Performance characteristics and design trades for an ISS Hybrid

Doppler Wind Lidar

G. D. Emmitt and S. WoodSimpson Weather Associates

Charlottesville, Va

ISS Winds Mission Science WorkshopMiami, 2011

Outline• Instrument design issues and data products• The DLSM and OSSEs• The global coverage• The sampling pattern• The key atmospheric variable for evaluating the expected

performance of a hybrid DWL– Clouds

• Cloud climatology• Cirrus in the tropics

– Aerosols• Background and enhanced

– Wind variability• Simulated performance profiles• Summary

Attribute Goal

Vertical depth of regard (km) 25Vertical resolution Tropopause to 25 km Top of BL to tropopause Surface to top of BL

42

1 (.25)Horizontal resolution (km) 350 (35)# of tracks 1# of perspectives within target volume

2

Horizontal component error (m/s) Above BL Within BL (includes sampling RMSE)

< 32 (1)

Numbers in () are desired

Data Goals for Wind Lidar on ISS

Instrument Design Issues

• Vertical coverage in cloudy regions (much of the globe)– Hybrid approach• Direct detection molecular for volumes with low

aerosol content (mid/upper troposphere and lower stratosphere)• Coherent detection for volumes with clouds and

sufficient aerosols (dust layers and lower troposphere)

– Number of telescopes– Dwell times

ISS Wind Lidar Concept

• Hybrid Doppler Wind Lidar:– Coherent detection (2 um) for aerosol and cloud

returns– Direct detection (.355 um) for molecular returns

• Two fixed telescopes provide forward and aft perspectives

• Variable dwell times allow a high spatial resolution (~ 28km) for the coherent system while allowing the direct system longer integration (~ 84 km)

Instrument parameters used in performance simulations for WISSCRS flown on the ISS

*At the fundamental 1.06um for direct detection. The utility wavelength is .355um** Includes: Pre LRE optics (.48), IF (.7) and LRE throughput (.74)

Parameter WISSCRS/ISSCoherent(2um)

WISSCRS/ISSDirect (.355um)

Orbit altitude (km) 350 350Orbit inclination (deg) 52 52Nadir angle (deg) 35 35Energy/pulse (J)* .25 .8PRF (Hz) 10 100Aperture (m) .5 .5EAP .49 15.7# perspectives per cycle 2 2Optical eff** .51 .25Heterodyne mixing eff .36Detector eff .8 .5Integration time (sec) 12(4, .1) 12Misalignment loss .42Beam split . .48 Filter throughput .17Edge sensitivity .007 Recycling factor/margin 1.6Conversion eff .45Wallplug eff * .014 .092Duty cycle (%) 100 100Power required (Watts) 178 869Vertical layer depth (m) 1000/500/250 2000/1000Beta(50) backscatter (/m/sr) 2.8 x 10-9

The DLSM

• Used since 1988 to simulate DWL performance for OSSEs using inputs from Nature Runs.– Clouds– Subgrid scale wind variability– Aerosol distributions

• Simulate both direct and coherent detection• Stress realistic characterization of random and

systematic errors.

Doppler Lidar Simulation Model

DLSM* simulations for use in OSSEs

• Emphasis on the tropics.• Scaling GWOS down to 350km• Uses GWOS instrument performance parameters

listed in a prior slide• Clouds in T511 Nature Run modified to conform to

ISCCP coverage statistics• Has been understating very thin to subvisual cirrus

effects which would be very positive for coherent detection coverage and slightly negative for direct detection accuracies.

* Doppler Lidar Simulation Model

ISS Wind Lidar Coverage for three orbits

The Sampling Pattern

• Illuminated volume– Coherent (250m long cylinder, ~ 2m diameter

footprint)• Spectral domain processing provides information on

turbulence intensity, BL depth, precipitation fall velocities

– Direct (2000m long, ~ 50m diameter)• Accuracy a function of intensity of return, presence of

clouds/aerosols could be derived.

• Sampling pattern, a ground perspective• Pattern over a hurricane

4 second dwell pattern for both direct and coherent (fore and aft perspectives)

4 second dwell pattern (fore and aft sampling)

Single shot coherent samples (~ 700meter intervals)

Clouds and Aerosols

532 nm Total Attenuated Backscatter

Seze, Pelon, Flamant, Vaughn, Trepte and Winker

Sub-visual Cirrus

• Until this year, simulations done using the DLSM in support of OSSEs have not included sub-visual cirrus derived from the nature run

• Recent published studies of very thin and subvisual cirrus have documented a climatology (5 years, in one case) of these upper tropospheric clouds.

• We have modified the DLSM to generate sub-visual cirrus from the ECMWF T511 Nature Run and are currently assessing the realism of the derivation.

Simulated thin and sub-visual cirrus

GWOS/ISS

Single shot threshold sensitivity

Natural Variability of 2 mm Backscatter

Backscatter (m-1sr -1)10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

Land

Mid-Upper Troposphere

Volcanic

Subvisual Cirrus

Maritime PBL

Continental PBL

Ocean

Background

Enhanced

Lower Troposphere

Surface GWOS/ISS

CALIPSO (derived from 532um)

Clouds

Simulated WISSCR’s performance using the DLSM with the T511

ECMWF Nature Run

Aerosol/cloud subsystem of the Wind Lidar on ISS20 – 10 N

Cirrus returnsOpaque clouds

Aerosols

Aerosol/cloud subsystem of the Wind Lidar on ISS10 -0 N

Aerosol/cloud subsystem of the Wind Lidar on ISS0 – 10 S

Aerosol/cloud subsystem of the Wind Lidar on ISS10 -20 S

Molecular subsystem of the Wind Lidar on ISS10 – 0 N

Molecular subsystem of the Wind Lidar on ISS 0 - 10 S

Molecular subsystem of the Wind Lidar on ISS10 -20 S

Summary• Platform attitude drift does not appear to be a major factor in DWL

data quality using pointing knowledge over pointing control.• Clouds will be a major factor in DWL coverage

– Direct detection (molecular) is negatively effected by the high clouds in the tropics.

– However, coherent provides winds within most of the high level clouds and within the lower troposphere below.

• The hybrid technology approach provides the best vertical coverage for science investigations in the tropics– A direct detection subsystem is critical to tropospheric/stratospheric

exchange investigations– The coherent subsystem is critical for accurate, high spatial resolution

measurements in cloudy scenes and in the lower troposphere.

Japanese JEM-EF

–Accommodates 9 experiment payloads–Nominal 500kg payloads–3kW 120VDC per payload–5 Mbits/second download data rates for

single payload– .8 x 1.0 x 1.8 meters –Access to cooling loop for thermal

management

JEM-EF

Key accommodation issues related to instrument performance

• High frequency vibrations (> 1 Hz)• Slow attitude changes (+- 10 degrees)• Power to PL (average and peak)• Thermal management• Orbital debris• Data rates (uplink and downlink)

Accommodations Summary

• The ISS offers an attractive orbit for focusing the Wind Lidar resources on the lower latitudes where ageostrophy is most dominant.

• Assuming that an ISS mission would be regarded as a research science with a focus on the tropics, instrument lifetime, duty cycle and data downloads would be negotiable.

• At this time, no accommodation “show stoppers” have been identified. Just completed a NASA evaluation within the IDL and MDL at GSFC

Molecular subsystem of the Wind Lidar on ISS52N – 52S

Aerosol/cloud subsystem of the Wind Lidar on ISS52N – 52S