Cold air surge at the surface: 2 November 2011

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Ano 2nov2011 SOLO VT.png

Positive (outbound) radial velocities of ~15 mps are associated with enhanced northerly low-level flow following a cold frontal passage. The arrow is located just above the leading edge of this stronger flow. Time lapse loops of the progress of this cold air surge have been prepared.

Contents

Introduction

Northeasterly low-level upslope flow increased at the CHILL radar site following a cold frontal passage during the afternoon hours of 1 November 2011. Mid-level clouds thickened and lowered by 00 UTC; precipitation began reaching the surface at the radar between 00 and 01 UTC on 2 November. This precipitation began as rain, but became increasingly mixed with wet snow after 01 UTC. By the following morning, about 6 inches of snow had accumulated at the CHILL site.

Radial velocity loop

The following series of RHI scans were taken on a programmed azimuth angle of 200 degrees. The low level northerly flow produced positive (outbound) radial velocities in a surface based layer. A localized surge in this northerly flow is marked by an arrow in each image frame. Above the surface layer, the southerly synoptic flow generated negative radial velocities. Near echo top, this inbound flow exceeded the radar's Nyquist limit and velocity aliasing occurred.


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Reflectivity loop

Images from the corresponding reflectivity loop are shown below. The reflectivity structure in the RHI plane indicates that precipitation originated aloft in the southerly synoptic flow layer. The trajectories of the precipitation particles curved as they descended from the stronger southerly flow in the upper levels. Reflectivity maximized in a thin layer near the surface where wet, melting snow particles existed.


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Linear Depolarization Ratio (Ldr) data

The two following plots show the Ldr data at the start and end of the times included in the above image lops. The data have been thresholded at a cross-polar signal level of -102 dBm to remove noise contamination. Water coated / soaked melting snow particles typically generate enhanced Ldr levels in excess of -20 dB (Kowalewski and Peters, JTEC 2010, p1555-1561). During the 46 minute time period spanned by the Ldr plots, the wet snow area shrank in depth and moved off towards the south. This is in agreement with the overall transition from rain through a wet rain / snow mixture to a pure, accumulating overnight snowfall that was observed to occur at the radar. As shown by these plots, important structures in the velocity and hydrometeor phase fields can exist at near-surface heights in cold season precipitation. Since radar beam height increases with range, these shallow features cannot be resolved at moderate to long ranges from the radar.

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References