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, JülichJülJüü ichchc 252 SeptemberSSeeptetetembembber 201220121 1 25.09.12 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 2 25.09.12 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 3 25.09.12 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 4 25.09.12 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 5 25.09.12 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 6 25.09.12 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 JuneJune UTC 20112011 N ESW Susanne Crewell, Kompaktkurs, Jülich 25 September 2012 7 25.09.12 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 8 25.09.12 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 9 25.09.12 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 10 25.09.12 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 11 25.09.12 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 12 25.09.12 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 Susanne Crewell, Kompaktkurs, Jülich 25 September 2012 13 25.09.12 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