25.09.12
Clouds, Precipitation and their Remote Sensing
Prof. Susanne Crewell AG Integrated Remote Sensing Institute for Geophysics and Meteorology University of Cologne
Susanne Crewell, Kompaktkurs, Jülich24. 25 September September 2012 2012
Intergovernmental Panel on Climate Change (IPCC) www.ipcc.ch Nobel price 2007
IPCC Fourth Assessment Report (FAR), 2007: "Warming of the climate system is unequivocal", and
"Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations".
Aerosols, clouds and their interaction with climate is still the most uncertain area of climate change and require multidisciplinary coordinated research efforts.
SusanneSusS sanna ne Crewell,Crewewellelll,K, Kompaktkurs,Kompakta kurs, JülJüJülichü ichchc 252 SeSSeptembereptetetembembber 220120121
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Why are clouds so complex? Cloud microphysical processes occur on small spatial scales and need to be parametrized in atmospheric models Cloud microphysics is strongly connected to other sub-grid scale processes (turbulence, radiation)
Cloud droplets 0.01 mm diameter 100-1000 per cm3
Condensation nuclei Drizzle droplets 0.001 mm diameter 0.1 mm diameter 1000 per cm3 1 per cm3
Rain drops ca. 1 mm diameter, 1 drops per liter
Susannesa Crewell, Kompaktkurs, Jülich 25 September 2012
Why are clouds so complex?
From hydrometeors
to single clouds to Einzelwolken to the global and cloud fields system
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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What are important cloud parameter?
Macro-physical Parameter Radiative Quantities cloud fraction extinction coefficient ε [m-1] cloud height optical thickness τ cloud contours z=∞ τ = ∫ ε (z') dz' λ λ 3D-structure z=0 transmission t = exp(-τ)
Micro-physical Parameter number concentration N Ice- and mixed phase clouds effective radius reff moments of phase liquid water content LWC the droplet spectrum shape radar reflectivity Z density
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Droplet spectra
Hawaii observations orographic
modelling Hawaii stratus
Passat Australia continental
moments of drop spectra cloud liquid water density [kg m-3] ∞ n 4π ∞ = ∫ = ρ ∫ 3 m(n) r N(r)dr LWC w r N(r)dr 0 3 0
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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Ice and mixed phase clouds
Bergeron-Findeisen
While everywhere sufficient cloud condensation nuclei for forming water droplets are available, much fewer ice nuclei exist
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
From small to large particles ....
0.1 μm aerosols 1.0 μm
cloud droplets 10 μm ice crystals
100 μm
1.0 mm rain drops snow 10 mm
turbulence microphysical models ~100 m numerical weather prediction (NWP) models ~10 km climateSusanne models Crewell, ~100 Kompaktkurs, km Jülich 25 September 2012
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Jülich ObservatorY for Cloud Evolution
• JOYCE aims at investigating the processes of cloud formation and cloud evolution (precipitation) • Various instruments set up at the Research Centre Jülich continuously monitor winds, temperature, water vapor, clouds, and precipitation over many years
geomet.uni-koeln.de/joyce
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
JOYCE: Scientific goals
Goals to disentangle water vapor variations due to advection and to local surface influence validate coupled models to better understand the development of boundary layer clouds including cloud radiation interaction to observe precipitation formation and improve parametrization schemes
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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JOYCE: Instrumente (24/7)
Scanning cloud Micro Rain radar MIRA-36s Radar
Pulsed Doppler Lidar Ceilometer Lidar
Scanning MWR Infrared HATPRO spectrometer AERI Doppler Sodar Total sky imager
Sun photo- Radiation sensors meter
Auxilliary instruments: 120-m meteorological mast, MAX-DOAS, GPS, polarimetric weather radar
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
How to remotely sense cloud parameters?
Active and passive techniques in different spectral regions use extinktion, absorption and scattering of electromagnetic radiation to indirectly sense cloud properties
Clouds are best „visible“ in atmospheric windows Microwaves (radiometry, radar & GNSS) Thermal infrared (satellite radiometry and spectrometry) Visible (reflected sun light, lidar, sun photometer)
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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How to determine cloud occurrence?
Total Sky Imager (Yes Inc.)
Specifications: Camera looks from above on spherical mirror Sun is blocked by black tape on mirror Temporal resolution 20 s
Products & retrievals: Cloud classification based on RGB components for each pixel (in-house algorithm): sky, thin- and opaque clouds (blue, light blue and white) Cloud fraction
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Total Sky Imager
Advantages: very reliable, intuitive, spatio-temporal structure Disadvantage: difficult to interprete due to geometry effects
18 UTC
12 UTC
06 1010 JJuneune UTC 22011011 N ESW Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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At which height do clouds occur?
