NOTES AND CORRESPONDENCE: Polarimetric Radar Observation of Multiple Freezing Levels

Abstract

An unusual multiple freezing-level event observed with polarimetric radar during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiments (IMPROVE-2) field program is described. The event occurred on 28 November 2001 when a warm front moved over the Oregon Cascade Mountains. As the front approached, an elevated melting layer formed above a preexisting melting layer near ground. Continued warming of the lower atmosphere eventually dissipated the lower melting layer. The polarimetric measurements are used to estimate the height of the freezing levels, document their evolution, and deduce hydrometeor habits. The measurements indicate that when the two freezing levels were first observed melting was incomplete in the upper melting layer and characteristics of particles that passed through the two melting layers were similar. As warming progressed, the character of particles entering the lower melting layer changed, possibly becoming ice pellets or frozen drops. Eventually, the refreezing of particles ended and only rain occurred below the elevated melting layer. The Doppler radial winds showed a well-defined wind maximum apparently associated with a “warm conveyor belt.” The jet intensified and descended through the elevated melting layer with time. However, the increase in wind speed did not appear connected with melting or result in precipitation enhancement.