### 2.2.3.1. Atmospheric Coupling[¶](#atmospheric-coupling "Permalink to this headline")

The current state of the atmosphere ([Table 2.2.4](#table-atmospheric-input-to-land-model)) at a given time step is used to force the land model. This atmospheric state is provided by an atmospheric model in coupled mode or from an observed dataset in land-only mode (Chapter [2.32](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Land-Only_Mode/CLM50_Tech_Note_Land-Only_Mode.html#rst-land-only-mode)). The land model then initiates a full set of calculations for surface energy, constituent, momentum, and radiative fluxes. The land model calculations are implemented in two steps. The land model proceeds with the calculation of surface energy, constituent, momentum, and radiative fluxes using the snow and soil hydrologic states from the previous time step. The land model then updates the soil and snow hydrology calculations based on these fluxes. These fields are passed to the atmosphere ([Table 2.2.5](#table-land-model-output-to-atmospheric-model)). The albedos sent to the atmosphere are for the solar zenith angle at the next time step but with surface conditions from the current time step.

Table 2.2.4 Atmospheric input to land model[¶](#id18 "Permalink to this table")   
| Field
 | variable name

 | units

 |
| --- | --- | --- |
| 1Reference height

 | \\(z'\_{atm}\\)

 | m

 |
| Atmosphere model’s surface height

 | \\(z\_{surf,atm}\\)

 | m

 |
| Zonal wind at \\(z\_{atm}\\)

 | \\(u\_{atm}\\)

 | m s\-1

 |
| Meridional wind at \\(z\_{atm}\\)

 | \\(v\_{atm}\\)

 | m s\-1

 |
| Potential temperature

 | \\(\\overline{\\theta \_{atm} }\\)

 | K

 |
| Specific humidity at \\(z\_{atm}\\)

 | \\(q\_{atm}\\)

 | kg kg\-1

 |
| Pressure at \\(z\_{atm}\\)

 | \\(P\_{atm}\\)

 | Pa

 |
| Temperature at \\(z\_{atm}\\)

 | \\(T\_{atm}\\)

 | K

 |
| Incident longwave radiation

 | \\(L\_{atm} \\, \\downarrow\\)

 | W m\-2

 |
| 2Liquid precipitation

 | \\(q\_{rain}\\)

 | mm s\-1

 |
| 2Solid precipitation

 | \\(q\_{sno}\\)

 | mm s\-1

 |
| Incident direct beam visible solar radiation

 | \\(S\_{atm} \\, \\downarrow \_{vis}^{\\mu }\\)

 | W m\-2

 |
| Incident direct beam near-infrared solar radiation

 | \\(S\_{atm} \\, \\downarrow \_{nir}^{\\mu }\\)

 | W m\-2

 |
| Incident diffuse visible solar radiation

 | \\(S\_{atm} \\, \\downarrow \_{vis}\\)

 | W m\-2

 |
| Incident diffuse near-infrared solar radiation

 | \\(S\_{atm} \\, \\downarrow \_{nir}\\)

 | W m\-2

 |
| Carbon dioxide (CO2) concentration

 | \\(c\_{a}\\)

 | ppmv

 |
| 3Aerosol deposition rate

 | \\(D\_{sp}\\)

 | kg m\-2 s\-1

 |
| 4Nitrogen deposition rate

 | \\(NF\_{ndep\\\_ s{\\it min}n}\\)

 | g (N) m\-2 yr\-1

 |
| 5Lightning frequency

 | \\(I\_{l}\\)

 | flash km\-2 hr\-1

 |

1The atmospheric reference height received from the atmospheric model \\(z'\_{atm}\\) is assumed to be the height above the surface as defined by the roughness length \\(z\_{0}\\) plus displacement height \\(d\\). Thus, the reference height used for flux computations (Chapter [2.5](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Fluxes/CLM50_Tech_Note_Fluxes.html#rst-momentum-sensible-heat-and-latent-heat-fluxes)) is \\(z\_{atm} =z'\_{atm} +z\_{0} +d\\). The reference heights for temperature, wind, and specific humidity (\\(z\_{atm,\\, h}\\), \\(z\_{atm,\\, {\\it m}}\\), \\(z\_{atm,\\, w}\\) ) are required. These are set equal to\\(z\_{atm}\\).