Lidar Ceilometer CT25K
Specifications: pulses at 905 nm temporal resolution 15 s range resolution ~15m, range 0-7 km
Products & retrievals: senitive to small particles cloud base height optical extinction assuming constant lidar ratio (in-house algorithm) aerosol layer height
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Lidar ceilometer
Advantages: very reliable, vertical structure Disadvantage: does not penetrate liquid water (cloud!)
Ice clouds
Rising PBL Rain Altitude (m above ground)
Aerosol
Fog
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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Remote sensing and sensor synergy
Lidar - backscatter coefficient prop. r2 - depolarisation information (phase!) - strong extinktion by water clouds
Cloud radar Radar Lidar - radar reflectivity factor
6 Radar Z = ∫ D N(D)dD Lidar
- Doppler-spectrum
- linear depolarisation ratio LDR Height LWC -liquid water content - influence by insects and drizzle
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Cloud radar
Sends (active!) out pulses of microwave radiation Measures backscattered radiation (Z @ 35 GHz) Time between emitted and received pulse information on the distance to backscatterer Cloud radar @ JOYCE
Sensitive towards cloud droplets, ice particles & precipitation Doppler radar radial velocity component can be measured Doppler spectrum can help to distinguish different targets Polarized receiver target discrimination constraint information on particle shape
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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Cloud radar radar reflectivity factor 7.8.2001 13:30 - 14:30
95 GHz GKSS cloud radar Doppler velocity MIRACLE
Lineare depolarisation ratio
backscatter proportional r6
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Doppler Cloud Radar MIRA-36
Elevation scan from 90° to 15°
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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Sensor Synergy: target categorization
Bit0: small liquid cloud drops (SCD) Only mean to derive complex vertical Bit1: falling hydrometeors Bit2: wet-bulb temperature < 0°C structure of multi-level, multi-phase clouds Bit3: melting ice Provides assumptions for radiative transfer
Bit4: aerosol and retrieval algorithms Bit5: insects
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Why is cloud liquid water so important? Liquid water path (LWP)
2007 2012 observations
Jiang et al, 2012 Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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MicroWave Radiometer (MWR)
HATPRO TOPHAT: Measures thermal emission of atmospheric gases and liquid water Brightness temperatures (TB) in 14 channels measurements Azimuth and elevation scanning Complete hemispheric scans during 7 min
Products: Temperature and humidity profiles Integrated Water Vapor (IWV) Liquid Water Path (LWP)
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Microwave radiometry
Standard atmosphere temperature profile water vapour profile liquid water path
liquid water path LWP=250 gm-2
scattering at cloud droplets is negligle in microwave spectral region extinction ≈l absorption α
∞ s T = T exp(−τ )+ ∫ T(s)α(s) exp(− ∫ α(s')ds') ds B Bcos 0 0
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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Cloud radar and microwave Interruption for scanning
radar reflectivity factor
doppler velocity But how to get the liquid water content profile ?
spectral width
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
The inverse problem
Remote sensing instruments measure Microwave spectrum indirect information, e.g. the measurement vector y includes radiances TB at different frequencies
Forward problem (radiative transfer) for a given atmospheric state x (temperature, humidity, cloud parameter) is well constrained y = F(x) Atmosperic profile
Inverse problem (retrieval algorithm), i.e. the determination of the atmospheric state is often ill-conditioned and requires the inclusion of empirical information
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Integrated Profiling Technique a variational approach towards multi- instrument retrieval measurement 1 + error
measurement 2 + error atmospheric state: temperature, humidity, hydrometeors measurement 3 + errors + error Inversion
OPTIMAL
a priori information ESTIMATION + error Löhnert et al., 2004 and 2008
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Liqud Water Content (LWC)
Application of LWC retrieved by IPT for evaluating regional climate models
Models show different liquid water paths and different peak altitudes
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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Further developments in synergy
• Combination of ground-based and satellite information spatial representation of supersites • Development of a “quasi-real-time” variational algorithm based on optimal estimation theory
Meteosat Seviri R SW Integration & minimization ProfilesPro TBIR of cost of function T, q, Z LWLWC, TBMW TBIR IIR reff
Cloud MRW AERI radar IRR
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
Challenges in sensor synergy
Goall: synchroneous scans radar – microwave radiometer
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Summary and conclusions
Clouds clouds have a strong effect on the Earths energy and water budget cloud processes are rather complex and involve scales from nm to km cloud feedbacks related to aerosols and changes in temperature and humidty are not well understood Observations better observations of clouds are urgently required sensor synergy observations and modelling need to be linked closely for further progress
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
How to sense the various cloud parameters?
Macro-physical Parameter Radiative Quantities cloud fraction extinction coefficient ε [m-1] cloud height optical thickness τ cloud contours z=∞ τ = ε (z') d z' λ ∫ λ 3D-structure z=0 transmission t = exp(-τ)
Micro-physical Parameter number concentration N Ice- and mixed phase clouds effective radius r eff phase liquid water content LWC shape radar reflectivity Z density
Susanne Crewell, Kompaktkurs, Jülich 25 September 2012
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