scholarly journals Improved simulation of clouds over the Southern Ocean in a General Circulation Model

2019 ◽  
Author(s):  
Vidya Varma ◽  
Olaf Morgenstern ◽  
Paul Field ◽  
Kalli Furtado ◽  
Jonny Williams ◽  
...  
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Supercooled Liquid ◽  
Short Wave ◽  
Global Climate Models ◽  
Ice Crystals ◽  
Wave Radiation ◽  
Short Wave Radiation

Abstract. The present generation of global climate models is characterized by insufficient reflection of short-wave radiation over the Southern Ocean due to a misrepresentation of clouds. This is a significant concern as it leads to excessive heating of the ocean surface, sea surface temperature biases, and subsequent problems with atmospheric dynamics. In this study we modify cloud micro-physics in a recent version of the Met Office's Unified Model and show that choosing a more realistic value for the shape parameter of atmospheric ice-crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation. In the model, for calculating the growth rate of ice crystals through deposition, the default assumption is that all ice particles are spherical in shape. We modify this assumption to effectively allow for oblique shapes or aggregates of ice crystals. Along with modified ice nucleation temperatures, we achieve a reduction in the annual-mean short-wave cloud radiative effect over the Southern Ocean by up to 4 W/m2, and seasonally much larger reductions. By slowing the growth of the ice phase, the model simulates substantially more supercooled liquid cloud. We hypothesize that such abundant supercooled liquid cloud is the result of a paucity of ice nucleating particles in this part of the atmosphere.

scholarly journals Improving the Southern Ocean cloud albedo biases in a general circulation model

2020 ◽  
Vol 20 (13) ◽  
pp. 7741-7751
Author(s):  
Vidya Varma ◽  
Olaf Morgenstern ◽  
Paul Field ◽  
Kalli Furtado ◽  
Jonny Williams ◽  
...  
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Short Wave ◽  
Cloud Microphysics ◽  
Global Climate Models ◽  
Ice Crystals ◽  
Wave Radiation ◽  
Short Wave Radiation

Abstract. The present generation of global climate models is characterised by insufficient reflection of short-wave radiation over the Southern Ocean due to a misrepresentation of clouds. This is a significant concern as it leads to excessive heating of the ocean surface, sea surface temperature biases and subsequent problems with atmospheric dynamics. In this study, we modify cloud microphysics in a recent version of the Met Office's Unified Model and show that choosing a more realistic value for the shape parameter of atmospheric ice crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation. In the model, for calculating the growth rate of ice crystals through deposition, the default assumption is that all ice particles are spherical in shape. We modify this assumption to effectively allow for oblique shapes or aggregates of ice crystals. Along with modified ice nucleation temperatures, we achieve a reduction in the annual-mean short-wave cloud radiative effect over the Southern Ocean by up to ∼4 W m−2 and seasonally much larger reductions compared to the control model. By slowing the growth of the ice phase, the model simulates substantially more supercooled liquid cloud.

scholarly journals Introducing Ice Nucleating Particles functionality into the Unified Model and its impact on the Southern Ocean short-wave radiation biases

2021 ◽  
Author(s):  
Vidya Varma ◽  
Olaf Morgenstern ◽  
Kalli Furtado ◽  
Paul Field ◽  
Jonny Williams
Keyword(s):  
Southern Ocean ◽  
Mineral Dust ◽  
Short Wave ◽  
Unified Model ◽  
Global Climate Models ◽  
Nucleation Temperature ◽  
Wave Radiation ◽  
Short Wave Radiation ◽  
Ocean Region ◽  
The Impact