2CAM provides convective and large-scale liquid and solid precipitation, which are added to yield total liquid precipitation \\(q\_{rain}\\) and solid precipitation \\(q\_{sno}\\). However, in CLM5, the atmosphere’s partitioning into liquid and solid precipitation is ignored. Instead, CLM repartitions total precipitation using a linear ramp. For most landunits, this ramp generates all snow below \\(0 ^{\\circ} C\\), all rain above \\(2 ^{\\circ} C\\), and a mix of rain and snow for intermediate temperatures. For glaciers, the end points of the ramp are \\(-2 ^{\\circ} C\\) and \\(0 ^{\\circ} C\\), respectively. Changes to the phase of precipitation are accompanied by a sensible heat flux (positive or negative) to conserve energy.

3There are 14 aerosol deposition rates required depending on species and affinity for bonding with water; 8 of these are dust deposition rates (dry and wet rates for 4 dust size bins, \\(D\_{dst,\\, dry1},\\, D\_{dst,\\, dry2},\\, D\_{dst,\\, dry3},\\, D\_{dst,\\, dry4}\\), \\(D\_{dst,\\, \\, wet1},D\_{dst,\\, wet2},\\, D\_{dst,wet3},\\, D\_{dst,\\, wet4}\\) ), 3 are black carbon deposition rates (dry and wet hydrophilic and dry hydrophobic rates, \\(D\_{bc,\\, dryhphil},\\, D\_{bc,\\, wethphil},\\, D\_{bc,\\, dryhphob}\\) ), and 3 are organic carbon deposition rates (dry and wet hydrophilic and dry hydrophobic rates, \\(D\_{oc,\\, dryhphil},\\, D\_{oc,\\, wethphil},\\, D\_{oc,\\, dryhphob}\\) ). These fluxes are computed interactively by the atmospheric model (when prognostic aerosol representation is active) or are prescribed from a time-varying (annual cycle or transient), globally-gridded deposition file defined in the namelist (see the CLM4.5 User’s Guide). Aerosol deposition rates were calculated in a transient 1850-2009 CAM simulation (at a resolution of 1.9x2.5x26L) with interactive chemistry (troposphere and stratosphere) driven by CCSM3 20th century sea-surface temperatures and emissions ([Lamarque et al. 2010](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#lamarqueetal2010)) for short-lived gases and aerosols; observed concentrations were specified for methane, N2O, the ozone-depleting substances (CFCs),and CO2. The fluxes are used by the snow-related parameterizations (Chapters [2.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Surface_Albedos/CLM50_Tech_Note_Surface_Albedos.html#rst-surface-albedos) and [2.8](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Snow_Hydrology/CLM50_Tech_Note_Snow_Hydrology.html#rst-snow-hydrology)).

4The nitrogen deposition rate is required by the biogeochemistry model when active and represents the total deposition of mineral nitrogen onto the land surface, combining deposition of NOy and NHx. The rate is supplied either as a time-invariant spatially-varying annual mean rate or time-varying for a transient simulation. Nitrogen deposition rates were calculated from the same CAM chemistry simulation that generated the aerosol deposition rates.

5Climatological 3-hourly lightning frequency at \\(\\sim\\)1.8° resolution is provided, which was calculated via bilinear interpolation from 1995-2011 NASA LIS/OTD grid product v2.2 ([http://ghrc.msfc.nasa.gov](http://ghrc.msfc.nasa.gov/)) 2-hourly, 2.5° lightning frequency data. In future versions of the model, lightning data may be obtained directly from the atmosphere model.

