Remote-sensing profiling instruments in Helsinki Testbed

Jani Poutiainen / FMI with Hannu Talvitie / Vaisala • • • • • • Wind profiler RASS Sodar Ceilometers Doppler lidar POSS • Not included: weather radars 12.2.2007

• • • • • • • • • •

Wind Profiler at Malmi Airport

LAP-3000 wind profiler (8/2005-11/2006) RASS for virtual temperature profiling (10/2005 11/2006) Remote access from Vaisala (Boulder and Vantaa) and FMI Pulse Doppler RAdio Detection And Ranging device Transmission pulse of electromagnetic energy with target backscattering.

The profiler computes height by using the time interval between transmission of the pulse and reception of the return signal. Wind speed and direction are obtained with Doppler principle, i.e. wave will shift in frequency because of the motion of the target relative to the observer.

Maximum backscattering power occurs when atmospheric irregularities are about half the size of the radar wavelength (Bragg scatter).

– perturbance in refractive index – backscatter from hydrometeors Local horizontal uniformity of the wind field is assumed.

For detailed info, see: http://testbed.fmi.fi/docs/Profiler-info.html

VAISALA LAP3000 SPECIFICATIONS

Operating frequency 1290 MHz Minimum height Maximum height Range resolution 75-150m 2-5km 60, 100, 200, 400m; Testbed configuration 102m Wind speed accuracy Wind direction accuracy Averaging time RF power output Antenna Aperture Direction Beamwidth MTBF <1 m/s <10 ° 3-60 minutes 600 W peak Electrically steerable micropatch phased-array panels 2.7 m 2 (4 panels) Zenith and ±15,5º from zenith in 4 orthogonal directions ~9 º 40,000 hours Other operating frequencies MHz 915, 920, 924, 1280, 1299, 1357.5

Approaching low pressure system Sea breeze

Effects on profiler range performance

Dependence on atmospheric conditions. Rapid and significant changes.

Affecting conditions: humidity, turbulence, precipitation, high winds and temperature.

Dependence on ground/sea clutter environment, available radio frequency emission bandwidth, aircrafts and birds.

Vertical profiles of mean availability for the wind measurements at various sites (Dibbern et al., 2003)

Radio acoustic sounding system (RASS) • Profiles of virtual temperature, i.e. temperature uncompensated for humidity or pressure.

• Continuous acoustic sine wave synchronized to RADAR frequency, about 2.6 kHz (half wavelength).

• Acoustic wave acts as an artificial target. • Profiler receives resulting backscatter and effectively measures the speed of sound. • Sound speed is related to air temperature thus temperature information can be calculated. • Profiler gathers wind velocity data for the largest portion of an averaging period and virtual temperature data for the remaining portion. In Testbed configuration, typically 27 min for wind and 3 min for temperature measurement. • Strong winds may transport the acoustic signal away from vertical alignment. • Atmosphere absorbs the acoustic signal, much determined by temperature, humidity and pressure.

Frequency Minimum height 2-3 kHz 75-150m Maximum height Range resolution In Testbed conf: Temperature accuracy Averaging time 1-1.5km

60, 100, 200, 400m lowest range 149 m, up to 400 ..1500 m, step 62 m 1 °C 3-60 minutes Sound pressure 1 m above transducer Aperture 132 dB 1.2 m2 (4 sources)

Data examples

Approaching cloud deck first

warms

upper layer (above 250 m) until cooling finally starts with clearing (17:30).

At the same time, closer to ground (150-250 m)

cooling

takes place with northerly winds.

Descending inversion

• • • • • Panu Maijala VTT

Sodar model: REMTECH PA1 Location at Malmi, 08-11/2006 Emission of acoustic pulse and detection of echo Doppler frequency shift. Echo signal relates to thermal turbulence in the atmosphere.

Sounds like a bird singing Linux based measurement system

Number of elements Type of elements Nominal central operating frequency (Hertz) 9 (up to 15 optional) frequencies are emitted on each tilted beam during one «beep» 52 Motorola/Remtech 2250 Antenna size (meters) (Supporting structure not included) Antenna weight including supporting structure 0.65 x 0.65

25 Kg Acoustic Power Maximum range 1 W 2000 m Average range in typical conditions 1300 m

Ceilometers in the Testbed domain

• 7 pcs of CT25K (FMI) • Cloud heights and amounts • 6 pcs of CL31 • Cloud heights, amounts and backscatter profile • • Nurmijärvi, Röykkä Mäntsälä, Purola • • • • Porvoo, Tirmo Helsinki, Vallila Vantaa, Vantaanlaakso Helsinki, Malmi

• Measurement of cloud height and amount, or vertical visibility. And backscatter profile.

