Mesoscale & Microphysical Structure of Winter Storms over the NEUS

A Climatology of Cool-Season Precipitation Structures Within the Cyclone Comma Head

ID-PRO demo for 12 Feb 2006

The figure above shows the reflectivity (dBZ) valid 1621 UTC on 12 February 2006. (a) Overlapping windows of 200×200 km boxes (gray boxes; single red box shown for emphasis) and 100 km step sizes (purple arrow) overlaid. (b) Background field of thresholds, calculated using the upper sextile within the overlapping windows from (a). (c) Binary field (black contours) isolated by ID-PRO, without applying image morphology. (d) Morphological thinning and image opening applied to (a). (e) Regions above the size threshold from (c) highlighted. (f) Resulting objects (blue “X”) and dotted fitted ellipses.

  • IDentification of PRecipitation Objects (ID-PRO) Algorithm
  • Highlights locally enhanced regions of reflectivity
  • Not limited to the previous definitions of primary band and multiband
  • New climatology of precipitation structures in a cyclone-relative frame
  • Characteristic environments of these precipitation structures

Winter storms have a variety of precipitation structures ranging from cells to banded precipitation within the cyclone comma head. Much of the previous research has explored primary snowbands, but there has been little investigation of the broader spectrum of snowband structures that may not fit the past definitions of primary or multibands. The current conceptual model of precipitation bands as described in Houze (2014) also does not account for findings from more recent field campaigns, and does not link this larger spectrum of structures to the various environmental conditions including lifting mechanism, stability, moisture, and shear.

A new algorithm, the IDentification of Precipitation Objects (ID-PRO), has been developed to identify precipitation structures in extratropical cyclones, building on work done by Ganetis et al. (2018). This algorithm divides a larger domain into several smaller boxes and computes the upper sextile reflectivity in each box, to focus on local enhancements of reflectivity. Composite radar over the Northeast U.S. and adjacent coastal waters from 1996-2023 is used for the analysis. ERA-5 is used to obtain surface cyclone tracks as constructed by Laura Tomkins (from NC State University) using the hourly sea-level pressure field. Both radar object data and ERA-5 analysis fields are re-gridded relative to the cyclone center to produce cyclone-centered composites. Results are then separated by the orientation of the cyclone track, and also by the 12-hour deepening rate.

Results show that a locally defined threshold improves object detection by separating large, amorphous regions and picking up several lower reflectivity objects, compared to older methods in the literature, although there is some sensitivity to thresholds chosen. The distribution of precipitation objects does not appear to be separated into distinct distributions of primary or multibands, as the histograms of area, length, or width resemble an exponential curve. Nevertheless, band-like objects most frequently occur northeast of the cyclone center for storms with a more zonal track, or north-northwest of the cyclone center for storms with a more meridional track. Larger bands are coincident with mid-level frontogenesis and slantwise instability, while upright instability is more common in regions of more amorphous bands.

Future projects could involve:

  • Subsetting the dataset to investigate snowfall.
  • Applying the algorithm to numerical model simulations to verify model depictions of precipitation objects.
  • Investigating precipitation objects in a different region of the United States.
ID-PRO conceptual model 1996-2023

Conceptual model of precipitation objects (a) for zonal-track cyclones, (b) meridional-track cyclones, and (c) deep-meridional cyclones, shaded by AR according to legend. The number of ellipses is roughly proportional to the relative frequency of objects for each cyclone type.

Presentations

Yeh, P. and B.A. Colle, 2024: A Comparison of Precipitation Objects in Midwest Cyclones during IMPACTS, AMS 1st Symposium on Cloud Physics, 28 Jan to 1 Feb 2024, Baltimore, MD.

Yeh, P. and B.A. Colle, 2023: Evolution of Multiband Structures Within the 16 February 2023 Midwest Cyclone During the IMPACTS Field Campaign, AMS 32nd Conference on Weather Analysis and Forecasting, 17-21 July 2023, Madison, WI.

Students Involved

Phillip Yeh (2019 – 2024) contact at phillip.yeh@stonybrook.edu

Multi-banding within the Comma-head of Northeast U.S. Extratropical Cyclones

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.

Research Website: http://flurry.somas.stonybrook.edu/band_study/index.html

Observed (upper-left) and simulated reflectivity for WRF simulations using the same physics but with different initial conditions that led to variances in simulated precipitation structures. The NARR-initialized WRF using YSU PBL and Morrison microphysics (upper-right) resolved multi-bands better than a whole suite of other configurations.
Cross section of observed (upper-left) and simulated reflectivity, saturation equivalent potential temperature, and circulation vectors through the multi-band environment for the 26-27 Dec 2010 case.

Presentations

Ganetis, S. and B. Colle, 2015: Evaluation of WRF Simulated Multi-bands over the Northeast U.S. Using Varied Initial Conditions and Physics, Northeast Regional Operational Workshop XVI, 4-5 November 2015, Albany, NY.

Ganetis, S., B. Colle, S. Yuter, N. Hoban, N. Corbin, 2015: Simulations of Multi-bands in the Comma Head of Northeast U.S. Winter Storms, AMS 16th Mesoscale Conference, 3-6 August 2015, Boston, MA.

Ganetis, S. and B. Colle, 2015: Evaluation of WRF Simulated Multi-bands over the Northeast U.S. Using Varied Initial Conditions and Physics,
AMS 27th Weather Analysis and Forecasting and 23rd Numerical Weather Prediction Conferences, 29 June – 3 July 2015, Chicago, IL.