Abstract. Insufficient reflection of short-wave radiation especially over the Southern Ocean region is still a leading issue in many present-day global climate models. One of the potential reasons for this observed bias is an inadequate representation of clouds. In a previous study, we modified the cloud micro-physics scheme in the Unified Model and showed that choosing a more realistic value for the capacitance or shape parameter of atmospheric ice-crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation over the Southern Ocean by brightening the clouds. However, attempts to modify the cloud phase by directly adjusting the micro-physics process rates like capacitance tend to affect both the hemispheres symmetrically whereas we seek to brighten only the high-latitude Southern Hemisphere clouds. In this study we implement a simple prognostic parametrisation whereby the heterogeneous ice nucleation temperature is made to vary three-dimensionally as a function of the mineral dust distribution in the model. As a result, those regions with less dust number density would have lower nucleation temperature compared to the default global value of −10 °C. By using mineral dust as an indicator for ice nucleating particles in the model, this parametrisation thus captures the impact of ice nucleating particles on the cloud distribution due to its general paucity over the Southern Ocean region. This approach thus improves the physics of the model with minimal complexity.

scholarly journals Quantitative Sensitivity Analysis of Physical Parameterizations for Cases of Deep Convection in the NASA GEOS-5

2016 ◽  
Vol 29 (2) ◽  
pp. 455-479 ◽  
Author(s):  
Derek J. Posselt ◽  
Bruce Fryxell ◽  
Andrea Molod ◽  
Brian Williams
Keyword(s):  
General Circulation ◽  
Process Model ◽  
Large Scale ◽  
Climate Models ◽  
Deep Convection ◽  
Input Parameter ◽  
Circulation Model ◽  
Parameter Sensitivity ◽  
Global Climate Models ◽  
Simulation Code

Abstract Parameterization of processes that occur on length scales too small to resolve on a computational grid is a major source of uncertainty in global climate models. This study investigates the relative importance of a number of parameters used in the Goddard Earth Observing System Model, version 5 (GEOS-5), atmospheric general circulation model, focusing on cloud, convection, and boundary layer parameterizations. Latin hypercube sampling is used to generate a few hundred sets of 19 candidate physics parameters, which are subsequently used to generate ensembles of single-column model realizations of cloud content, precipitation, and radiative fluxes for four different field campaigns. A Gaussian process model is then used to create a computationally inexpensive emulator for the simulation code that can be used to determine a measure of relative parameter sensitivity by sampling the response surface for a very large number of input parameter sets. Parameter sensitivities are computed for different geographic locations and seasons to determine whether the intrinsic sensitivity of the model parameterizations changes with season and location. The results indicate the same subset of parameters collectively control the model output across all experiments, independent of changes in the environment. These are the threshold relative humidity for cloud formation, the ice fall speeds, convective and large-scale autoconversion, deep convection relaxation time scale, maximum convective updraft diameter, and minimum ice effective radius. However, there are differences in the degree of parameter sensitivity between continental and tropical convective cases, as well as systematic changes in the degree of parameter influence and parameter–parameter interaction.

scholarly journals Cloud Vertical Distribution across Warm and Cold Fronts in CloudSat–CALIPSO Data and a General Circulation Model

2010 ◽  
Vol 23 (12) ◽  
pp. 3397-3415 ◽  
Author(s):  
Catherine M. Naud ◽  
Anthony D. Del Genio ◽  
Mike Bauer ◽  
William Kovari
Keyword(s):  
Relative Humidity ◽  
Southern Hemisphere ◽  
General Circulation Model ◽  
General Circulation ◽  
Cloud Cover ◽  
Climate Models ◽  
Storm Track ◽  
Circulation Model ◽  
Global Climate Models ◽  
Cold Fronts

Abstract Cloud vertical distributions across extratropical warm and cold fronts are obtained using two consecutive winters of CloudSat–Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations and National Centers for Environmental Prediction reanalysis atmospheric state parameters over the Northern and Southern Hemisphere oceans (30°–70°N/S) between November 2006 and September 2008. These distributions generally resemble those from the original model introduced by the Bergen School in the 1920s, with the following exceptions: 1) substantial low cloudiness, which is present behind and ahead of the warm and cold fronts; 2) ubiquitous high cloudiness, some of it very thin, throughout the warm-frontal region; and 3) upright convective cloudiness near and behind some warm fronts. One winter of GISS general circulation model simulations of Northern and Southern Hemisphere warm and cold fronts at 2° × 2.5° × 32 levels resolution gives similar cloud distributions but with much lower cloud fraction, a shallower depth of cloudiness, and a shorter extent of tilted warm-frontal cloud cover on the cold air side of the surface frontal position. A close examination of the relationship between the cloudiness and relative humidity fields indicates that water vapor is not lifted enough in modeled midlatitude cyclones and this is related to weak vertical velocities in the model. The model also produces too little cloudiness for a given value of vertical velocity or relative humidity. For global climate models run at scales coarser than tens of kilometers, the authors suggest that the current underestimate of modeled cloud cover in the storm track regions, and in particular the 50°–60°S band of the Southern Oceans, could be reduced with the implementation of a slantwise convection parameterization.

