Airborne Doppler Observations of a Cold Front in the Vicinity of Vancouver Island

Cheng-Ku Yu Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

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Nicholas A. Bond Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington

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Abstract

This study uses airborne Doppler radar and flight-level measurements from the Coastal Observations and Simulations with Topography experiment to examine the detailed mesoscale structure of an oceanic cold front upstream of Vancouver Island on 13 December 1993. These aircraft observations show that there were dramatic differences in frontal structure and movement between nearshore and offshore regions, presumably due to the effects of blocking by the terrain. The aircraft observations are considered in two parts, since the behavior of the front evolved over the flight period. During the early and middle portion of the flight, the low-level flow east of the front was out of the south-southeast in the nearshore region, rather than southerly as found farther offshore. The nearshore segment of the front was oriented south–north and appeared to be stationary. The zone of orographic influence was found to extend to a region ∼20 km offshore and confined primarily to the lowest 1.5 km (MSL). In distinct contrast to the nearshore segment, the offshore segment of the front was oriented southwest–northeast and retrograded slowly northwestward. It exhibited more deep inflow and its associated slope was less steep. During the latter part of the flight, the offshore portion of the front moved eastward as is more typical of a cold front. Meanwhile, the nearshore segment of the front remained virtually stationary, and thus a significant distortion developed in the front. This distortion featured a local minimum in low-level convergence along the front and, hence, also reduced precipitation rates.

The structure and evolution of this front was related to the interactions between synoptic, orographic, and boundary layer effects. The low-level portion of the front did not resemble a classic gravity current in its structure or propagation. The frontal updraft at low levels appeared to be less due to the cooler air behind the front undercutting the warmer air, but rather more due to Ekman pumping, that is, frictional convergence, associated with the cyclonic vorticity concentrated at the front.

Corresponding author address: Dr. Cheng-Ku Yu, Dept. of Atmospheric Sciences, National Taiwan University, 61, Ln. 144, Sec. 4, Keelung Rd., Taipei, Taiwan. Email: yuku@hp735.as.ntu.edu.tw

Abstract

This study uses airborne Doppler radar and flight-level measurements from the Coastal Observations and Simulations with Topography experiment to examine the detailed mesoscale structure of an oceanic cold front upstream of Vancouver Island on 13 December 1993. These aircraft observations show that there were dramatic differences in frontal structure and movement between nearshore and offshore regions, presumably due to the effects of blocking by the terrain. The aircraft observations are considered in two parts, since the behavior of the front evolved over the flight period. During the early and middle portion of the flight, the low-level flow east of the front was out of the south-southeast in the nearshore region, rather than southerly as found farther offshore. The nearshore segment of the front was oriented south–north and appeared to be stationary. The zone of orographic influence was found to extend to a region ∼20 km offshore and confined primarily to the lowest 1.5 km (MSL). In distinct contrast to the nearshore segment, the offshore segment of the front was oriented southwest–northeast and retrograded slowly northwestward. It exhibited more deep inflow and its associated slope was less steep. During the latter part of the flight, the offshore portion of the front moved eastward as is more typical of a cold front. Meanwhile, the nearshore segment of the front remained virtually stationary, and thus a significant distortion developed in the front. This distortion featured a local minimum in low-level convergence along the front and, hence, also reduced precipitation rates.

The structure and evolution of this front was related to the interactions between synoptic, orographic, and boundary layer effects. The low-level portion of the front did not resemble a classic gravity current in its structure or propagation. The frontal updraft at low levels appeared to be less due to the cooler air behind the front undercutting the warmer air, but rather more due to Ekman pumping, that is, frictional convergence, associated with the cyclonic vorticity concentrated at the front.

Corresponding author address: Dr. Cheng-Ku Yu, Dept. of Atmospheric Sciences, National Taiwan University, 61, Ln. 144, Sec. 4, Keelung Rd., Taipei, Taiwan. Email: yuku@hp735.as.ntu.edu.tw

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