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Observations of Snow Crystal Type and Degree of Riming during Snow Events in the Southern Appalachian Mountains. Benjamin Sherwood, Department of Geography & Planning, Appalachian State University Baker Perry, Department of Geography & Planning, Appalachian State University. Introduction

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Observations of Snow Crystal Type and Degree of Riming during Snow Events in the Southern Appalachian Mountains

Benjamin Sherwood, Department of Geography & Planning, Appalachian State University

Baker Perry, Department of Geography & Planning, Appalachian State University

Introduction

Snow crystal type and degree of riming often vary considerably among different types of snow storms and even within the same storm due to various cloud microphysical factors that remain poorly categorized. Variations in snow crystal type and degree of riming are important influences on new snowfall density, a critical parameter in accurately forecasting snowfall. We categorize snow crystal type and degree of riming using snow microscopy field observations. Observations from a Parsiveldisdrometer are also used to identify periods of graupel associated with heavy riming of crystals. We also analyze surface and upper-air atmospheric conditions with available observations from several meteorological stations, upper-air soundings, and a Micro Rain Radar (MRR) vertically-pointing radar

Data and Methods

Riming and Graupel

Riming refers to “the capture and freezing of supercooled cloud droplets by falling snow crystals…the degree of riming is based on a visual rating of the rime coverage of the crystal surface area and expressed on a scale ranging from 0 (unrimed) to 5 (graupel).” (Mosimann et al., 1994).

Microscopy

Snow microscopy observations are used to classify snow crystal type throughout snow events. Snow crystals are classified based on crystal pattern as well as the degree of riming (in accordance with Mosimann et al., 1994) occurring on individual snow crystals throughout a snow event. These observations are plotted along with data from a Parsivel Disdrometer and Micro Rain Radar.

Parsivel Disdrometer

A Parsivel Disdrometer is a horizontally oriented laser that measures particle characteristics as they fall through the beam. This device is used to classify precipitation types based on individual particle size and fall speed. The device can also record precipitation intensity based on the amount of particles falling through the laser at any time throughout a storm. Precipitation types are recorded as synoptic codes. The code most relevant to this study is a value of 87 which indicates graupel.

Micro Rain Radar (MRR)

An “MRR is a vertically pointing microwave profiler for the measurement of rain rate, liquid water content and drop size distribution from near ground to several hundred metres.” For this project the MRR device was used to plot real-time cloud micro physical characteristics based on the reflectivity and falling velocity of particles in the atmosphere. The MRR works by measuring backscatter produced from the reaction of the “vertically pointing microwave” as it comes into contact with particles in the atmosphere (Biral). This data is stored by the device and is transformed into plots containing reflectivity in decibels (dBZ) as well the velocity of falling particles. The MRR plots were used as the backgrounds of plots created from the Parsivel data in order to show correlations between snow characteristics and cloud micro physical conditions.

Results

Microscopy observations tend to suggest that riming intensity is greatest at the beginning of snow events. Throughout storm events riming intensity tends to decrease. This phenomenon is also observed in data collected by the ParsivelDisdrometer throughout storms. The code for graupel (87) is most frequently observed at the beginning of snow events and tends to decline in frequency throughout snow events. Plots showing Parsivel, microscopy, and MRR data suggest a correlation between precipitation intensity and riming intensity. Times of increased precipitation intensity occur around times of increased riming and tend to decrease as riming increases.

Conclusion

As referenced from Mosimann et al., 1994 snow crystals collect supercooled cloud droplets as they fall through the atmosphere producing riming. A decrease in riming intensity would suggest a decrease in the availability of moisture in the atmosphere for riming to occur. This idea also supports the phenomenon that precipitation intensity decreases as riming decreases. Establishing relationships between atmospheric conditions (including cloud microphysical characteristics) and snow crystal type and degree of riming during snow storms is crucial to understanding the complexities of snow storms and improving their forecasting.

References:

Mosimann, Lukas, Ernest Weingartner, Albert waldvogel. "An anylsis of accreted drop sizes and mass on rimed snow crystals." Journal of the atmospheric sciences. 51.11 (1994): 1548-1558.

"Vertically pointing micro rain radar." Biral. Biral, n.d. Web. 17 Apr 2012. <http://www.biral.com/meteorological-sensors/precipitation-sensors/vertical-pointing-complete-micro-rain-radar>.

February 2-3, 2009

March 8, 2008

MRRdBZ

47.0

44.0

MRRdBZ

MRRdBZ

41.0

38.0

47.0

47.0

35.0

MRR dBz with Microscopy and Parsivel; Precipitation Intensity, SYNOP Code and Radar Reflectivity(dBz)

44.0

44.0

32.0

41.0

41.0

mm/h

29.0

38.0

38.0

26.0

mm/h

35.0

35.0

23.0

32.0

32.0

20.0

29.0

29.0

17.0

26.0

26.0

14.0

23.0

23.0

11.0

20.0

20.0

8.0

17.0

17.0

14.0

14.0

11.0

11.0

8.0

8.0

SYNOP Code of 87 = Graupel

SYNOP Code of 87 = Graupel

Riming Scale

MRRdBZ

47.0

44.0

41.0

38.0

35.0

32.0

29.0

26.0

23.0

20.0

17.0

14.0

11.0

March 1-2, 2009

8.0

February 26-28, 2008

Unrimed dendrite (0)

Graupel (5)

mm/h

mm/h

SYNOP Code of 87 = Graupel

SYNOP Code of 87 = Graupel

Riming Scale

Riming Scale