[Seminar on Aug 12] Detection and Prediction of Atmospheric Persistent Ridges and Blocks
Dr. Ping Liu
Stony Brook University, USA
Room 1218, Building 3, IAP
10:00 am, Aug 12, 2019
Atmospheric persistent ridges and blocks are major causes to extreme weather events such as heatwaves and droughts. These systems are difficult to predict from onset to decay, partly because their detection is less satisfactory. Several popular approaches are compared, particularly in detecting blocks at 500-hPa geopotential heights Z. The comparison indicates that 1) time anomalies Z' by removing long-term climatologies deviate away from actual ridges and blocking highs and they are less-ideal base fields for detection, and 2) the traditional criteria for blocks in reversals of meridional Z gradients against a single latitude along each longitude (central blocking latitude or CBL) detect notably different blocking climatologies. A new approach using zonal anomalies Z* is developed for detecting persistent ridges. This approach further separates blocks by requiring reversals of Z gradients at any latitude inside a detected high component. The blocks newly detected in both longitude and latitude occur substantially more often than those detected before, and have clear seasonality and quasi-barotropicity in both the Northern and Southern Hemispheres. Their maximum intensity, moving speed and lifetime have similar log-normal distributions with some exceptions. This new approach is also applied to analyze the medium-range forecasts of persistent ridges in the NOAA GEFS ensemble prediction system. The presentation is based upon three collaborative papers in press or under review.