Definition of instruments for a cloud observing station.

1
Definition of instruments for a cloud observing
station.
2
Specification for cloud observing station.

• Fundamental instruments.
• Vertically pointing Dopplerised mm wave radar.
• Vertically pointing lidar.
• Multiple frequency vertically pointing microwave
  radiometer.
• Operating conditions.
• All weather. 24h/365days
• Resume reliable operation after rainfall.
• Unmanned but visits during the working week.
• Data from operational forecast model.
• Height of freezing level, T and P profile for
  categorisation.
• Humidity temperature profile - correct 94GHz
  attenuation.
• Horizontal wind (for turbulence) or local wind
  profiler?

3
Specification for cloud observing station.

• Fundamental instrument limits.
• Low level water clouds extinguish the lidar
  upper clouds not seen.
• Radar may not detect water clouds, or ice clouds
  with small Xls.
• Sampling during rainfall.
• Radomes wet during rain unreliable data -
  quantifiable attenuation.
• Response of normal rain gauges not rapid enough.
• Rainfall CAN be detected from the Doppler signal
  of the falling drops.
• To account for the lack of data during precip,
  when comparing with model clouds, the model must
  be sub-sampled to remove rain periods
• Use operational radars to get IWC from Z in
  precipitation.

4
Basic and Advanced Specification .

• Basic for CloudNET analysis cloud fraction
  and water content.
• mm wave Doppler radar.
• Ceilometer to detect water clouds.
• Dual frequency microwave radiometers for LWP.
• More advanced cloud station.
• High sensitivity mm wave Doppler radar
• Sensitive lidar.
• Microwave radiometer for LWP and humidity
  profiling.

5
Cloud radar frequency?

• 35Ghz
• Mature technology
• Low attenuation by oxygen, water vapour and
  liquid water clouds.
• Large insect return. Recognise using Doppler.
• 94GHz.
• New technology. Tube lifetime?
• Attenuation by O2 water vapour ? 2dB liquid
  1dB /100g/m-2.
• Low return from insects.
• 10Ghz (X-band)
• Low cost cheaper components.
• Sufficient sensitivity?

6
Performance of 35GHz tube.
Adjust drive voltage
1.5dB
Oct 05
Nov 04
7
Performance of three 94GHz tubes
gt10dB loss
?11 months
Conclusion Use 35GHz
8
Pulse coding
Z errors up to 1dB if target high Doppler and
high Doppler width. i.e only in ppn.
8bit pulse coding 10dB more sensitive than
single pulse. No range side lobes.
9
Radar sensitivity? Cabauw large antenna,
-55dBZ at 1km for 30secs and 60m. Sufficent?
STRATOCUMULUS always seen by lidar. 9200 hours
cumulative frequency. at 50dBZ still see only
56 of Sc.
BUT can always rely on simple lidar to see Sc.
10
Radar sensitivity ice clouds?Use molecular
backscatter from sensitive lidar to find optical
depth,want to see ? 0.05 is cloud detected
by radar?
Sensitivity 55dBz at 1km OK. Miss v few low ?
clouds
Sensitivity Z-45dBZ at 1km miss some clouds with
? 0.05
11
Radar sensitivity v cost.

• Sensitivity Z-45dBZ _at_ 1km is useful
• misses some high clouds.
• Sensitivity -55dBZ at 1km ideal
• This is achieved with commercial systems
  using
• coded pulse. Cost is about 500K euro.
• (magnetron or coded pulse??)
• Studies suggest FM/CW system would be cheaper.
• With careful design minimal range sidelobes.
• Sensitivity of 50dBZ at 1km for about 100,000
  euro?
• Needs to be studied further.
• X-band possibility.

12
LIDAR SENSITIVITY cloudnet algorithm only needs
lidar to see water clouds very easy with simple
system.
Cost 20,000 Euro

• MICROWAVE RADIOMETER
• Current systems need Tb to 0.5K 150,000 euro
• Calibration with black bodies, tip curves.
• Relax the specification - tolerate 5K drift in
  Tb?
• Must be reasonably stable during cloudy periods.
• What would be the price? 50,000 Euro??

13
BASIC SPEC radar (-45dBZ_at_1km)100K, ceilometer
20K radiometers
50K - total 170K-200K??
P, T, q and wind profiles from model or wind
from local profiler. Z in raining ice clouds
from operational radar At this price met
services could install a network, initially for
model evaluation, ultimately for data
assimilation.
14
Lidar high sensitivity How sensitive a
lidar to see thin high ice clouds? Several
available 532nm/ 10km molecular? 6 10-7 /m/sr
with snr 6 i.e. ? 10 7/m/sr
Comparison of fractional cloud cover inferred
from radar -45kdBZ at 1 km and sensitive lidar
(10 7/m/sr at 10km. SOLID line
observed Dotted line model NEED GOOD LIDAR TO
SEE ICE IN THE MODEL
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High Sensitivity Lidar.

• uv or green - sensitivity 10 7/m/sr at 10km.
• Sees thin ice clouds with low optical depth.
• Use molecular return for unambiguous optical
  depth.
• Also good for profiling aerosols
• direct optical depth of aerosols
• depolarisation channel
• for shape of ice crystals and aerosols
• Cost of new systems now being advertised
• weather proof
• Unmanned 150,000 euro?
• Need to demonstrate their performance in the
  field.

16
High Sensitivity Specification

• Radar 55dBZ _at_ 1km 500k
• Lidar 150k
• Profiling Radiometers 150k
• TOTAL 800k???
• SUITABLE FOR A FEW SPECIALISED SITES.