Student Seminar: Ya-Chien Feng
Speaker 1: Ya-Chien Feng
Title: Better use of ground targets for radar refractivity retrievals
Abstract
Radar refractivity retrievals provide insight on the high-resolution distribution of near-surface moisture that is considered important to study convective and boundary layer processes. These retrievals are based on the fact that the phase of a stationary target changes with the refractivity index of air along the radar beam path. Accurate refractivity retrievals are critical for quantitative applications, such as assimilating refractivity into numerical models or radar network implementation. To obtain a good quality refractivity field at a given height above the terrain, it is necessary to know the vertical gradient of refractivity (dN/dh) between the radar and the target.
A new method for determining the vertical profile of refractivity is proposed based on the power pattern of selected point-like ground targets. The temporal evolution of the returned power from the targets records the diurnal variation of the dN/dh associated with the near-surface mixing conditions that affect the radar beam propagation. The resulting dN/dh estimation shows skill based on in-situ tower observation. The newly obtained information shall improve the original refractivity retrieval method, especially on complex topography.
Speaker 2: Bryn Ronalds
Title: On the relationship between North Atlantic baroclinic growth rate regimes and surface cyclogenesis
Abstract
Baroclinic instability is the primary mechanism for midlatitude surface cyclogenesis. A metric used to measure this is the baroclinic growth rate, which is a calculation of both the static stability and horizontal temperature gradients over the bottom layer of the troposphere (850-600 hPa). An areal coverage of the North Atlantic basin (25-60°N and 0-80°W) by high moist baroclinic growth rates (σm) was computed in order to identify large-scale ideal conditions for cyclogenesis, and a time series of the standardized anomalies was obtained. Concentrating solely on the cold season months (DJF), all synoptically independent events with standardized anomalies greater than two and length greater than four consecutive days were identified (n=7). It was found that each was associated with significantly anomalous cold thicknesses over a large portion of North America prior to the event, and warm thickness anomalies on the equatorward side of the jet during the event. Each event also had very deep low pressure centres over the northeast quadrant of the North Atlantic. Most of the cyclones within the basin deepened with a rate greater than 1 Bergeron, therefore fitting the definition of bombogenesis, and were located within the poleward exit region of an anomalously strong and zonally extended tropopause jet.