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

Ensemble simulations of explosive cyclogenesis are examined in a lengthy ru n 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 starting from s lightly perturbed initial conditions. The perturbations are randomly select ed to represent equal approximations to the truth, given typical analysis d ifferences between major centers. Ensembles are generated starting two, thr ee, four, and five days prior to maximum depth. Two lows are contrasted, th e deepest low near Kamchatka and a marginally explosive low over the centra l Pacific. The early development of both systems was suppressed by their presence in t he confluent entrance region of the Pacific winter jet. An intense low near Kamchatka eventually developed in each member of the ensemble at all proje ctions, but the details of development varied from member to member and wer e related to the involvement of a surface perturbation coming up into the s ystem from low latitudes. In contrast, cyclogenesis over the central Pacifi c occurred in some members of the ensemble but not at all in others. The di fference in behavior of the two systems is reflected in a localized enhance ment of the error growth of the planetary and synoptic scales for the centr al Pacific low and is related to the smaller horizontal scale of the centra l Pacific low. Probabilistic estimates of precipitation quantity and surface wind speeds p roduced by the ensemble showed moderate skill at day 5 with respect to clim atology, mainly away from the regions of most vigorous synoptic activity, w hen 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.