Simulating mixed-phase cloud properties with ICON around the CAPRICORN field campaign at the kilometre scale

2020 ◽  
Author(s):  
Veeramanikandan Ramadoss ◽  
Alain Protat ◽  
Yi Huang ◽  
Steven Siems ◽  
Anna Possner
Keyword(s):  
Southern Ocean ◽  
Climate Models ◽  
Cold Front ◽  
Meteorological Data ◽  
Short Wave ◽  
Atmospheric Composition ◽  
Wave Radiation ◽  
Limited Area ◽  
Field Campaign ◽  
Radiative Balance

<p>Stratocumulus clouds are low-level boundary layer clouds that cover 23% of the ocean surface on a global average, with a mean coverage of 25% to 40% in the mid-latitude oceans. These clouds affect Earth's radiative balance due to their strong radiative cooling effect. Many climate models underestimate the reflection of short wave radiation over the Southern Ocean (SO) which results in a positive mean bias of 2K in the annual mean SST in the mid-latitudes of the southern hemisphere. The organization, cloud field properties and the cloud radiative effects of these clouds occur at the lee of cold front in the SO are analyzed in this study. At this conference, we will present preliminary results.<br>Real case simulations are performed in this study by using ICON - LAM (Icosahedral Nonhydrostatic - Limited Area Model) with two-way nesting domains of resolutions 4.9 km to 2.4 km to 1.2 km. The initial and lateral boundary conditions for the model are derived from IFS meteorological data. CAPRICORN (Clouds, Aerosols, Precipitation, Radiation, and Atmospheric Composition over the Southern Ocean) field campaign that took place during March and April 2016 has continuously observed the open-cell and stratocumuli using shipborne radars and lidars on 26 and 27 March 2016 at the lee of a cold front between 47ºS 144ºE and 45ºS 146ºE (South of Tasmania). The results are evaluated quantitatively and qualitatively with the shipborne observations and HIMAWARI satellite retrievals respectively.</p>

scholarly journals Hydroclimate variability in Scandinavia over the last millennium – insights from a climate model-proxy data comparison

2017 ◽  
Author(s):  
Kristina Seftigen ◽  
Hugues Goosse ◽  
Francois Klein ◽  
Deliang Chen
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Climate Model ◽  
Global Climate ◽  
Model Performance ◽  
Circulation Model ◽  
Global Climate Models ◽  
Last Millennium ◽  
Atmospheric Variability ◽  
Out Of Sample

Abstract. The integration of climate proxy information with General Circulation Model (GCM) results offers considerable potential for deriving greater understanding of the mechanisms underlying climate variability, as well as unique opportunities for out-of-sample evaluations of model performance. In this study, we combine insights from a new tree-ring hydroclimate reconstruction from Scandinavian with projections from a suite of forced transient simulations of the last millennium and historical intervals from the CMIP5 and PMIP3 archives. Model simulations and proxy reconstruction data are found to broadly agree on the modes of atmospheric variability that produces droughts/pluvials in the region. But despite these dynamical similarities, large differences between simulated and reconstructed hydroclimate time series remain. We find simulated interannual components of variability to be overestimated, while the multidecadal/longer timescale components generally are too weak. Specifically, summertime moisture variability and temperature are weakly negatively associated at inter-annual timescales but positively correlated at decadal timescales, revealed from observational and proxy evidences. On this background, the CMIP5/PMIP3 simulated timescale dependent relationship between regional precipitation and temperature is considerably biased, because the short-term negative association is overestimated, and the long-term relationship is significantly underestimated. The lack of adequate understanding for mechanisms linking temperature and moisture supply on longer timescales has important implication for future projections. Weak multidecadal variability in models also implies that inference about future persistent droughts and pluvials based on the latest generation global climate models will likely underestimate the true risk of these events.

