Impact of Graupel on the Structure and Bulk Microphysical Aspects of a Vigorous Narrow Cold-Frontal Rainband in 3-D Simulation Experiments

Narrow cold-frontal rainband (NCFR) convection is simulated with a three-dimensional anelastic time-dependent cloud model to investigate the impact of graupel on storm structure and domain-integrated bulk microphysical characteristics of the NCFR. The model adapts Kessler’s warm rain parameterization, and the three-class ice phase parameterization of Rutledge and Hobbs for other precipitation processes, but using a temperature-weighted diagnostic partitioning of cloud phase. Precipitating hydrometeors generally include rain, graupel and snow, each assumed spherical with inverse exponential size distributions.

Two comparative 168-min simulations are run, one retaining the full microphysics and the other excluding graupel. Both are initialized identically, closely adapting a rawinsounding taken prior to a vigorous central California NCFR on 5 February 1978, and superimposing a deep cold pool as forcing. Of particular interest are the various hydrometeor masses as well as the sources and sinks for the precipitating hydrometeors, each integrated over the domain volume, with main emphasis on the last hour of the simulations (108-168 min).

Both simulations quickly spin up a strong quasi-steady NCFR that propagates rapidly akin to a density current. This propagation increases slightly with time, as does the near-surface cooling behind the updraft, though the cloud top heights are modest and the updraft has very little thermal buoyancy. In a storm-relative frame, the updraft tilts rearward over the cold pool. Without graupel, both the updraft and downdraft extrema, especially the former, are modestly strengthened, and the near-surface cold pool enhancement takes longer to develop.

There are several qualitative microphysical similarities between the two NCFR simulations. Cloud contributes far less mass than precipitation does, cloud ice is over an order of magnitude less abundant than cloud water, the precipitation mass is much more ice than liquid, and rain is the second most abundant hydrometeor. Also, accretion of cloud water is a significant secondary source of rain, evaporation and accretion by snow are mutually comparable major rain sinks, conversion from cloud ice strongly dominates snow production, riming and deposition are appreciable secondary snow sources, and sublimation is a secondary snow sink.

However, the no-graupel NCFR simulation also differs significantly from the full-physics case. Total hydrometeor mass is over twice as large without graupel, though rain mass is somewhat less. The most abundant hydrometeor is snow rather than graupel, and much more lopsidedly so, contributing an order of magnitude more mass; this is largely because, with full physics, accretion of snow is by far the largest graupel source. The top source of rain is graupel melt in the full-physics NCFR, but snowmelt when graupel is absent. Excluding graupel reduces the dominance of conversion of cloud ice to snow, while considerably increasing the secondary roles of riming and deposition. With full physics, snow depletion is overwhelmingly due to accretion by graupel while sublimation is over 40 times less, but without graupel the snow is lost mainly by melting and its sublimation is only about three times slower than the snowmelt.