The Trouble with Water: Condensation, Circulation and Climate

This is the final version. Available on open access from Springer Verlag via the DOI in this recordThis article discusses at a basic level a few of the problems that arise in geophysical fluid dynamics and climate that are associated with the presence of moisture in the air, its condensation and release of latent heat. Our main focus is Earth’s atmosphere, but we also discuss how these problems might manifest themselves on other planetary bodies, with particular attention to Titan where methane takes on the role of water. Geophysical fluid dynamics has traditionally been concerned with understanding the very basic problems that lie at the foundation of dynamical meteorology and oceanography. Conventionally, and a little ironically, the subject mainly considers ‘dry’ fluids, meaning it does not concern itself overly much with phase changes. The subject is often regarded as dry in another way, because it does not consider problems perceived as relevant to the real world, such as clouds or rainfall, which have typically been the province of complicated numerical models. Those models often rely on parameterizations of unresolved processes, parameterizations that may work very well but that often have a semiempirical basis. The consequent dichotomy between the foundations and the applications prevents progress being made that has both a secure basis in scientific understanding and a relevance to the Earth’s climate, especially where moisture is concerned. The dichotomy also inhibits progress in understanding the climate of other planets, where observations are insufficient to tune the parameterizations that weather and climate models for Earth rely upon, and a more fundamental approach is called for. Here, we discuss four diverse examples of the problems with moisture: the determination of relative humidity and cloudiness; the transport of water vapor and its possible change under global warming; the moist shallow water equations and the Madden–Julian Oscillation; and the hydrology cycle on other planetary bodies.Leverhulme TrustNatural Environment Research Council (NERC)Newton Fun

Atmospheric Response to SST anomalies. Part 2: Background-state dependence, teleconnections and local effects in summer

This is the author accepted manuscriptThe code required to run the Isca model framework, including the setup used in the present work, is provided on GitHub at www.github.com/ExeClim/Isca. Information on running the model is also provided at www. exeter.ac.uk/iscaIn this paper and its companion, Part I, we explore the response of the atmosphere to sea-surface temperature anomalies in different geographical locations and seasons. In Part 1we focussed on northern-hemisphere winter (DJF) whereas in this paper, Part 2, we focus on summer (JJA) and inter-seasonal comparisons. We use two different configurations of the same idealised atmospheric model, constructed using two different configurations of continents and topography. These configurations give rise to slightly different background wind fields and variability within the same season, and therefore give a measure of how robust a response is to small changes in the background-state. We characterise the types of responses that are found to SST anomalies in the midlatitudes and tropics in JJA, and compare these with the corresponding responses in DJF. We find that the responses to midlatitude SST anomalies in JJA are generally on a much smaller spatial scale than those in DJF. Responses in the tropical Pacific are much less dependent on season, although teleconnections between the tropical Pacific and the North Atlantic are not found in JJA as robustly as they are in DJF. Given insight from our model results, however, we do find some summer periods in reanalysis data where there is a strong association between the tropical Pacific and the summer North-Atlantic Oscillation. We discuss the reasons for these effects and the implications for Northern Hemisphere seasonal prediction in summer.SIT is supported by the Natural Environment Research Council (grant number NE/M006123/1) and GKV also acknowledges support from the Royal Society (Wofson Foundation), the Leverhulme Trust and the Newton Fund/CSSP

Atmospheric Circulation and Thermal Phase-Curve Offset of Tidally and Non-Tidally Locked Terrestrial Exoplanets

This is the final version. Available from IOP Publishing via the DOI in this recordUsing an idealised general circulation model, we investigate the atmospheric circulation of Earth-like terrestrial planets in a variety of orbital configurations. We relax the common assumption of the planet being tidally-locked, and look at the role atmospheric dynamics can have in the observed thermal phase curve when the substellar point is non-stationary. In slowly rotating planets, a moving forcing can induce strong jets in the upper troposphere, both prograde and retrograde, sensitive to the speed and direction of the diurnal forcing. We find that, consistent with previous shallow water model experiments, the thermal phase curve offset is sensitive to the velocity of the substellar point moving across the surface of the planet. For a planet with a known orbital period, the results show that the observed hotspot on the planet could be either east or west of the substellar point, depending on whether the planet is tidally-locked or not