scholarly journals The impact of precipitation evaporation on the atmospheric aerosol distribution in EC-Earth v3.2.0

2018 ◽  
Vol 11 (4) ◽  
pp. 1443-1465 ◽  
Author(s):  
Marco de Bruine ◽  
Maarten Krol ◽  
Twan van Noije ◽  
Philippe Le Sager ◽  
Thomas Röckmann
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Climate Models ◽  
Circulation Model ◽  
Global Climate Models ◽  
Orthogonal Polarization ◽  
Earth Model ◽  
Climate Projections ◽  
Aerosol Distribution ◽  
The Impact

Abstract. The representation of aerosol–cloud interaction in global climate models (GCMs) remains a large source of uncertainty in climate projections. Due to its complexity, precipitation evaporation is either ignored or taken into account in a simplified manner in GCMs. This research explores various ways to treat aerosol resuspension and determines the possible impact of precipitation evaporation and subsequent aerosol resuspension on global aerosol burdens and distribution. The representation of aerosol wet deposition by large-scale precipitation in the EC-Earth model has been improved by utilising additional precipitation-related 3-D fields from the dynamical core, the Integrated Forecasting System (IFS) general circulation model, in the chemistry and aerosol module Tracer Model, version 5 (TM5). A simple approach of scaling aerosol release with evaporated precipitation fraction leads to an increase in the global aerosol burden (+7.8 to +15 % for different aerosol species). However, when taking into account the different sizes and evaporation rate of raindrops following Gong et al. (2006), the release of aerosols is strongly reduced, and the total aerosol burden decreases by −3.0 to −8.5 %. Moreover, inclusion of cloud processing based on observations by Mitra et al. (1992) transforms scavenged small aerosol to coarse particles, which enhances removal by sedimentation and hence leads to a −10 to −11 % lower aerosol burden. Finally, when these two effects are combined, the global aerosol burden decreases by −11 to −19 %. Compared to the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations, aerosol optical depth (AOD) is generally underestimated in most parts of the world in all configurations of the TM5 model and although the representation is now physically more realistic, global AOD shows no large improvements in spatial patterns. Similarly, the agreement of the vertical profile with Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite measurements does not improve significantly. We show, however, that aerosol resuspension has a considerable impact on the modelled aerosol distribution and needs to be taken into account.

Predicting Climate Change through Response Operators in a Coupled General Circulation Model

2020 ◽  
Author(s):  
Valerio Lembo ◽  
Valerio Lucarini ◽  
Francesco Ragone
Keyword(s):  
Climate Change ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Global Climate Models ◽  
Meridional Overturning Circulation ◽  
Climate System ◽  
Global Ocean ◽  
Climate Response ◽  
Ocean Heat Uptake

<p>Global Climate Models are key tools for predicting the future response of the climate system to a variety of natural and anthropogenic forcings. Typically, an ensemble of simulations is performed considering a scenario of forcing, in order to analyse the response of the climate system to the specific forcing signal. Given that the the climate response spans a very large range of timescales, such a strategy often requires a dramatic amount of computational resources. In this paper we show how to use statistical mechanics to construct operators able to flexibly predict climate change for a variety of climatic variables of interest, going beyond the limitation of having to consider specific time patterns of forcing. We perform our study on a fully coupled GCM - MPI-ESM v.1.2 - and for the first time we prove the effectiveness of response theory in predicting future climate response to CO<sub>2</sub> increase on a vast range of temporal scales. We specifically treat atmospheric  (surface temperature) and oceanic variables (strength of the Atlantic Meridional Overturning Circulation and of the Antarctic Circumpolar Current), as well as the global ocean heat uptake.</p>

On the impact of sea ice in a global ocean circulation model

1997 ◽  
Vol 25 ◽  
pp. 111-115 ◽  
Author(s):  
Achim Stössel
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Convective Activity ◽  
The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

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