AceCAST Version 1.3 New Feature Highlight: Fractional Sea Ice

AceCAST Version 1.3 New Feature Highlight: Fractional Sea Ice

By: Taylor Trask, Meteorology Solutions Architect, TempoQuest

An image showing the fractional sea ice activation setting under the physics section in the WRF input file.

The fractional sea ice capability is one of several fundamental options implemented in TempoQuest’s current release of their cutting-edge weather software product, AceCAST (version 1.3). This feature enables users to treat sea ice as a fractional field to better represent polar climates and is a critical option to use when performing climate projections both for operational and research applications.

Sea ice is frozen water on the ocean surface near the poles which can significantly influence global climate. More incoming solar radiation is reflected off the bright surface of sea ice since the proportion of radiation that is returned by the sea ice is greater than that for liquid water (ie., sea ice has a higher albedo). During the winter months, there is an abundance of sea ice which causes regional ocean surface temperatures to decrease and sea ice to further increase, since colder temperatures enhance sea ice expansion. However, during the spring and summer months, the opposite occurs where warmer temperatures gradually melt sea ice and reduce the sea ice surface areas available to reflect sunlight, and thus, also decreases the albedo since more radiation can be absorbed by the ocean. This results in rising ocean temperatures since nothing inhibits the darker ocean surface from absorbing the incoming solar energy. This seasonal cycle of sea ice expansion and melting influences ocean currents. Ocean currents transport cold water from the poles to the tropics and warm water from the tropics to the poles, which ultimately regulates the global climate since air temperature is influenced by conductive heat transfer between the water and the air.

The global climate is sensitive to the influences of the thermodynamic (heat transfer) processes associated with the sea ice seasonal cycle, thus it is important to implement the fractional sea ice option in any climate simulation, especially those over polar regions. Fractional sea ice is different from the default behavior, which only determines if there is sea ice (or not) – meaning there are no “in between” values.

To use this capability in an AceCAST simulation, a user must use input data that includes fractional sea ice representation. These input data can be found at the National Snow and Ice Data Center. Additionally, to activate the fraction sea ice feature, a user must set the options fractional_seaice = 1 and seaice_threshold = user defined value under the physics section in the input file (namelist.input). The fractional_seaice option simply activates the representation of fractional sea ice within a model simulation. The seaice_threshold setting tells the model to check each grid point to see if it is a water point based on the condition that this point has a skin temperature that is colder than the value (in Kelvin) set in this setting. If a grid point is colder than the threshold setting, then make that grid point a sea ice point. The default setting for seaice_threshold is 0, and therefore, this value must be adjusted to fit the criteria for any simulation. Lastly, the user must also set the following prior to running their simulation:

  1. Use XICE in place of SEAICE in the associated Variable Table (Vtable)
  2. Use a compatible surface layer physics options:
    1. sf_sfclay_physics = 1, 2, or 5,
  3. Use a compatible land surface physics options:
    1. sf_surface_physics = 2 or 3