Upload
ashly-janes
View
216
Download
1
Tags:
Embed Size (px)
Citation preview
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