The Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model version 4 (PSU/NCAR MM4) system shows that the simplified bucket method pioneered by Manabe (1969) to parameterize surface evapotranspiration (ET) has an apparent tendency to overestimate surface ET during nighttime and daytime due to (1) the inappropriate assignment of a parameter called moisture availability (M) in the method, and (2) the use of the saturation mixing ratio at the skin temperature as the surface mixing ratio when the long-term observational data from the Atmospheric Radiation Measurement (ARM) program are used for verification. It is also noted that the degree of overestimating latent heat fluxes decreases with the forecasting time. This is the so-called ¡¥spinup problem¡¦ that is common in many numerical models owing to the inadequate assignment of the initial skin temperature and the associated saturation surface mixing ratio.
A Penman-Monteith (PM) method of estimating potential ET is implemented into the modeling system and is shown to lead to a more reasonable estimation (less overestimation) of ET. The degree of overestimating or underestimating latent heat flux by the PM method is mainly controlled by the se.tting of stomatal resistance given a fixed M. Less surface evaporative cooling, as implied by the PM method, leads to a warmer skin temperature and, consequently, a stronger estimation of daytime sensible heat flux by the model. Compared with the bucket method, the PM method leads to a lower moisture supply from the model's ground surface; thus, there is less probability of low-level cloud formation. A more reasonable estimation of net radiation at the ground surface is then proven to be associated with the use of the PM method. This method restricts the moisture supply from the ground surface and enables the model to make a prediction of the amount and tendency of the mixing ratio at the lowest model level (about 40 meters above ground level), which is in more agreement with the corresponding observations.