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Exploring methods to extract vertical motions in winter storms

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Title: Exploring methods to extract vertical motions in winter storms
Author(s): Rosenow, Andrew
Advisor(s): Rauber, Robert M.
Department / Program: Atmospheric Sciences
Discipline: Atmospheric Sciences
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: M.S.
Genre: Masters
Subject(s): Winter storm wind profiler vertical motions radial velocity snow bands Profiling of Winter Storms (PLOWS) Profiling of Winter Storms lower bound method Rapid Update Cycling (RUC) potential instability Contoured Frequency by Altitude (CFAD) cloud radar
Abstract: Methods of retrieving vertical motions in wintertime cyclones are tested for the 915 MHz profiler, a profiling radar on the University of Alabama-Huntsville Mobile Integrated Profiling System, used during several winter cyclones observed during the Profiling of Winter Storms (PLOWS) project. First, the profiler radial velocity measurements from two storms are compared statistically to radial velocity measurements of the Wyoming Cloud Radar (WCR) aboard the NSF/NCAR C-130 using Contoured Frequency by Altitude Diagrams (CFADs). The WCR analysis shows a narrow spread in velocities with 1-2 ms-1 wide distributions in the lower stratiform portions of the cloud, and wider distributions with 3+ ms-1 spread in velocities in the top two km below cloud top where convective tops were observed. In the more convective region, a wider velocity distribution spanned the depth of the convective towers which were 7-8 km deep. The profiler shows similar presentation of the storms. However, the profiler did not sense the top 1-2 km of echoes that the WCR sensed, which implies the profiler lacks sufficient sensitivity to detect the cloud top convective region. This suggests that the profiler velocities are better suited to detect regions of deeper convection, rather than the cloud top convection common in the stratiform region. The first method to extract vertical air motions from the profiler, the lower bound method, uses Doppler spectra to calculate vertical air motions. The lower bound method indicated that for the cyclone that impacted the Midwestern US on 11-12 February, 2009, the warm side of the wraparound region has the strongest vertical motions, ranging from 1-6 ms-1, with magnitudes of 1-3 ms-1 in the cold side of the wraparound and warm frontal shield. However, examination of another cyclone from 8-9 December, 2009 shows that the profiler has significant side lobe return, making the lower bound method unsuitable for use. The second method first estimates reflectivity-weighted mean terminal velocities from ice particle spectra and then subtracts this mean terminal velocity from the measured radial velocity. The radial velocity is not sensitive to side lobes like the lower bound method, making this method more robust. This analysis shows a similar field of vertical motion as the lower bound method, but with slightly weaker peak vertical velocities of 4 ms-1. The strongest vertical motions are on the warm side of the wraparound region, with the deepest updrafts extending up to 5 km in depth. The vertical motions also show that in the warm frontal region as well as the cold side of the wraparound, almost all the upward air motion is within 1 km of cloud top. The vertical motions are then related to RUC profiles of equivalent potential temperature with respect to ice over the instrument site. These profiles were created from hourly forecasts from the model run prior to the arrival of the feature of interest. The time periods of deeper convection with vertical motions greater than 2 ms-1 corresponded to periods with potential instability.
Issue Date: 2012-02-06
Genre: thesis
URI: http://hdl.handle.net/2142/29696
Rights Information: Copyright 2011 Andrew Rosenow
Date Available in IDEALS: 2012-02-06
Date Deposited: 2011-12
 

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