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    "# Stabilizing cloud feedback dramatically expands the habitable zone of tidally locked planets\n",
    "## Yang, Cowan, Abbot\n",
    "## 2013"
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   "source": [
    "## Introduction\n",
    "* Previous HZ studies - mostly based on 1D models\n",
    "* 1D models cannot predict cloud coverage or altitude, which is essential for radiative effects, such as **albedo (A)**\n",
    "* The impact of 3D cloud behaviour on the inner edge of HZ has not been considered"
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   "source": [
    "Add figures from Joshi 2003 or smth"
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   "source": [
    "## Global climate models"
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   "source": [
    "* Community Atmospheric Model: CAM3, CAM4, and CAM5 coupled with slab ocean (d=50m)\n",
    "* Fully coupled model: CCSM3 (d=4000m)\n",
    "* Simulated clouds: marine stratus, layered clouds, shallow and deep convective clouds in both liquid and ice phases\n",
    "* CAM4: new convection scheme\n",
    "* CAM5: new cloud scheme, attempt at aerosol-cloud interactions"
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   "source": [
    "check the schemes"
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   "source": [
    "* Radiation scheme: accurate for CO2 concentrations < 0.1 bar and WV column content < 1200 kg/m2\n",
    "* Cloud infrared scattering: not included (negligible)"
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    "* Star spectra: M-star (K-star) with Teff=3400K (4500K)\n",
    "* Stellar flux: 1000-2600 W/m2\n",
    "* Geothermal flux: 0"
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   "source": [
    "### Planetary constants\n",
    "* $R= 2 R_E$\n",
    "* $g= 1.4 g_E$\n",
    "* Orbital period: 60 days\n",
    "* Obliquity and eccentricity are zero\n",
    "* 3 rotation periods: 1:1, 2:1, 6:1"
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   "source": [
    "### Atmospheric composition\n",
    "* p = 1 bar\n",
    "* N2 and H2O only\n",
    "* Tests with CO2 (?)\n",
    "* CAM5 aerosols: set to 20th century Earth"
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    "### Land-sea set-up in CCSM3 simulations (with slightly different planet parameters)\n",
    "\n",
    "* aqua-planet\n",
    "* one ridge on the eastern terminator\n",
    "* two ridges on terminators"
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   "source": [
    "## The stabilizing cloud feedback"
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   "source": [
    "<img style=\"float: center;\" width=\"80%\" src=\"figures/YangEtAl2013-fig1.png\">"
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   "source": [
    "<img style=\"float: center;\" width=\"40%\" src=\"figures/YangEtAl2013-fig1.png\">\n",
    "\n",
    "### Non-TL\n",
    "* Planetary albedo is similar to Earth\n",
    "* Contr. to TL, A decreases when solar flux increases => destabilising feedback"
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    "<img style=\"float: center;\" width=\"40%\" src=\"figures/YangEtAl2013-fig1.png\">\n",
    "\n",
    "### TL\n",
    "* Most of the day side is covered in clouds\n",
    "* Thick clouds where the insolation is greatest -> higher planetary albedo\n",
    "* this effect is greater for M- and K-star spectra\n",
    "* Surface temperature on planets such as HD 85512b and GJ 163c can be moderate due to such high albedo"
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   "source": [
    "#### with cloud scheme switched off\n",
    "* Big drop in albedo, increase in temperature\n",
    "* Thus, clouds account for 73 K of cooling"
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   "source": [
    "#### greenhouse effect\n",
    "* Diff. between upward LW flux at sfc. and TOA:\n",
    "  - $G = T _ { \\mathrm { em } } ^ { \\mathrm { sff } } - T _ { \\mathrm { ef } }$,\n",
    "  - $T _ { \\mathrm { ef } } = \\left( F _ { \\uparrow } ^ { \\mathrm { top } } / \\sigma \\right) ^ { ( 1 / 4 ) }$,\n",
    "  - $T^{srf} _ { \\mathrm { em } } = \\left( F _ { \\uparrow } ^ { \\mathrm { srf } } / \\sigma \\right) ^ { ( 1 / 4 ) }$\n",
    "* smaller on TL planets due to low-level temp. inversion on the nightside"
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   "source": [
    "<img style=\"float: center;\" width=\"60%\" src=\"figures/YangEtAl2013-fig2.png\">"
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   "source": [
    "## Thermal phase curves\n",
    "<img style=\"float: center;\" width=\"80%\" src=\"figures/YangEtAl2013-fig3.png\">"
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    "Clouds create a significant dayside OLR minimum.\n",
    "\n",
    "Can there be a false positive? - A planet without clouds would have to have A ~ 0.8, which is implausible for planets around M-stars"
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   "source": [
    "## Sensitivity tests"
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   "source": [
    "* cloud particle size\n",
    "* cloud fraction parameters\n",
    "* surface pressure\n",
    "* CO2 concentration\n",
    "* planetary radius, rotation rate\n",
    "* surface gravity\n",
    "* oceanic mixed layer depth\n",
    "* convective scheme\n",
    "* model resolution\n",
    "* land-sea distribution, ocean heat transport"
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    "Robust results across different simulations. Similar albedo of ~0.5 (except for the one with dynamical ocean)"
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    "* **cloud particle size**\n",
    "* cloud fraction parameters\n",
    "* surface pressure\n",
    "* CO2 concentration\n",
    "* **planetary radius**,  **rotation rate**\n",
    "* surface gravity\n",
    "* oceanic mixed layer depth\n",
    "* convective scheme\n",
    "* model resolution\n",
    "* land-sea distribution, **ocean heat transport**\n",
    "\n",
    "\n",
    "Robust results across different simulations. Similar albedo of ~0.5 (except for the one with dynamical ocean)"
   ]
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   "source": [
    "<img style=\"float: center;\" width=\"70%\" src=\"figures/YangEtAl2013-fig4.png\">"
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   "source": [
    "<img style=\"float: center;\" width=\"80%\" src=\"figures/YangEtAl2013-tab1.png\">"
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   "source": [
    "### Three main parameters that reduce albedo"
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   "source": [
    "1. Liquid droplet size"
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   "source": [
    "2. Small orbital period or large planetary radius => smaller Rossby deformation radius => rapidly rotating regime => concentrated convection and fewer dayside clouds"
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    "3. Addition or increase of OHT => smaller day-night temperature gradient => weaker circulation => fewer clouds\n",
    "  * OHT has to be large (x10 of Earth's)\n",
    "  * Obstruction by continents weakens this effect"
   ]
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