evaporation or condensation, respectively. In order for latent heat to be present, there

must be a vapor gradient between the surface and the atmosphere. It is formulated as

(Kahle 1977, Balick et al. 1981b)

(4.8)

qg is the mixing ratio of the air at the surface, and qa is the mixing ratio of the air at 2.0 *m*,

and *W *is the corrected windspeed (*m/s*). If *W *is below 2.0 *m/s *then *W = 2.0 m/s *else

the turbulent Schmidt number, *r*ch, is replaced by the turbulent Prandtl number, *r*ce (0.71),

calculating the bulk Richardson number in Equation (4.7). Balick et al. (1981) give the

latent heat of evaporation as

⎡*T *- *T*

⎤

- 273.15⎥ .

(4.9)

⎣ 2

⎦

The mixing ratio *q*g = *M * g qs (*T *) with *M*g the moisture factor ( 0 ≤ *M * g ≤ 1 ) and *q*s the

saturated mixing ratio. The value assigned to the moisture factor depends on the degree

of saturation of the soil. If it is raining, *M * g = 1 otherwise it is equal to the surface soil

moisture content (refer to Chapter 7). Balick et al. (1981) quantify the mixing ratios as

0.622*e*d

(4.10)

⎡17.269(*T*a - 273.15) ⎤

⎥

⎣

⎦

where *e*d is the vapor pressure (*Pa*) and *RH*s,a is the relative humidity. *RH*s = 1.0 and

⎧ 17.269

⎩21.8745 *over ice */ *snow*

⎧35.86

⎩ 7.66 * over ice */ *snow*

Jordan (1991) quantifies the precipitation heat flux as

(4.11)

3600(sec/ *hr*)

32