usefulness of vertical velocity measurements in clouds with a 1290 mhz profiler henk klein baltink...
TRANSCRIPT
Usefulness of vertical Usefulness of vertical velocity measurementsvelocity measurements
in clouds with a 1290 MHz in clouds with a 1290 MHz profilerprofiler
Henk Klein Baltink
Atmospheric Research Section
21 October 2002 Cloudnet Workshop KNMI 2
Quiet-air fall velocityspectral line
Spectral broadeningby turbulence2w
wi -w
wi +w
wi
Resulting spectrum
Mean vertical speed w
Problem: Velocity power spectrum <> DSD
21 October 2002 Cloudnet Workshop KNMI 3
DSD retrieval from single cloudradar spectrumGossard (et al) (1994,1997) Babb et al (1999), Klugmann (2001):
- use specific spectral shape in the small droplet range to estimate w
- cloud peak is at mean vertical speed
Babb et al (2000):
- double sided constrained inversion technique
21 October 2002 Cloudnet Workshop KNMI 4
1290 MHz Windprofiler vertical speed
Origin backscatter: Rayleigh vs. Bragg
• in rain: Rayleigh• scatter from higher levels (ice clouds): mainly Rayleigh• in drizzle/light rain: often both• in water clouds: (only?) Bragg• in clear-air: Bragg
21 October 2002 Cloudnet Workshop KNMI 5
Windprofiler operational acquisition cycle
• alternating low (100 m) and high mode (400 m)• per mode 1 vertical and 4 oblique beams• dwelltime per beam 25 sec.• spectral resolution 30 cm/s• not co-located with cloudradar• beam-width: 6°
quantative use of w and w limited?
21 October 2002 Cloudnet Workshop KNMI 6
Case: 27-July-2002
21 October 2002 Cloudnet Workshop KNMI 7
Backscatter profiler and CT75
21 October 2002 Cloudnet Workshop KNMI 8
Low mode vertical beam, CT75 Base
21 October 2002 Cloudnet Workshop KNMI 9
35 GHz Cloudradar Backscatter
21 October 2002 Cloudnet Workshop KNMI 10
Follow-up (?):
• assessment usefulness “w” on a case-by-case base • enhanced backscatter in water clouds: related to cloud geometry/top ?• use of co-planar oblique beams for estimation of w• optimize acquisition cycle (or use SA-technique ?)• discrimination between Rayleigh/Bragg scatter