Classification of Low Level Jets at Cabauw

Fred Bosveld, Peter Baas and Henk Klein BaltinkKNMI

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 statisticshttp://www.knmi.nl/~bosveld -> research -> ERA40/RACMO2http://www.ecmwf.int/research/demeter/d/help/catalog/research/era/diagnostics/fluxes

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 1995 • 1994-1996 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

Hodograph for 19-20 aug 19961600 - 600

0

4

8

12

16

-16 -12 -8 -4 0

u [m/s]

v [m

/s]

200m311m426m

21

06

03

00

18

18

06

03

00

21

18

06

0300

21

Development LLJet 19-20 aug 1996 in time

0

400

800

1200

0 5 10 15 20

windspeed (m/s)

z (m

)

19:00

01:00

07:00

LLJ Occurence

Frequency of occurence

0

20

40

60

80

100

f1q1 f1q2 f1q3 f2q1 f2q2 f2q3 f3q1 f3q2 f3q3

class

%

2m/s

20%

LLJ Height

Distribution of height

0

100

200

300

400

500

f1q1 f1q2 f1q3 f2q1 f2q2 f2q3 f3q1 f3q2 f3q3

class

hei

gh

t [m

]

2m/s

20%

LLJ Veering

Turning of the jet compared to 10m wind

-20

0

20

40

60

80

f1q1 f1q2 f1q3 f2q1 f2q2 f2q3 f3q1 f3q2 f3q3

class

turn

ing

[°]

2m/s

20%

LLJ Strength

Speed of jet relative to v_geo (jetstrength)

0.0

0.5

1.0

1.5

2.0

2.5

f1q1 f1q2 f1q3 f2q1 f2q2 f2q3 f3q1 f3q2 f3q3

class

[-]

2m/s

20%

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