b4b: Documentation changes for issue #3722 (dewpoint temperature restriction on bare ground dew flux)

doc/source/tech_note/Fluxes/CLM50_Tech_Note_Fluxes.rst

@@ -603,6 +603,24 @@ where :math:`\Phi_{air}` (mm\ :sup:`3` mm\ :sup:`-3`) is the air filled pore spa
 
 where :math:`T_{1}` (K) is the temperature of the top soil layer and :math:`T_{f}` (K) is the freezing temperature of water (:numref:`Table Physical Constants`).
 
+In :eq:`5.67`, if :math:`q_{atm} - q_{soil} < 0`, then :math:`r_{soil}=0`. Furthermore, if :math:`q_{atm} - q_{soil} < 0` and :math:`T_{g} > T_{atm,\,dp}`, then :math:`E_{g}=E_{soil}=E_{sno}=E_{h2osfc}=0`. This limits sporadic large dew fluxes that can result in unrealistically high surface temperatures passed to the atmospheric model (The RRTMGP component in particular, which returns an error and stops the model if the surface temperature is greater than 355K).  See discussion beginning 12/30/2025 in https://github.com/ESCOMP/CTSM/issues/3589).
+
+:math:`T_{atm,\, dp}` is the dewpoint temperature at the forcing height (K) determined from Equation (7) in :ref:`Lawrence (2005) <Lawrence2005>`
+
+.. math::
+   :label: 5.81aa
+
+   T_{atm,\, dp} = \frac{B_{1} \ln(e_{atm}/C_{1})} {A_{1}-\ln(e_{atm}/C_{1})} + T_{f}
+
+where :math:`e_{atm}` is the vapor pressure at the forcing height (Pa) restricted to be no less than the value corresponding to 1% relative humidity
+
+.. math::
+   :label: 5.81ab
+
+   e_{atm} = \max(\frac{q_{atm} P_{atm}} {q_{atm}+0.622}, 0.01e_{atm,\, sat})
+
+where :math:`e_{atm,\, sat}` is the saturated vapor pressure at the forcing height (section :numref:`Saturation Vapor Pressure`).  Over liquid water (:math:`T_{g} >= T_{f}`), :math:`A_{1}=17.625`, :math:`B_{1}=243.04`, :math:`C_{1}=610.94` (:ref:`Lawrence (2005) <Lawrence2005>`), and over ice (:math:`T_{g} < T_{f}`), :math:`A_{1}=22.587`, :math:`B_{1}=273.86`, :math:`C_{1}=611.21` (:ref:`Alduchov and Eskridge (1996) <AlduchovandEskridge1996>`).
+
 The roughness lengths used to calculate :math:`r_{am}`, :math:`r_{ah}`, and :math:`r_{aw}` are :math:`z_{0m} =z_{0m,\, g}`, :math:`z_{0h} =z_{0h,\, g}`, and :math:`z_{0w} =z_{0w,\, g}`. The displacement height :math:`d=0`. The momentum roughness length is :math:`z_{0m,\, g} =0.0023` for glaciers without snow (:math:`f_{sno} =0) {\rm }`, and :math:`z_{0m,\, g} =0.00085` for bare soil surfaces without snow (:math:`f_{sno} =0) {\rm }` (:ref:`Meier et al. (2022) <Meieretal2022>`).
 
 For bare soil and glaciers with snow ( :math:`f_{sno} > 0` ), the momentum roughness length is evaluated based on accumulated snow melt :math:`M_{a} {\rm }` (:ref:`Meier et al. (2022) <Meieretal2022>`). For :math:`M_{a} >=1\times 10^{-5}`
@@ -651,20 +669,19 @@ The numerical solution for the fluxes of momentum, sensible heat, and water vapo
 
 #. The following system of equations is iterated three times:
 
-#. Friction velocity :math:`u_{*}`  (:eq:`5.32`, :eq:`5.33`, :eq:`5.34`, :eq:`5.35`)
+   #. Friction velocity :math:`u_{*}`  (:eq:`5.32`, :eq:`5.33`, :eq:`5.34`, :eq:`5.35`)
 
-#. Potential temperature scale :math:`\theta _{*}`  (:eq:`5.37` , :eq:`5.38`, :eq:`5.39`, :eq:`5.40`)
+   #. Potential temperature scale :math:`\theta _{*}`  (:eq:`5.37` , :eq:`5.38`, :eq:`5.39`, :eq:`5.40`)
 
-#. Humidity scale :math:`q_{*}`  (:eq:`5.41`, :eq:`5.42`, :eq:`5.43`, :eq:`5.44`)
+   #. Humidity scale :math:`q_{*}`  (:eq:`5.41`, :eq:`5.42`, :eq:`5.43`, :eq:`5.44`)
 
