doc: update CLM:update_T documentation

Author: jjokella

doc/content/setup_tsmp/input_enkfpf.md

@@ -472,9 +472,46 @@ CLM (standalone only).
 `CLM:update_T`: (integer) Flag for updating of CLM's ground and
 vegetation temperature.
 
--  0: No update of ground and vegetation temperature
+`0`
+: No update of ground and vegetation temperature
+
+`1`
+: Update of ground and vegetation temperature, based on
+  Kustas2009-observation operator
+
+`2`
+: Update of ground and vegetation temperature based on TSKIN in
+  observation operator
+
+`3`
+: Update of first-layer-soil-temperature and vegetation temperature
+  based on TSKIN in observation operator
+
+#### Kustas2009 observation operator
+
+Variable names in this subsection reflect the conventions from
+Kustas2009, not from CLM model.
+
+Eq(7) from Kustas2009
+(<http://dx.doi.org/10.1016/j.agrformet.2009.05.016>)
+
+{attribution="Kustas2009"}
+>\begin{align*}
+>T_R(\phi) \approx \left[ f_C(\phi) T_{C}^{4} + (1 - f_C(\phi)) T_{S}^{4} \right]^{\frac{1}{4}}
+>\end{align*}
+>
+> where $T_C$ is canopy temperature, $T_{S}$ is soil temperature, and
+> $f_C(\phi)$ is the fractional vegetation cover observed at the
+> radiometer view angle $\phi$. For a canopy with a spherical leaf
+> angle distribution and leaf area index $LAI$,
+>
+>\begin{align*}
+> f_C(\phi) = 1 - \exp\left(\frac{-0.5 \, \Omega \, LAI}{\cos\phi}\right)
+>\end{align*}
+>
+> where the factor $\Omega$ indicates the degree to which vegetation
+> is clumped as in row crops or sparsely vegetated shrubland canopies.
 
--  1: Update of ground and vegetation temperature
 
 ### CLM:print_swc ###