Density of air (\\(\\rho \_{atm}\\) ) (kg m\-3) is also required but is calculated directly from \\(\\rho \_{atm} =\\frac{P\_{atm} -0.378e\_{atm} }{R\_{da} T\_{atm} }\\) where \\(P\_{atm}\\) is atmospheric pressure (Pa), \\(e\_{atm}\\) is atmospheric vapor pressure (Pa), \\(R\_{da}\\) is the gas constant for dry air (J kg\-1 K\-1) ([Table 2.2.7](#table-physical-constants)), and \\(T\_{atm}\\) is the atmospheric temperature (K). The atmospheric vapor pressure \\(e\_{atm}\\) is derived from atmospheric specific humidity \\(q\_{atm}\\) (kg kg\-1) as \\(e\_{atm} =\\frac{q\_{atm} P\_{atm} }{0.622+0.378q\_{atm} }\\).

The O2 partial pressure (Pa) is required but is calculated from molar ratio and the atmospheric pressure \\(P\_{atm}\\) as \\(o\_{i} =0.209P\_{atm}\\).

Table 2.2.5 Land model output to atmospheric model[¶](#id19 "Permalink to this table")   
| Field
 | Variable name

 | units

 |
| --- | --- | --- |
| 1Latent heat flux

 | \\(\\lambda \_{vap} E\_{v} +\\lambda E\_{g}\\)

 | W m\-2

 |
| Sensible heat flux

 | \\(H\_{v} +H\_{g}\\)

 | W m\-2

 |
| Water vapor flux

 | \\(E\_{v} +E\_{g}\\)

 | mm s\-1

 |
| Zonal momentum flux

 | \\(\\tau \_{x}\\)

 | kg m\-1 s\-2

 |
| Meridional momentum flux

 | \\(\\tau \_{y}\\)

 | kg m\-1 s\-2

 |
| Emitted longwave radiation

 | \\(L\\, \\uparrow\\)

 | W m\-2

 |
| Direct beam visible albedo

 | \\(I\\, \\uparrow \_{vis}^{\\mu }\\)

 | 

 |
| Direct beam near-infrared albedo

 | \\(I\\, \\uparrow \_{nir}^{\\mu }\\)

 | 

 |
| Diffuse visible albedo

 | \\(I\\, \\uparrow \_{vis}\\)

 | 

 |
| Diffuse near-infrared albedo

 | \\(I\\, \\uparrow \_{nir}\\)

 | 

 |
| Absorbed solar radiation

 | \\(\\vec{S}\\)

 | W m\-2

 |
| Radiative temperature

 | \\(T\_{rad}\\)

 | K

 |
| Temperature at 2 meter height

 | \\(T\_{2m}\\)

 | K

 |
| Specific humidity at 2 meter height

 | \\(q\_{2m}\\)

 | kg kg\-1

 |
| Wind speed at 10 meter height

 | \\(u\_{10m}\\)

 | m s\-1

 |
| Snow water equivalent

 | \\(W\_{sno}\\)

 | m

 |
| Aerodynamic resistance

 | \\(r\_{am}\\)

 | s m\-1

 |
| Friction velocity

 | \\(u\_{\*}\\)

 | m s\-1

 |
| 2Dust flux

 | \\(F\_{j}\\)

 | kg m\-2 s\-1

 |
| Net ecosystem exchange

 | NEE

 | kgCO2 m\-2 s\-1

 |

1\\(\\lambda \_{vap}\\) is the latent heat of vaporization (J kg\-1) ([Table 2.2.7](#table-physical-constants)) and \\(\\lambda\\) is either the latent heat of vaporization \\(\\lambda \_{vap}\\) or latent heat of sublimation \\(\\lambda \_{sub}\\) (J kg\-1) ([Table 2.2.7](#table-physical-constants)) depending on the liquid water and ice content of the top snow/soil layer (section 5.4).

2There are \\(j=1,\\ldots,4\\) dust transport bins.

