Combining Radar and Aircraft Observations to Investigate the Kinematic and Microphysical Properties of the 20 May 2011 Squall Line
Open Access
- Author:
- Elliott, Kyle A
- Graduate Program:
- Meteorology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 22, 2016
- Committee Members:
- Matthew Robert Kumjian, Thesis Advisor/Co-Advisor
- Keywords:
- radar
aircraft
meteorology
squall line
midlatitude
dual-wavelength ratio
reflectivity
correlation coefficient
Cloud Imaging Probe
Rear Inflow Jet
thermodynamics
microphysics
mesoscale
sublimation
aggregation
vapor deposition
MC3E - Abstract:
- Little is known about the microphysical structure of squall line precipitation in the mid and upper levels of the troposphere. With the evolution of dual-frequency approaches and dual-polarization radar data, an assessment of the microphysical properties above the melting layer has become feasible. A greater knowledge of the microphysical processes present in midlatitude, continental squall lines will allow cloud modelers to better represent these processes in numerical simulations. Conventional and dual-polarization radar data collected during the Midlatitude Continental Convective Clouds Experiment (MC3E) are analyzed in order to assess the evolution of the squall line that progressed across the Southern Great Plains on 20 May 2011. Thermodynamic and microphysical observations collected by the University of North Dakota (UND) Citation aircraft complement the radar analyses and allow for an assessment of the rear inflow jet’s effects on the storm, particularly its microphysical structure. Dual-polarization radar measurements of co-polar correlation coefficient (CC) values between 0.6 and 0.8 signify the presence of the melting layer between 3 and 4 km above ground level. These low values indicate a mixture of melting snowflakes and completely melted liquid drops within the layer. C- and Ka-band dual-wavelength ratios (DWRC,Ka) in the 3-5 dB range indicate the presence of ice crystal aggregates within the 3.5 – 5.5 km layer. Downward-relative radar reflectivity factor (Z) gradients < 5 dB km-1 near the -15 °C level argue against the presence of dendritic crystal growth. In-situ cloud particle images further suggest the absence of dendritic growth. Only small, roundish particles and a few aggregates appear in Cloud Imaging Probe (CIP) imagery, suggesting that vigorous vapor diffusional growth of crystals into nonspherical shapes was not present. Relative humidity values well below 100% with respect to ice in the ice-phase layer of the storm suggest sublimation, which can explain the hydrometeors’ roundish appearances. Near the back edge of the stratiform precipitation, westerly storm-relative zonal wind speeds as great as 15 m s-1 at mid-levels inject dry, ambient air into the storm. The appearance of this rear inflow jet is coincident with decreases in the relative humidity with respect to ice of an additional 10-30% and leads to an enhancement in sublimation. As a result of the sublimation, mid-level temperatures drop 1-3 °C. Because all but aggregational growth is inhibited in subsaturated conditions, particle size and number concentrations drop dramatically and lead to an erosion of radar precipitation echoes. Therefore, the rear inflow jet is seen to play a key role in the evolution of both the thermodynamic and microphysical environments in trailing stratiform precipitation.