Vertical Structures of Precipitation in Cyclones Crossing the Oregon Cascades

Socorro Medina, Ellen Sukovich, Robert A. Houze
2007 Monthly Weather Review  
Analysis of the vertical structure of radar echoes in extratropical cyclones moving over Oregon from the Pacific Ocean indicates characteristic precipitation processes in each sector of the storms as they pass over the Cascade Mountains. During the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2), conducted in November and December 2001, sixteen consecutive shortwave baroclinic troughs passed over the Cascade Mountains
more » ... Cascade Mountains of western Oregon. This study is based on high-resolution continuous observations of these storms by a vertically pointing radar located on the windward slope of the Cascade range during IMPROVE-2. In the early sector of an extratropical cyclone, a Leading Edge Echo (LEE) region appears aloft and descends toward the surface. Updraft cells inferred from the vertically pointing Doppler radial velocity are often absent or weak unless the air undergoing ascent at upper levels is unstable. If instability exists, it does not affect the shape of the overall structure of the LEE but it does lead to the LEE region being occasionally populated by embedded convective cells aloft. In the middle sector, the precipitation is more intense and the radar echo comprises a thick vertically-continuous layer extending from the mountainside up to a height of ~5-6 km for several hours. When the middle sector passes over the windward slope of the Cascades, the vertical structure of the precipitation exhibits a Double Maximum Echo (DME). One maximum is associated with the radar reflectivity bright band. A second region of high reflectivity is located ~1-2.5 km above the bright band. The secondary reflectivity maximum aloft is not observed in the middle sector of the storm prior to its passage over the windward slope of the Cascades. Apparently, the secondary reflectivity maximum appears over the Cascade windward slope as a result of the interaction of the baroclinic system with the terrain in the form of a 3 mountain wave anchored to the crest of the range. A layer of updraft cells (> 0.5 m s -1 ) occurs in a region of turbulent motions located between the two reflectivity maxima which is thought to be important in enhancing the growth and fallout of precipitation over the windward slope of the Cascades. In the late (postfrontal) sector of the storm, the precipitation takes the form of generally isolated Shallow Convective Echoes (SCE). This region is characterized by low echoes tops and, in some cases, by upward motion near the top of the shallow cells. The individual precipitating cells observed in the SCE region become broader after interacting with the windward Cascade slopes, suggesting that orographic uplift in the postfrontal sector enhances the convective cells. In the SCE region the precipitation decreases very sharply over the leeward side of the Cascades. The three recognizable types of echo regions (LEE, DME, and SCE) have been found to recur from storm to storm during IMPROVE-2, with remarkable similarity. However, these echo regions do exhibit some variability in their detailed structure, apparently in association with variations in the large-scale environment.
doi:10.1175/mwr3470.1 fatcat:lnfybaslx5eaje4s2j2adych5a