LM-PAFOG : three-dimensional fog forecasting with
the "Lokal Modell" of the German Weather Service

The presence of fog and low clouds in the lower atmosphere can have a critical impact on both airborne and ground transports.
High quality predictions of fog formation and dissipation, together with the associated changes in visibility,
would therefore be of immense operational value in the field. However, the many physical processes involved in fog formation interact in a complex and highly non linear fashion.
These interactions are not adequately resolved by current operational mesoscale models.
Numerical simulations require high horizontal and vertical resolutions combined with a sophisticated cloud microphysics.
A new microphysical parameterization based on the one dimensional fog forecast model,PAFOG (Bott & Trautmann, 2002) , was implemented in the “Lokal Modell” (LM), nonhydrostatic mesoscale model of german weather service (Steppler et al., 2003).
The implementation of cloud condensation nuclei as a new prognostic variable, into the dynamical core of LM, integrates the new microphysics into the three-dimensional frame. LM-PAFOG runs over a small, local area (100x100 pixels) with a horizontal resolution of 2.8km. The high vertical resolution is concentrated near the ground : In LM-PAFOG 25 of 35 levels are located in the first 2000 meters.
The current research involves fog events around the Lindenberg area (Germany) for the last quarter of 2005. The results have been compared with satellite data and measurements taken at the Observatory of the german weather service.

Reference :
Bott, A., Trautmann, T. (2002), PAFOG - a new efficient forecast model of radiation
fog and low-level stratiform clouds. Atmospheric Research, 64, 191-203.
Steppeler, J., Doms, G., Schettler, U., Bitzer, H.W., Gassmann, A., Damrath, U., Gregoric,
G. (2003) Meso-gamma scale forecasts using nonhydrostatic model LM’, Meteorol.
Atmos. Phy. 82, 75-96.

par M. Masbou (1,2), A. Bott (1)
(1) Meteorological Institute, University of Bonn, Germany (mmasbou@uni-bonn.de), (2)
Laboratoire de Meteorologie Physique, Blaise Pascal University, Clermont-Ferrand, France

Le vendredi 17 novembre 2006 – 14 heures

Salle de réunion du LaMP / Bt 5 de Physique

Structure and dynamic origin of boundary-layer convergence zones and convective initiation

During the warm season the daytime continental boundary-layer is marked by thermals, cores of buoyant, rising air. Aircraft and airborne radar data, collected in the central Great Plains of North America during the 2002 International H2O Project (IHOP), show that insects prefer to reside in thermals. Radar data were combined with gust-probe-derived vertical air motion data to show that insects tend to oppose the thermals' ascending motion, thus explaining the presence of radar-detected insect plumes. Sometimes these plumes aggregate to form
a single line, known as a radar 'fine-line'. Such fine-lines are about 1 km wide and sometimes over 100 km long. In clear-air mode, operational 10-cm radars are sensitive enough to see these fine-lines. At least in North America, forecasters now routinely monitor these lines as potential loci of deep-convection initiation.

Several radar fine-lines, all with a humidity contrast, were sampled in IHOP. This talk uses soundings, aircraft and airborne mm-wave radar observations to dynamically interpret the existence and vertical structure of these fine-lines as they formed within the well-developed, convective boundary layer. In all cases the fine-line represents a shallow convergence zone. This convergence sustains a sharp contrast in humidity, and usually in potential temperature, across the fine-line. The question addressed herein is whether, at the scale examined here (~10 km), the airmass contrast itself, in particular the horizontal density (virtual potential temperature) difference and resulting solenoidal circulation, is responsible for the sharp definition of the boundary and the sustained convergence, leading to a radar fine-line.

par Bart Geerts

Department of Atmospheric Science
University of Wyoming, USA

Le jeudi 9 novembre 2006 – 14 heures
Salle de réunion de l’OPGC / Bt 8 de Physique

Weather Radar Research Applications and some Operational Considerations

Meteorological radars are finding more applications as their use continues to expand. I will give an overview of radar derived precipitation measurement techniques, compare stratiform rainfall melting level radar data with an aircraft 2D Cloud probe example, show radar detection of an Icelandic volcano eruption, and describe how radar can measure surface level water vapor as a complement to profiles derived from GPS measurements. I will finish with a brief description of the two weather radar networks - the US NEXRAD and the French ARAMIS networks.

Dr. R. Jeffrey Keeler
rjkeeler@ieee.org
r-J-K Consulting, LLC
Tel/Fax: 303-444-7675
1112 Maxwell Ave Mobile: 303-579-1043 Boulder, Colorado
USA 80304
Le vendredi 20 octobre 2006 – 14 heures Salle de réunion de l’OPGC / Bt 8 de Physique