Mesoscale banding is known to affect the timing, intensity and magnitude of snowfall associated with developing coastal cyclones off of the East Coast of the United States. While much attention has been given in the literature to single-banded systems, less is understood about the dynamics of multi-banded precipitation structures within the comma head of a coastal cyclone outside of theoretical work. The East Coast event of 26-27 December 2010 exhibited multi-banded structure in which 10 finescale bands were subjectively identified embedded within the mean flow moving towards a larger primary band. Other events, such as the 8-9 February 2013 event, exhibited a brief period of multi-banding that gave way to a very dynamic primary band. Questions remain about the dynamical, thermodynamical and thus microphysical processes that differed between the two observed precipitation structures and about the relative frequency and predictability of multi-banded cases relative to cases that solely exhibit single bands.
This work aims to answer the following fundamental questions regarding single and multi-bands in the comma head region of extratropical cyclones in the Northeast U.S.:
How do the multi-band intensity, spacing, and longevity depend on the basic ingredients for band formation (i.e. forcing for ascent, stability, and moisture)?
Are there any favored differences in environmental stability and methods of forcing for lift between multi-bands and single bands?
How well can a mesoscale model simulate multi-bands?
What are the roles of diabatic processes including latent heating (condensation/deposition/freezing) and cooling (evaporation/sublimation/melting) on the evolution of the thermal environment of single and multi-bands and are they the same?
One goal of this research is to create a multi-year dataset consisting of 50 cool season (October through March) banding events to have a dataset with high spatial and temporal resolution with which to examine the evolution of forcing and stability of the band environment for both single bands and multi-bands. With horizontal grid spacing on the order of 1 km, the life cycle of multi-bands will be investigated to complement previous work completed for single bands. By comparing data for both single bands and multi-bands, differences in the environmental stability and forcing can be determined. For example, it is hypothesized that multi-bands form as larger clusters of convection but become elongated parallel to the thermal wind to form bands and tend to aggregate into a single, larger band further downstream in a region of maximum low-to-mid-level deformation. Multi-bands may form in a region of greater instability than single bands and are the result of shallower mesoscale circulations, confined to the boundary layer, which may be the result of low-level convergence as compared to the deeper lower-tropospheric frontogenesis that is likely forcing the ascent resulting in the single band. Initially, vertical convection may generate cells which become more slantwise after latent heating may increase conditional symmetric instability that is released then released by the circulations.
Another goal of this research is to complete a process-oriented analysis using a mesoscale model run down to 400-m grid spacing for a subset of case studies to determine how multi-bands grow, develop and control their spacing. The role of generating cells aloft will also be investigated. How multi-bands impact any pre-existing single bands will also be determined. For example, Novak et al. (2009; 2010) determined that upstream convection/PV anomalies acted to weaken the pre-existing single band but if the pre-existing convection is multi-banded in nature and merges with the primary band it is unknown whether that would have the same dissipative effect.
Finally, the case studies will be used to determine the roles of diabatic processes including latent heating (condensation, deposition, freezing) and cooling (evaporation, sublimation, melting) on the evolution of single and multi-bands. Diabatic processes are necessary to maintain the convection associated with both single and multi-bands. The release of latent heat counteracts the stabilization of the band environment by the thermally direct frontogenetical circulation. The latent cooling of the environment may coincide with synoptic-scale cold air advection that may enhance the existing temperature gradient. During band formation, latent heating may play a larger role for multi-bands than for single bands because of the differences in forcing for ascent (i.e. weaker low-level convergence versus stronger low-to-mid-level frontogenesis).
Collaboration with the Cloud and Precipitation Processes and Patterns group at NCState headed by Dr. Sandra Yuter will investigate the microphysical evolution by using radar data from sites in the region especially Upton, NY (OKX) and vertically-pointing radar at Stony Brook University (SBU) as well as in-situ observations at SBU of crystal habit when available.
This study will provide a more focused look at understanding the small-scale nature and predictability of multi-bands within the comma-head of developing East Coast extratropical cyclones and shine light on their relative frequency and impact on cyclone precipitation evolution.