-#. Roughness lengths for sensible :math:`z_{0h,\, g}`  and latent heat
-   :math:`z_{0w,\, g}`  (:eq:`5.81a` , :eq:`5.81b` , :eq:`5.82`)
+   #. Roughness lengths for sensible :math:`z_{0h,\, g}`  and latent heat :math:`z_{0w,\, g}`  (:eq:`5.81a` , :eq:`5.81b` , :eq:`5.82`)
 
-#. Virtual potential temperature scale :math:`\theta _{v*}`  ( :eq:`5.17`)
+   #. Virtual potential temperature scale :math:`\theta _{v*}`  ( :eq:`5.17`)
 
-#. Wind speed including the convective velocity, :math:`V_{a}`  ( :eq:`5.24`)
+   #. Wind speed including the convective velocity, :math:`V_{a}`  ( :eq:`5.24`)
 
-#. Monin-Obukhov length :math:`L` (:eq:`5.49`)
+   #. Monin-Obukhov length :math:`L` (:eq:`5.49`)
 
 #. Aerodynamic resistances :math:`r_{am}` , :math:`r_{ah}` , and
    :math:`r_{aw}`  (:eq:`5.55`, :eq:`5.56`, :eq:`5.57`)
@@ -697,6 +714,8 @@ where
 
    \frac{dq_{g} }{dT_{g} } =\left(1-f_{sno} -f_{h2osfc} \right)\alpha _{soil} \frac{dq_{sat}^{T_{soil} } }{dT_{soil} } +f_{sno} \frac{dq_{sat}^{T_{sno} } }{dT_{sno} } +f_{h2osfc} \frac{dq_{sat}^{T_{h2osfc} } }{dT_{h2osfc} } .
 
+Note that :math:`\frac{\partial E_{g} }{\partial T_{g} } = 0` if :math:`q_{atm} - q_{soil} < 0` and :math:`T_{g} > T_{atm,\, dp}`. 
+
 The partial derivatives :math:`\frac{\partial r_{ah} }{\partial T_{g} }` and :math:`\frac{\partial r_{aw} }{\partial T_{g} }`, which cannot be determined analytically, are ignored for :math:`\frac{\partial H_{g} }{\partial T_{g} }` and :math:`\frac{\partial E_{g} }{\partial T_{g} }`.
 
 .. _Sensible and Latent Heat Fluxes and Temperature for Vegetated Surfaces:

doc/source/tech_note/References/CLM50_Tech_Note_References.rst

@@ -11,6 +11,10 @@ Aber, J.D., Melillo, J.M. and McClaugherty, C.A., 1990. Predicting long-term pat
 
 Aber, J.D., Goodale, C.L., Ollinger, S.V., Smith, M.-L., Magill, A.H., Martin, M.E., Hallett, R.A., and Stoddard, J.L. 2003. Is nitrogen deposition altering the nitrogen status of northeastern forests? BioScience 53:375-389.
 
+.. _AlduchovandEskridge1996:
+
+Alduchov, O.A., and Eskridge, R.E. 1996. Improved Magnus form approximation of saturation vapor pressure. J. Appl. Meteor. 35:601-609.
+
 .. _Alietal2016:
 
 Ali, A. A., C. Xu, A. Rogers, R. A. Fisher, S. D. Wullschleger, E. Massoud, J. A. Vrugt, J. D. Muss, N. McDowell, and J. Fisher, 2016: A global scale mechanistic model of photosynthetic capacity (LUNA V1. 0). Geosci. Mod. Dev., 9:587-606.
@@ -687,6 +691,10 @@ Lavigne, M.B., and Ryan, M.G. 1997. Growth and maintenance respiration rates of
 
 Law, B.E., Sun, O.J., Campbell, J., Van Tuyl, S. and Thornton, P.E. 2003. Changes in carbon storage and fluxes in a chronosequence of ponderosa pine. Global Change Biology, 9: 510-514.
 
+.. _Lawrence2005:
+
+Lawrence, M.G. 2005. The relationship between relative humidity and the dewpoint temperature in moist air. Bull. Amer. Meteorol. Soc. 86:225-234. DOI:10.1175/BAMS-86-2-225.
+
 .. _Lawrenceetal2007:
 
 Lawrence, D.M., Thornton, P.E., Oleson, K.W., and Bonan, G.B. 2007. The partitioning of evapotranspiration into transpiration, soil evaporation, and canopy evaporation in a GCM: Impacts on land-atmosphere interaction. J. Hydrometeor. 8:862-880.