Ensemble simulations of explosive cyclogenesis at ranges of 2-5 days

Abstract

Ensemble simulations of explosive cyclogenesis are examined in a lengthy run of a global general circulation model with the perfect ensemble context. Attention is focused on the day when the deepest low appeared. An ensemble of 31 members is obtained by integrating 30 additional runs strating from slightly perturbed initial conditions. The perturbations are randomly selected to represent equal approximations to the truth, given typical analysis differences between major centers. Ensembles are generated starting two, three, four, and five days prior to maximum depth. Two lows are contrasted, the deepest low near Kamchatka and a marginally explosive low over the central Pacific. The early development of both systems was suppressed by their presence in the confluent entrance region of the Pacific winter jet. An intense low near Kamchatka eventually developed in each member of the ensemble at all projections, but the details of development varied from member to member and were related to the involvement of a surface perturbation coming up into the system from low latitudes. In contrast, cyclogenesis over the central Pacific occurred in some members of the ensemble but not at all in others. The difference in behavior of the two systems is reflected in a localized enhancement of the error growth of the planetary and s synoptic scales for the central Pacific low and is related to the smaller horizontal scale of the central Pacific low. Probabilistic estimates of precipitation quantity and surface wind speeds produced by the ensemble showed moderate skill at day 5 with respect to climatology, mainly away from the reginos of most vigorous synoptic activity, when verified against individual ensemble members. Skill would be reduced if the ensemble mean proved to be more seriously in error as is the case for a forecast verified against observations.