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Resolved Snowball Earth Clouds 15 JUNE 2014 A B B O T 4391 Resolved Snowball Earth Clouds DORIAN S. ABBOT Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois (Manuscript received 2 December 2013, in final form 23 February 2014) ABSTRACT Recent general circulation model (GCM) simulations have challenged the idea that a snowball Earth would be nearly entirely cloudless. This is important because clouds would provide a strong warming to a high- albedo snowball Earth. GCM results suggest that clouds could lower the threshold CO2 needed to deglaciate a snowball by a factor of 10–100, enough to allow consistency with geochemical data. Here a cloud-resolving model is used to investigate cloud and convection behavior in a snowball Earth climate. The model produces convection that extends vertically to a similar temperature as modern tropical convection. This convection produces clouds that resemble stratocumulus clouds under an inversion on modern Earth, which slowly dissipate by sedimentation of cloud ice. There is enough cloud ice for the clouds to be optically thick in the longwave, and the resulting cloud radiative forcing is similar to that produced in GCMs run in snowball conditions. This result is robust to large changes in the cloud microphysics scheme because the cloud longwave forcing, which dominates the total forcing, is relatively insensitive to cloud amount and particle size. The cloud-resolving model results are therefore consistent with the idea that clouds would provide a large warming to a snowball Earth, helping to allow snowball deglaciation. 1. Introduction Additionally, in at least some areas banded iron for- mations, which indicate anoxic ocean conditions, were Between 600 and 800 million years ago there were at deposited during these glaciations (Kirschvink 1992; least two periods during which paleomagnetic evidence Hoffman and Li 2009). The basic paleomagnetic evidence indicates that glaciers flowed from continents into the that glacial deposits (Evans and Raub 2011) and overlying ocean at or near the equator1 (Kirschvink 1992; Hoffman carbonates (Hoffman and Li 2009) were at low paleo- et al. 1998). These events are associated with large carbon latitudes has been confirmed by recent meta-analyses isotopic excursions and the glacial sequences are capped incorporating all available data. with thick layers of carbonates (Hoffman and Schrag The (hard) snowball Earth hypothesis (Kirschvink 2002), both of which indicate that these glaciations were 1992; Hoffman et al. 1998) explains these data through associated with large perturbations of the carbon cycle the onset of global glaciation, with the entire ocean and were qualitatively different from the recent Pleisto- covered in kilometer-thick ice, the slow accumulation of cene glaciations. Moreover, geochemical measurements CO in the atmosphere due to greatly reduced silicate indicate that the CO during these events reached 0.01– 2 2 weathering (Walker et al. 1981) leading to warming and 0.1 volume mixing ratio (vmr; Kasemann et al. 2005; Bao deglaciation after millions of years, and the deposition et al. 2008, 2009), although there is some debate about of cap carbonates during the resulting extremely warm the details (Sansjofre et al. 2011; Cao and Bao 2013). period when strong weathering would reduce the at- mospheric CO2. Alternative hypotheses involving thin tropical ice or open tropical oceans have been proposed 1 There are potentially more such episodes about two billion (Hyde et al. 2000; McKay 2000; Pollard and Kasting years ago for which the data are poorer. 2005; Abbot et al. 2011), mainly to explain the apparent survival of photosynthetic and animal life in the ocean Corresponding author address: Dorian S. Abbot, 5734 S. Ellis through the snowball events (e.g., Love et al. 2009; Ave., Chicago, IL 60637. Bosak et al. 2011), although oases in a hard snowball E-mail: [email protected] might also be able to permit this survival (e.g., Campbell DOI: 10.1175/JCLI-D-13-00738.1 Ó 2014 American Meteorological Society 4392 JOURNAL OF CLIMATE VOLUME 27 et al. 2011; Tziperman et al. 2012). In this paper I will cloud-resolving model explicitly simulates nonhydrostatic focus on the hard snowball Earth hypothesis in the spirit cloud-scale motions, although it still requires a cloud of fully investigating its implications to evaluate its microphysical scheme, whereas GCMs rely on subgrid- consistency with available data, while acknowledging scale parameterizations to model convection and cloud that future data may prove that it is not the correct behavior. Since clouds and convection in a cloud-resolving model for all of the global glaciations. model are less parameterized than in a GCM they may be Pierrehumbert (2004, 2005) performed the first gen- more likely to yield an accurate representation