A stochastic Lagrangian basis for a probabilistic parameterization of moisture condensation in eulerian models

This is the final version. Available from American Meteorological Society via the DOI in this record.In this paper, we describe the construction of an efficient probabilistic parameterization that could be used in a coarse-resolution numerical model in which the variation of moisture is not properly resolved. An Eulerian model using a coarse-grained field on a grid cannot properly resolve regions of saturation-in which condensation occurs-that are smaller than the grid boxes. Thus, in the absence of a parameterization scheme, either the grid box must become saturated or condensation will be underestimated. On the other hand, in a stochastic Lagrangian model of moisture transport, trajectories of parcels tagged with humidity variables are tracked, and small-scale moisture variability can be retained; however, explicitly implementing such a scheme in a global model would be computationally prohibitive. One way to introduce subgrid-scale saturation into an Eulerian model is to assume the humidity within a grid box has a probability distribution. To close the problem, this distribution is conventionally determined by relating the required subgrid-scale properties of the flow to the grid-scale properties using a turbulence closure. Here, instead, we determine an assumed probability distribution by using the statistical moments from a stochastic Lagrangian version of the system. The stochastic system is governed by a Fokker-Planck equation, and we use that, rather than explicitly following the moisture parcels, to determine the parameters of the assumed distribution. We are thus able to parameterize subgrid-scale condensation in an Eulerian model in a computationally efficient and theoretically well-founded way. In two idealized advection-condensation problems, we show that a coarse Eulerian model with the subgrid parameterization is well able to mimic its Lagrangian counterpart.Engineering and Physical Sciences Research Council (EPSRC)Royal Society (Wolfson Foundation)Leverhulme TrustNatural Environment Research Council (NERC

Zonal-mean atmospheric dynamics of slowly-rotating terrestrial planets

This is the final version. Available from the American Meteorological Society via the DOI in this record.The zonal-mean atmospheric flow of an idealized terrestrial planet is analyzed using both numerical simulations and zonally symmetric theories, focusing largely on the limit of low planetary rotation rate. Two versions of a zonally symmetric theory are considered, the standard Held-Hou model, which features a discontinuous zonal wind at the edge of the Hadley cell, and a variant with continuous zonal wind but discontinuous temperature. The two models have different scalings for the boundary latitude and zonal wind. Numerical simulations are found to have smoother temperature profiles than either model, with no temperature or velocity discontinuities even in zonally symmetric simulations. Continuity is achieved because of the presence of an overturning circulation poleward of the point of maximum zonal wind, which allows the zonal velocity profile to be smoother than the original theory without the temperature discontinuities of the variant theory. Zonally symmetric simulations generally fall between the two sets of theoretical scalings, and have a faster polar zonal flow than either. Three-dimensional simulations that allow for eddy motion fall closer to the scalings of the variant model. At very low rotation rates the maximum zonal wind falls with falling planetary rotation rate, even in the three-dimensional simulations, and collapses completely at zero rotation. Nevertheless, the low-rotation limit of the overturning circulation is strong enough to drive the temperature profile close to a state of nearly constant potential temperature.Leverhulme Trus

