THE ERICA IOP-5 STORM .2. SENSITIVITY TESTS AND FURTHER DIAGNOSIS BASED ON MODEL OUTPUT

This paper continues the study of the ERICA IOP 5 storm begun in a com panion paper. The latter documented the storm development, utilizing b oth conventional and special observations, and presented the results o f a successful simulation of the storm by the Pennsylvania State Unive rsity-NCAR Mesoscale Model MM4. At 24 h into the simulation, the MM4 p redicted a central pressure of 984 mb, close to the observed value, wh ereas the Nested Grid Model (NGM) of the National Meteorological Cente r forecasted a depth of only 997 mb for the same hour. Here the result s of experiments designed to test the sensitivity of the development t o latent heating, surface energy fluxes, Gulf Stream position, and gri d size are first presented. A high sensitivity to latent heating and a moderate sensitivity to the other parameters are found. A comparison with other cases in the literature reveals that the sensitivity to lat ent heating, and to the fluxes, was unusually large. In view of this f inding, further diagnosis is made of the behavior of a number of moist ure-sensitive parameters in the model, namely, the potential vorticity (PV), the stability of the storm environment to vertical and slantwis e ascent, and the surface energy fluxes. The diagnosis revealed that ( i) large diabatically produced PV, capable of substantially impacting the storm intensity, appeared in the lower portion of the warm-frontal cloud mass, (ii) the storm environment was neutral or even unstable t o vertical ascent (near and ahead of the cold front) and to slantwise ascent (in and above the warm-frontal zone), and (iii) the movement of the storm near and parallel to the Gulf Stream allowed heated and moi stened air to be continuously ingested into the storm. In seeking clue s to the cause of the superior performance of the MM4, additional expe riments are carried out in which a Kuo-type convection scheme, such as employed in the NGM, replaces the Grell scheme used in the previous M M4 simulations. It is concluded that approximately 60% of the differen ce between the MM4 and NGM predictions can be accounted for by the uti lization of the Grell scheme and a finer grid (30 km vs 85 km) and tha t the effects of grid size and convective parameterization are highly coupled in this case. The remaining difference is attributed to other elements of the predictions that are not further investigated. An expe riment conducted on a slowly deepening ERICA storm (IOP 7) demonstrate s that, in this case at least, the MM4 shows no tendency to produce ex cessive deepening of ocean storms.