Pj. Roebber et Lf. Bosart, THE SENSITIVITY OF PRECIPITATION TO CIRCULATION DETAILS - PART-I - ANANALYSIS OF REGIONAL ANALOGS, Monthly weather review, 126(2), 1998, pp. 437-455
In this paper, the sensitivity of precipitation associated with ordina
ry baroclinic continental winter storms to small variations in the syn
optic-scale circulation is explored using ensembles of observed region
al analogs. When seeking time-coherent analogs for periods of 1-2 days
in length on the basis of anomaly correlation scores at both the surf
ace and 500 hPa, the observed precipitation structures are strongly de
pendent on circulation details, reflecting subtle differences at both
the synoptic scale and mesoscale. It has also been found that when see
king time-coherent regional analogs on the basis of the observed preci
pitation distributions, such analogs cannot be identified. However, co
herent precipitation structures that are identified for shorter period
s (order 1 day) are found to occur in association with highly dissimil
ar circulations. Thus, the sensible weather that results from a partic
ular circulation is crucially dependent on the details of the how. The
authors believe that these findings provide some explanation for the
relatively slow advance of precipitation forecast skill in the face of
continuously improving forecasts of the large-scale circulation. The
study of the evolution of the base case and its closest circulation an
alog is split into two successive 24-h periods. During the first 24 h,
the substantial differences in the observed precipitation are linked
to the complex interplay between precipitation events prior to the per
iod of interest (which led to the generation of upwind sources of mois
ture) and synoptic-scale dynamics (primarily differences in the struct
ure and intensity of the subtropical jet, which led to slight deviatio
ns in the low level flow between the two cases and subsequent differen
ces in moisture advection from the Gulf of Mexico and moisture converg
ence). During the second 24 h, differences in the observed precipitati
on are tied to differences in the convective response despite similarl
y potentially unstable environments, reflecting differences in synopti
c and mesoscale triggering mechanisms. To more fully resolve the relat
ive importance of these mechanisms, experiments with a mesoscale numer
ical model will be reported in a subsequent paper.