Hylke de Vries Post doc at KNMI, Global Climate office: B3.42 PO BOX 201, 3730 AE De Bilt, The Netherlands T: +31-302206-559 E: hylke.de.vries -at- knmi.nl Welcome. I work as a postdoc at KNMI. My main subject of research is currently midlatitude atmospheric blocking and its impact on extreme winter temperature (see examples below). Collaborators at KNMI are Rein Haarsma, Wilco Hazeleger and Wim Verkley and Tim Woollings from University of Reading. Other scientific areas I am interested in include baroclinic waves and potential vorticity dynamics, to which blocking appears to be intricately related (see below). In the past I worked at Reading University with John Methven, Brian Hoskins and Tom Frame trying to extend the counter-propagating Rossby wave (CRW) perspective on baroclinic instability. Last update: 28/02/2012

Winter example Example of midlatitude winter blocking situation. Shown are geopotential height in contours and temperature anomalies in shading. The geopotential height contours clearly resemble an Omega structure. Temperature advection associated with the blocking leads to significantly higher than usual temperatures to the West of the high pressure (over Greenland), and colder than usual conditions over much of the continent.

Summer example Example of summer blocking situation that lasted for many weeks throughout july 1976 and led to the driest july on record. The temperature response appears to be less extreme than in winter, with the (anomalously) high temperatures occurring right under the high pressure. The tripole-like structure of the geopotential height is well known in idealized fluid dynamical (numerical) experiments to be meta-stable. Also the theory of "modons" may be very relevant here.

Potential vorticity dynamics of midlatitude blocking In order to understand more of the dynamics of atmospheric blocking it can be helpful to track features that are prominently visible in mature blocks. One of these are large-scale negative potential vorticity (PV) anomalies. In the animation below, I've created a tracking algorithm to follow negative PV anomalies (shading). The coloring of the contours indicates whether sufficient thresholds for amplitude, size and persistence have been exceeded.