Regime Change Behaviour During Asian Monsoon Onset

This is the final version of the article. Available from American Meteorological Society via the DOI in this record.As the ITCZ moves off the equator on an aquaplanet, the Hadley circulation transitions from an equinoctial regime with two near symmetric, significantly eddy-driven cells, to a monsoon-like regime with a strong, thermally direct cross-equatorial cell, intense low-latitude precipitation, and a weak summer hemisphere cell. Dynamical feedbacks appear to accelerate the transition. This study investigates the relevance of this behavior to monsoon onset by using primitive-equation model simulations ranging from aquaplanets to more realistic configurations with Earth’s continents and topography. A change in the relationship between ITCZ latitude and overturning strength is identified once the ITCZ moves poleward of about ∼ 7 ◦ . Monsoon onset is associated with off-equatorial ascent, in regions of non-negligible planetary vorticity, and this is found to generate a vortex stretching tendency that reduces upper level absolute vorticity. In an aquaplanet, this causes a transition to the cross-equatorial, thermally direct regime, intensifying the overturning circulation. Analysis of the zonal momentum budget suggests a stationary wave, driven by topography and land-sea contrast, can trigger a similar transition in the more realistic model configuration, with the wave extending the ascent region of the Southern Hemisphere Hadley cell northward, and enhanced overturning then developing to the south. These two elements of the circulation resemble the East and South Asian monsoons.The work was supported by the UK-China Research & Innovation Partnership Fund, through the Met Office Climate Science for Service Partnership (CSSP) China, as part of the Newton Fund. GKV also acknowledges support from the Royal Society (Wolfson Foundation), the Leverhulme Trust, and NERC

Mapping the Energy Cascade in the North Atlantic Ocean: The Coarse-graining Approach

This is the final version of the article. Available from AMS via the DOI in this record.A coarse-graining framework is implemented to analyze nonlinear processes, measure energy transfer rates and map out the energy pathways from simulated global ocean data. Traditional tools to measure the energy cascade from turbulence theory, such as spectral flux or spectral transfer rely on the assumption of statistical homogeneity, or at least a large separation between the scales of motion and the scales of statistical inhomogeneity. The coarse-graining framework allows for probing the fully nonlinear dynamics simultaneously in scale and in space, and is not restricted by those assumptions. This paper describes how the framework can be applied to ocean flows. Energy transfer between scales is not unique due to a gauge freedom. Here, it is argued that a Galilean invariant subfilter scale (SFS) flux is a suitable quantity to properly measure energy scale-transfer in the Ocean. It is shown that the SFS definition can yield answers that are qualitatively different from traditional measures that conflate spatial transport with the scale-transfer of energy. The paper presents geographic maps of the energy scale-transfer that are both local in space and allow quasi-spectral, or scale-by-scale, dynamics to be diagnosed. Utilizing a strongly eddying simulation of flow in the North Atlantic Ocean, it is found that an upscale energy transfer does not hold everywhere. Indeed certain regions, near the Gulf Stream and in the Equatorial Counter Current have a marked downscale transfer. Nevertheless, on average an upscale transfer is a reasonable mean description of the extra-tropical energy scale-transfer over regions of O(10^3) kilometers in size.Financial support was provided by IGPPS at Los Alamos National Laboratory (LANL) and NSF grant OCE-1259794. HA was also supported through DOE grants de-sc0014318, de-na0001944, and the LANL LDRD program through project number 20150568ER. MH was also supported through the HiLAT project of the Regional and Global Climate Modeling program of the DOE’s Office of Science, and GKV was also supported by NERC, the Marie Curie Foundation and the Royal Society (Wolfson Foundation). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231

Processes and Timescales in Onset and Withdrawal of 'Aquaplanet Monsoons'