Ganetis, S., B. Colle, S. Yuter, N. Corbin, 2014: Observed and Simulated Multi-bands in Northeast U.S. Winter Storms, Regional Operational Workshop XV, 12-13 November 2014, Albany, NY

Students Involved

Sara Ganetis (2013 – 2017) contact at sara.ganetis@stonybrook.edu

The Thermodynamic and Microphysical Evolution of an Intense Snowband during the 8-9 February 2013 Northeast U.S. Blizzard

The Northeast U.S. extratropical cyclone of 8 – 9 February 2013 exhibited blizzard conditions and an intense snowband to the north and west of the surface cyclone center. The snowband occurred in three distinct phases during its > 12 h lifetime. Phase 1 is defined as the time during which the band developed in an environment along a mixed-phase transition zone in an area of strong low-to-midlevel frontogenesis and pivoted over central Long Island and southern Connecticut where it remained quasi-stationary. During this phase, the environment around the band cooled to < 0°C except for a 200-hPa shallow layer approximately 75 km wide by 200 km long coincident with the snowband that remained above-freezing for > 5 h. Phase 2 is defined as the time during which the band exhibited heavy snowfall rates exceeding 7.5 – 10 cm/h with very large and varied hydrometeors. The above-freezing layer altered the microphysical character of the snowband such that radar reflectivity of > 55 dBZ was observed over central Long Island. Ground observations from the nearby Stony Brook University corroborated the dual-polarization measurements to conclude that the intense band was associated with large, aggregated hydrometeors likely growing via wet-growth processes in strong updrafts. Within 1 h, the intense and varied snowband appeared to weaken to ~ 35 dBZ and was characterized by less dense, similar hydrometeors indicative of a colder environment while still maintaining heavy snowfall rates (6.5 – 6.7 cm/h) which constituted phase 3.
KOKX reflectivity (shaded according to scale, dBZ) for (a) 2129 UTC 8 Feb, (b) 0042UTC 9 Feb, (c) 0340 UTC 9 Feb. KOKX correlation coefficient (ρhv) (shaded according to scale) for (d) 2129 UTC 8 Feb, (e) 0042 UTC 9 Feb, (f) 0340 UTC 9 Feb. KOKX differential reflectivity (dB, shaded according to scale) for (g) 2129 UTC 8 Feb, (h) 0042 UTC 9 Feb, (i) 0340 UTC 9 Feb.
(a) WRF simulated reflectivity (dBZ, shaded according to scale) and (b) KOKX observed reflectivity (dBZ, shaded according to scale) time-height diagram at SBU from 1500 UTC 8 Feb – 0800 UTC 9 Feb. (c) Field observations at SBU of the microphysical evolution of ice habit (shaded vertical bars), riming (mean: solid, high: dashed, low: dotted-dash) from 1830 UTC 8 Feb – 1030 UTC 9 Feb 2013. The time of each observed phase is indicated along the bottom of the image.
The thermodynamic and microphysical evolution of this snowband has been examined to determine the dominant processes responsible for maintaining the layer of above-freezing temperatures in the vicinity of the snowband. The Weather Research and Forecasting (WRF) model was used to analyze the temperature evolution. Trajectory analysis was completed by comparing backwards trajectories launched from three thermally distinct points, one within the warm region of the snowband and two in the colder environment to the west and east of the band. The trajectory that terminated within the warmer snowband environment underwent rapid ascent during which condensation and deposition warmed the air more relative to the other trajectories before undergoing rapid descent just downwind of the band environment. A potential temperature budget revealed that although the contributions of vertical advection and diabatic heating should counteract one another, the magnitude of the vertical advection term was larger than the diabatic term which supported a warmer layer coincident with strong subsidence just downstream of the band. Finally, sensitivity tests were performed in which the temperature tendency contributions from diabatic heating or cooling were excluded separately from the model integration starting at the end of phase 1. It was found that the latent cooling did contribute in conjunction with large-scale cold air advection to cool the above-freezing layer to support the change in microphysical character of the snowband and latent heating maintained the temperature gradient, instability and frontogenesis affecting the longevity of the band.

Publications

Ganetis, S.A, B.A. Colle, 2015: The thermodynamic and microphysical evolution of an intense snowband during the Northeast U.S. blizzard of 8-9 February 2013. In press to Mon. Wea. Rev.

Presentations

Ganetis, S., B.A. Colle, M.J. Sienkiewicz, D.S. Schultz, P. Heinselman, D.R. Novak, J. Picca (talk, 2013), Examination of the Thermodynamic and Microphysical Evolution of the Northeast Blizzard of 8 – 9 February 2013, Northeast Regional Operational Workshop XIV, 10-11 December 2013, Albany, NY.


Ganetis, S., B.A. Colle, M.J. Sienkiewicz, D.S. Schultz, D.R. Novak, P. Heinselman, J. Picca (talk, 2013), Examination of the Thermodynamic and Microphysical Evolution of the Northeast US Blizzard of 8-9 February 2013, NOAA/NWS New York, NY Winter Weather Workshop, 4 December 2013, Upton, NY.

Ganetis, S., B.A. Colle, M.J. Sienkiewicz, D.S. Schultz, D.R. Novak, P. Heinselman, J. Picca (talk, 2013), Evolution of an Intense Mesoscale Snowband During the 8-9 February 2013 Northeast U.S. Blizzard, 15th Conference on Mesoscale Processes, 6-9 August 2013, Portland, OR. 

Students Involved

Sara Ganetis (2013 – 2017) contact at sara.ganetis@stonybrook.edu