Classification of Low Level Jets at Cabauw Fred Bosveld, Peter Baas and Henk Klein Baltink KNMI Contents: Motivation Observations Definition of Low Level Jet Classification Conclusions

Motivation Relevance of Low Level Jet (LLJ) Stable boundary layer dynamics Aviation Transport of atmospheric constituents Bring observations to modellers Condens long-term observations at various sites into relevant statistics suitable for evaluation of atmospheric models.

Diurnal statistics -> research -> ERA40/RACMO2

Classification Night time wind veering 6 hours after sunset Wind speed classes: F200 < 5 m/s 5 < F200 < 10 m/s F200 > 10 m/s Iso thermal radiation: Qiso < 3 (cloudy) 3 < Qiso < 6 (half cloudy) Qiso> 6 (clear sky)

Low Level Jet Observations Wind profiler/RASS and 200 m tower Geowind from surface pressure network Here we use and 2001-today available

LLJ Definition Lowest wind maximum with above this level wind speed decrease: at least 2 m/s. at least 20 % Classification: Isothermal long wave cooling Geowind (0-5; 5-10; >10 m/s) Parameters: Occurence Height Strength Turning of wind Z U

Example

LLJ Occurence

LLJ Height

LLJ Veering

LLJ Strength

Conclusions General Generate statistics from observations for model evaluation Find relevant parameter classes –Time –Radiative cooling and surface pressure gradient Low level jets occurs at significant number of nights Typical heights from 100 m at low to over 300 m at high geowinds. Turning of the wind range from 25 to 40, at low geowinds even more. Strength decreases with increasing geowind. Future work Sensitivity of results on LLJ Criteria Application for other sites