• LIDAR technology (LIDAR = Light detection and ranging). • Laser pulses are sent out in a vertical or near-vertical direction. • The backscatter is caused by haze, fog, mist, virga, precipitation and clouds. • Knowing light speed and time delay between the launch and the detection of signal gives cloud base.

Lens Receiver Mirror Transmitter

Measurement range Measurement resolution Reporting interval Measurement interval Laser source Center wavelength Operating Mode

Vaisala CL31

0 ... 7.5 km 10 m or 5 m 2 ... 120 s, selectable 2 s default; 16 s in Testbed configuration Indium Gallium Arsenide (InGaAs) Diode Laser 910 ± 10 nm at 25 °C Pulsed Nominal pulse properties at full range measurement: Energy 1.2 μWs ± 20% Peak power 11 W typical Width, 50% Repetition rate Average power Max Irradiance Beam divergence 110 ns typical 10.0 kHz 12.0 mW 760 μW/cm² measured with 7 mm aperture ±0.4 mrad x ±0.7 mrad

Noora Eresmaa FMI

Mixing height estimation and Testbed-data

CL31-ceilometers

• The 2-step algorithm is under development • An idealized backscattering profile formula

B

(

z

) is fitted to the measured profile by the 

B

(

z

) 

B SL



B ML



B SL



B ML

*

erf

(

z z SL

) 2 2

s SL



B ML

 2

B U



B ML

2 

B U

*

erf

(

z



z ML s ML

) •

z SL

corresponds to the lower mixing layer (Step1);

z ML

to the upper one (Step2).

An example of the ceilometer profile fitting (15 April 2006 at 10:00 UTC in Vallila, Helsinki)

Multiple aerosol layers on 16.3.2006

Christoph Münkel Vaisala

Ground fog measurement with Vaisala Ceilometer CL31

Strong and clearly structured signal from ground fog patches.

Karen Bozier Univ. of Salford

Doppler lidar – University of Salford & Halo photonics

• Atmospheric backscatter and vertical wind velocity measurement. Measurements of the eddy dissipation rate and the integral length scale of turbulent eddies. • Construction: optical base unit, the weather-proof antenna and the signal processing and data acquisition unit. • Selectable parameters such as the length of the range gate, maximum range and number of pulses accumulated for each measurement and the spectral resolution. • Investigation of the day-night boundary layer transition.

Parameters

Operating wavelength

Description/Value

1.55 mm Pulse Repetition Frequency 20 kHz Energy per pulse Beam divergence 10 mJ 50 mrad ~ 5cm at 1 km Range gate Minimum range 30 m 30 m Maximum range up to 7 km dependant on atmospheric conditions Temporal resolution 0.1 – 30 s

9th August 2006 between 10:40 – 11:00 UTC. Karen Bozier Univ. of Salford

1200 1100 1000 900 800 700 600 500 400 300 200 100 0 -1000 8:57:00 8:58:04 8:58:52 8:59:56 9:01:00 9:02:04 9:03:08 -500 0 500 1000 1500

Backscatter profile in relative units

2000 2500 3000

The picture above shows modeled distribution of particulate matter concentrations on the August 9th 2006 at 09:00 UTC (source: http://www.fmi.fi/ ). The unit in the given color scale is µg/m3. The picture below presents ceilometer Vaisala CL31 backscatter profiles at Helsinki Malmi site on the same day (08:57-09:03 UTC).

• • • •

POSS – Precipitation occurrence sensor system

Järvenpää 12/05 Malmi 12/05-05/06 Jokioinen 06/06 Customs declaration until 11/08, Communications Regulatory Authority permission for extension in process.

Kenneth Wu Met Service Canada • • • • Precipitation detection Type identification (dominant type): Drizzle ( L), Rain (R), Snow (S), Hail (A), Indeterminate ( P) Intensity and rate estimation: Very light (-- ), Light ( - ), Moderate ( ), Heavy ( + ) Rain drop size distribution (DSD) • • • Doppler shifted frequency is proportional to the velocity component in the boresight of the antenna.

Mean power of the Doppler signal depends on: raindrop size, antenna pattern, transmitter power and window transmission losses.

Both amplitude and frequency from a single particle vary as it passes through the measurement volume.

• • • • • • • Microwave (10.525 GHz, 3cm) CW Doppler bi-static radar 43 mw nominal output power Measurement range approx. 2 m Measurement volume 30 m 3 Heated TX/RX window with bird deterrent Maximum total power consumption: 200W Operates under all weather conditions