevail over the Southern Ocean strongly suppress cloud droplet freezing, reduce precipitation, and enhance cloud reflectivity. The results help explain why a strong radiation bias occurs mainly in this remote region away from major sources of ice-nucleating particles. The results present a substantial challenge to climate models to be able to simulate realistic ice-nucleating particle concentrations and their effects under specific meteorological conditions.

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The Role of Poleward-Intensifying Winds on Southern Ocean Warming

Journal of Climate ◽

10.1175/2007jcli1764.1 ◽

2007 ◽

Vol 20 (21) ◽

pp. 5391-5400 ◽

Cited By ~ 92

Author(s):

John C. Fyfe ◽

Oleg A. Saenko ◽

Kirsten Zickfeld ◽

Michael Eby ◽

Andrew J. Weaver

Keyword(s):

Surface Water ◽

Southern Ocean ◽

Co2 Emissions ◽

Climate Model ◽

Mesoscale Eddy ◽

Ocean Warming ◽

Time Varying ◽

Model Simulations ◽

Warm Surface Water ◽

Climate Model Simulations

Abstract Recent analyses of the latest series of climate model simulations suggest that increasing CO2 emissions in the atmosphere are partly responsible for (i) the observed poleward shifting and strengthening of the Southern Hemisphere subpolar westerlies (in association with shifting of the southern annular mode toward a higher index state), and (ii) the observed warming of the subsurface Southern Ocean. Here the role that poleward-intensifying westerlies play in subsurface Southern Ocean warming is explored. To this end a climate model of intermediate complexity was driven separately, and in combination with, time-varying CO2 emissions and time-varying surface winds (derived from the fully coupled climate model simulations mentioned above). Experiments suggest that the combination of the direct radiative effect of CO2 emissions and poleward-intensified winds sets the overall magnitude of Southern Ocean warming, and that the poleward-intensified winds are key in terms of determining its latitudinal structure. In particular, changes in wind stress curl associated with poleward-intensified winds significantly enhance pure CO2-induced subsurface warming around 45°S (through increased downwelling of warm surface water), reduces it at higher latitudes (through increased upwelling of cold deep water), and reduces it at lower latitudes (through decreased downwelling of warm surface water). Experiments also support recent high-resolution ocean model experiments suggesting that enhanced mesoscale eddy activity associated with poleward-intensified winds influences subsurface (and surface) warming. In particular, it is found that increased poleward heat transport associated with increased mesoscale eddy activity enhances the warming south of the Antarctic Circumpolar Current. Finally, a mechanism involving offshore Ekman sea ice transport (modulated by enhanced mesoscale activity) that acts to significantly limit the human-induced high-latitude Southern Hemisphere surface temperature response is reported on.

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