Alternative methods for specification of observed forcing in single‐column models and cloud system models

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Abstract
We discuss alternative methods for prescribing advective tendencies in single‐column models (SCMs) and cloud system models. These include “revealed forcing,” in which the total advective tendency is prescribed from observations; “horizontal advective forcing,” in which the horizontal advective tendencies are prescribed, together with the observed vertical motion which is combined with the predicted sounding to determine the tendencies due to vertical advection; and “relaxation forcing,” in which the horizontal advective tendencies are computed by relaxing the sounding toward the observed upstream sounding, with a relaxation timescale determined by the time required for the wind to carry parcels across the grid column. When relaxation forcing is used, the horizontal advective tendencies can be diagnosed from the model output and compared with the corresponding observed tendencies. We present SCM results to illustrate these three forcing methods, based on data from several field experiments in both the tropics and the midlatitudes. Each method is shown to have its strengths and weaknesses. Overall, the results presented here do not show unambiguous differences between revealed forcing and horizontal advective forcing. The two methods appear to be generally comparable. Revealed forcing may therefore be preferred for its simplicity. Relaxation forcing guarantees realistic soundings of the state variables but can produce large errors in parameterized processes which are driven by rates (e.g., fluxes) rather than states. In particular, relaxation forcing gives large errors in the precipitation rate in this model. We demonstrate that relaxation forcing leads to unrealistically high (low) precipitation in versions of the model which tend to produce unrealistically dry (humid) soundings. The observed horizontal advective tendencies in the tropics are so weak, especially for temperature, that small absolute errors in the diabatic tendencies diagnosed with relaxation forcing can lead to large relative errors in the diagnosed horizontal advective tendencies.