of clouds eral circulation model (GCM) simulations applied spe- in a climate different from the modern climate, although cifically to the hard snowball Earth, using the Fast cloud-resolving models must be run on small domains and Ocean Atmosphere Model (FOAM). He found that therefore cannot calculate the effects of large-scale mo- the snowball was nowhere near deglaciating at CO2 5 tions, which can drive convective behavior. I find that 0.2 vmr, which is inconsistent with the available geo- SAM produces cloud condensate and a CRF similar to chemical estimates of the CO2 (0.01–0.1 vmr; Kasemann that of most GCMs (Abbot et al. 2012, 2013). This con- et al. 2005; Bao et al. 2008, 2009). This led to the pro- clusion is robust to large changes in the cloud micro- posal that dust could significantly lower the snowball physical scheme. This work confirms the GCM results albedo (Abbot and Pierrehumbert 2010; Abbot and that clouds would likely provide significant warming to Halevy 2010; Le Hir et al. 2010; see also Schatten and a snowball Earth. Endal 1982) and a series of investigations in other GCMs (Le Hir et al. 2007; Abbot and Pierrehumbert 2010; Le Hir et al. 2010; Hu et al. 2011; Pierrehumbert 2. SAM et al. 2011). The GCMs yielded sometimes conflicting a. Model description results. For example, Abbot and Pierrehumbert (2010) found that FOAM could only deglaciate at CO2 5 I use the System for Atmospheric Modeling 0.1 vmr when the albedo was reduced in the tropics to (Khairoutdinov and Randall 2003), version 6.10.4. SAM account for a mixture of dust with snow and ice, whereas is a cloud-resolving model that solves the anelastic the Community Atmosphere Model (CAM) deglaciated equations of motion and has liquid water and ice moist in some configurations without reducing the albedo at static energy, total nonprecipitating water, and total CO2 5 0.1 vmr. precipitating water as prognostic thermodynamical vari- This led to a comparison of six different GCMs run in ables. Cloud condensate is diagnosed as occurring when consistent snowball Earth conditions (Abbot et al. 2012, an air parcel reaches saturation and supersaturation is not 2013). Most of these GCMs calculate clouds using allowed. Longwave and shortwave radiative fluxes are subgrid-scale schemes, but one, the superparameterized computed using the radiation scheme from the National Community Atmosphere Model (SP-CAM), has an Center for Atmospheric Research (NCAR) Community embedded two-dimensional cloud-resolving model2 in Atmosphere Model, version 3 (Collins et al. 2004). This each grid box. The main finding of Abbot et al. (2012, radiation scheme is accurate to within a few watts per 2013) was that all of the other GCMs produced a much meter squared at CO2 5 0.1 vmr (Abbot et al. 2012). I use higher cloud radiative forcing (CRF) than FOAM, the single-moment cloud-microphysics scheme described which led to a warming of the tropics by 7–11 K relative in Khairoutdinov and Randall (2003).Mymainresults to FOAM, which is equivalent to an increase of the CO2 are robust to large changes in the microphysical scheme by a factor of 10–100 in the snowball climate. Although parameters (section 4). I also tested the double-moment the complex processes involved in deglaciation were not scheme described in Morrison et al. (2009), but found that modeled, this is roughly enough warming to allow de- the model was unstable when I used it in the configuration glaciation to occur at a CO2 level consistent with the described here. geochemical data even without the albedo-reducing ef- I ran the model on a doubly periodic square domain fects of dust (which could still be relevant). with a side length of 128 km that ranged vertically from Here I perform similar tests to those performed using the surface to a model top height of 17.5 km. I used GCMs by Abbot et al. (2012, 2013), but instead I use a horizontal grid size of 1 km, a variable vertical grid size a cloud-resolving model [the System for Atmospheric (368 m above 2 km and finer below this) for a total of 59 Modeling (SAM); Khairoutdinov and Randall 2003]. A vertical levels, and a time step of 10 s. I confirmed that the model had converged with respect to these param- eters (appendix B). I applied a small linear shear of 21 21 2 The cloud-resolving model embedded in SP-CAM is actually 20.01 m s mb (1 mb 5 1 hPa) to avoid convective a version of SAM, the model used in this paper. self-aggregation and domain-size dependence of results 15 JUNE 2014 A B B O T 4393 TABLE 1. This table compares the surface temperature (Ts) and top-of-atmosphere shortwave (CRFsw), longwave (CRFlw), and total 24 (CRF) cloud radiative forcing for the reference SAM snowball simulation with CO2 5 10 vmr, a SAM snowball simulation with CO2 5 2 10 2 vmr, and the reference modern tropics SAM simulation.
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