.. _recipes_snowalbedo:

Emergent constraint on snow-albedo effect
=========================================

Overview
--------

The recipe recipe_snowalbedo.yml computes the springtime snow-albedo
feedback values in climate change versus springtime values in the seasonal
cycle in transient climate change experiments following Hall and Qu (2006).
The strength of the snow-albedo effect is quantified by the variation in net
incoming shortwave radiation (Q) with surface air temperature (T\ :sub:`s`\) due
to changes in surface albedo :math:`\alpha_s`:

.. math::

   \left( \frac{\partial Q}{\partial T_s} \right) = -I_t \cdot \frac{\partial \alpha_p}{\partial \alpha_s} \cdot \frac{\Delta \alpha_s}{\Delta T_s}

The diagnostic produces scatterplots of simulated springtime
:math:`\Delta \alpha_s`/:math:`\Delta T_s` values in climate change (ordinate)
vs. simulated springtime :math:`\Delta \alpha_s`/:math:`\Delta T_s` values in the
seasonal cycle (abscissa).

Ordinate values: the change in April :math:`\alpha_s` (future projection - historical)
averaged over NH land masses poleward of 30°N is divided by the change in
April T\ :sub:`s` (future projection - historical) averaged over the same region.
The change in :math:`\alpha_s` (or T\ :sub:`s`) is defined as the difference between
22nd-century-mean :math:`\alpha_s`: (T\ :sub:`s`) and 20th-century-mean :math:`\alpha_s`. Values of
:math:`\alpha_s` are weighted by April incoming insolation (I\ :sub:`t`) prior to averaging.

Abscissa values: the seasonal cycle :math:`\Delta \alpha_s`/:math:`\Delta T_s`
values, based on 20th century climatological means, are calculated by
dividing the difference between April and May :math:`\alpha_s`: averaged over NH continents
poleward of 30°N by the difference between April and May T\ :sub:`s` averaged over the
same area. Values of :math:`\alpha_s`: are weighted by April incoming insolation prior to
averaging.


Available recipes and diagnostics
---------------------------------

Recipes are stored in recipes/

    * recipe_snowalbedo.yml

Diagnostics are stored in diag_scripts/emergent_constraints/

    * snowalbedo.ncl: springtime snow-albedo feedback values vs. seasonal cycle


User settings in recipe
-----------------------

#. Script snowalbedo.ncl

   *Required settings for script*

   * exp_presentday: name of present-day experiment (e.g. "historical")
   * exp_future: name of climate change experiment (e.g. "rcp45")

   *Optional settings for script*

   * diagminmax: observational uncertainty (min and max)
   * legend_outside: create extra file with legend (true, false)
   * styleset: e.g. "CMIP5" (if not set, this diagnostic will create its own
     color table and symbols for plotting)
   * suffix: string to be added to output filenames
   * xmax: upper limit of x-axis (default = automatic)
   * xmin: lower limit of x-axis (default = automatic)
   * ymax: upper limit of y-axis (default = automatic)
   * ymin: lower limit of y-axis (default = automatic)

   *Required settings for variables*

   * ref_model: name of reference data set

   *Optional settings for variables*

   none

   *Required settings (scripts)*

   none

   *Optional settings (scripts)*


Variables
---------

* tas (atmos, monthly mean, longitude latitude time)
* rsdt (atmos, monthly mean, longitude latitude time)
* rsuscs, rsdscs (atmos, monthly mean, longitude latitude time)


Observations and reformat scripts
---------------------------------

* ERA-Interim (tas - esmvaltool/cmorizers/data/formatters/datasets/era_interim.py)
* ISCCP-FH (rsuscs, rsdscs, rsdt - esmvaltool/cmorizers/data/formatters/datasets/isccp_fh.ncl)


References
----------

* Flato, G., J. Marotzke, B. Abiodun, P. Braconnot, S.C. Chou, W. Collins, P.
  Cox, F. Driouech, S. Emori, V. Eyring, C. Forest, P. Gleckler, E. Guilyardi,
  C. Jakob, V. Kattsov, C. Reason and M. Rummukainen, 2013: Evaluation of
  Climate Models. In: Climate Change 2013: The Physical Science Basis.
  Contribution of Working Group I to the Fifth Assessment Report of the
  Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K.
  Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and
  P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom
  and New York, NY, USA.

* Hall, A., and X. Qu, 2006: Using the current seasonal cycle to constrain
  snow albedo feedback in future climate change, Geophys. Res. Lett., 33,
  L03502, doi:10.1029/2005GL025127.


Example plots
-------------

.. figure:: /recipes/figures/ipccwg1ar5ch9/fig-9-45a.png
   :align: center

   Scatterplot of springtime snow-albedo effect values in climate
   change vs. springtime :math:`\Delta \alpha_s`/:math:`\Delta T_s` values in
   the seasonal cycle in transient climate change experiments (CMIP5 historical
   experiments: 1901-2000, RCP4.5 experiments: 2101-2200). Similar to IPCC AR5
   Chapter 9 (Flato et al., 2013), Figure 9.45a.