East Asia and Western Pacific METEOROLOGY AND CLIMATE

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It is interesting to note that an initial impetus to develop the MM4 system in the mid
1970s was to produce an atmospheric and chemical model to study regional air pollution. It is
also worth noting that a version of the MM4-RADM system, the European Acid Deposition Model
(EURAD) has been adapted for the use in Europe (Ebel el a].. 1989). Hass et al.
(I989)successfully simulated the transport and depositing of radioactive material in Europe
following the Chernobyl accident using a version of EURAD.
3.0 SENSITIVITY STUDIES AND OBSERVING-SYSTEMS SIMULATION
A number of recent studies have examined the sensitivity of regional-scale, limited area
models to variations in initial conditions, lateral boundary conditions (LBC), and treatment of
physical processes. Anthes et al. (1989) analyzed results from 72-h simulations and forecasts for
12 cases to understand the contribution to model error or uncertainty introduced by these factors.
The simulations and forecasts were verified for both the 12 individual cases and for the ensemble
average of the 12 cases, using several objective measures of skill. The differences in these skill
scores were tested for their statistical significance.
The results showed that the use of observed LBC exerts a strong control on the growth of
errors over a domain size of 3600 x 4800 km. The errors showed little growth beyond about 36 h,
so that the 72-h simulations were nearly as accurate as the 36-h simulations. On these time and
space scales, the quality of the LBC were more important than any other factor tested in the
temporal evolution of the model errors. The results showed that the large-scale atmospheric
structure has a major effect on the evolution of small-scale features in the model.
Anthes e_t ah. (1989) introduced the concept of climatological use and verification of
regional models. The climatological skill of the control version of MM4 was quite good. The skill
scores of the ensemble average simulations were considerably better than the average scores of the
individual simulations. The model has small bias errors and the horizontal structure of the
model-simulated atmosphere is similar to the observed structure for scales of motion resolved by
the upper-air observational network over the United States.
In a related study, Warner et al. (1989) performed a series of observing-systems
simulation experiments (OSSEs) with MM4 in order to determine the effect of horizontal and
vertical data resolution, data location, and measurement error on mesoscale forecast accuracy. In
agreement with previous results, the error generally decreased with increasing forecast time.
Major contributors to the observed error reduction included nonlinear effects, geostrophic
adjustment and surface forcing, in addition to the use of identical LBC.
Although the studies by Anthes et aJL (1989) and Warner gt al. (1989) showed that small,
random errors in the initial conditions had relatively minor effect on the simulations, one should
not conclude that models are insensitive to changes in the quality or density of initial data. When
the initial data contribute information that improves the initial analysis in a horizontally and
vertically coherent manner, they can have a major positive effect on the forecast. For example,
Douglas and Warner (1987) showed that incorporating 110 temperature and humidity soundings
derived from the visible and infrared spin-scan radiometer sounder (VAS) into the MM4 produced
large changes in the analysis over the North Pacific Ocean, and that these changes resulted in a
positive impact on the forecast of cyclogenesis over the North Pacific.
A challenge of model initialization is to incorporate the divergence and vertical motion
fields into the model's initial conditions in a way that balances these fields with the diabatic and
other forcing in the model initial data. When precipitation exists at the initial time, the adiabatic
cooling associated with upward motion in the precipitation system is nearly balanced by the
diabatic heating associated with latent heat release. This balance suggests that observed
precipitation rates could be used in a dynamical initialization procedure to generate realistic
vertical motion and related divergence fields in the numerical model. Wang and Warner (1988)
tested a scheme to use observed precipitation rates in the initialization of MM4, the use of the