This is the final version. Available from American Meteorological Society via the DOI in this record.Data availability: The research materials supporting this publication can be accessed by contacting Ruth Geen ([email protected]).Aquaplanets with low heat capacity slab ocean boundary conditions can exhibit rapid changes in the regime of the overturning circulation over the seasonal cycle, which have been connected to the onset of Earth’s monsoons. In spring, as the ITCZ migrates off the Equator, it jumps poleward and a sudden transition occurs from an eddy-driven, equinoctial regime with two weak Hadley cells, to a near angular momentum conserving, solstitial regime with a strong, cross-equatorial winter hemisphere cell. Here, the controls on the transition latitude and rate are explored in idealised moist aquaplanet simulations. It is found that the transition remains rapid relative to the solar forcing when year length and slab ocean heat capacity are varied, and, at Earth’s rotation rate, always occurs when the ITCZ reaches approximately 7°. This transition latitude is, however, found to scale inversely with rotation rate. Interestingly, the transition rate varies non-monotonically with rotation, with a maximum at Earth’s rotation rate, suggesting that Earth may be particularly disposed to a fast monsoon onset. The fast transition relates to feedbacks in both the atmosphere and the slab ocean. In particular, an evaporative feedback between the lower-level branch of the overturning circulation and the surface temperature is identified. This accelerates monsoon onset and slows withdrawal. Lastly, comparing eddy-permitting and axisymmetric experiments shows that, in contrast with results from dry models, in this fully moist model the presence of eddies slows the migration of the ITCZ between hemispheres.UK-China Research and Innovation Partnership FundRoyal SocietyLeverhulme Trus

Reduced high-latitude land seasonality in climates with very high carbon dioxide

This is the final version. Available from the American Meteorological Society via the DOI in this record Code and data availability: The code to reproduce the figures is available at https://github.com/matthewjhenry/simple-seasonality-arctic and the data is available at https://zenodo.org/record/4529135.Observations of warm past climates and projections of future climate change show that the Arctic warms more than the global mean, particularly during winter months. Previous work has attributed this reduced Arctic land seasonality to the effects of sea ice or clouds. In this paper, we show that the reduced Arctic land seasonality is a robust consequence of the relatively small surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission, without recourse to other processes or feedbacks. We use a General Circulation Model (GCM) with no clouds or sea ice and a simple representation of land. In the annual mean, the equator-to-pole surface temperature gradient falls with increasing CO2, but this is only a near-surface phenomenon and is not caused by the change in total meridional heat transport, which is virtually unaltered. The high-latitude land has about twice as much warming in winter than in summer, whereas highlatitude ocean has very little seasonality in warming. A surface energy balance model shows how the combination of the smaller surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission gives rise to the reduced seasonality of the land surface. The increase in evaporation over land also leads to winter amplification of warming over land, although amplification still occurs without it. While changes in clouds, sea ice, and ocean heat transport undoubtedly play a role in high-latitude warming, these results show that enhanced land surface temperature warming in winter can happen in their absence for robust reasons.Natural Environment Research Council (NERC

Variations on a pathway to an early Eocene climate

This is the final version. Available from Wiley via the DOI in this record. The climate of the early Eocene was characterized by much higher temperatures and a smaller equator-to-pole surface temperature gradient than today. Comprehensive climate models have been reasonably successful in simulating that climate in the annual average. However, good simulations of the seasonal variations, and in particular much warmer Arctic winters over land, have proven more difficult. Further, while increased greenhouse gases seems necessary to achieve an Eocene climate, it is unclear whether there is a unique combination of factors that leads to agreement with all available proxies. Here we use a very flexible General Circulation Model to examine the sensitivity of the modeled climate to differences in CO2 concentration, land surface properties, ocean heat transport, and cloud extent and thickness. Even in the absence of ice or changes in cloudiness, increasing the CO2 concentration leads to a polar-amplified surface temperature change because of increased water vapor levels combined with the lack of convection at high latitudes, with the nonlinear dependence of longwave radiation on temperature amplifying the increase in winter over land. Additional low clouds over Arctic land generally decrease summer temperatures and further increase winter temperatures (except at very high CO2 levels). An increase in the land surface heat capacity, plausible given large changes in vegetation, also decreases the Arctic land seasonality. Thus, different combinations of factors—high CO2 levels, changes in low-level clouds, and an increase in land surface heat capacity—can lead to a simulation within the proxy uncertainty range of the majority of proxy data.Natural Environment Research CouncilNational Science Foundatio