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Research of RG Crewell

Improved observations

Development of state-of-the-art microwave radiometers

Passive Microwave radiometers on-board weather satellites are the most important measurements for today's weather forecasts. However, they also bear a great potential for atmospheric research when operated from the surface. They are the only instruments that can deliver continuous information on the amount of liquid water contained in the clouds. In addition they can be used for temperature and water vapor profiling, as well as for ice and snowfall cloud microphysical property retrieval. Research on ground-based microwave radiometers has focused on:

  • Calibration accuracy 1,2,3
  • The Microwave Radiometer for Cloud Carthography MICCY 4
  • Humidity And Temperature PROfiling with the HATPRO instrument series5,6,7
  • Liquid water retrieval of clouds 8,9
  • Boundary layer characteristics: water vapor and its horizontal inhomogeneity10,11,12 and assessment with GPS slant paths13
  • Polarimetric measurements at 90/150 GHz with a Dual Polarization Radiometer DPR14,15,16 for studying snowfall microphysics
  • A novel microwave radiometer for Assessment of Atmospheric Propagation Conditions for 10 and 90 GHz Frequency Bands ATPROP17,18
  • HAMP - the microwave package on the High Altitude and LOng range research aircraft HALO19
Cooperation: Radiometer Physics, GmbH (GER), NOAA SSL (USA), McGill University (CAN)
Ground-based instrumentation and retrievals

Next to microwave radiometers, a suite of instruments operating in different parts of the visible, infrared and microwave part of the electromagnetic spectrum are employed to investigate surface/atmosphere exchange processes, atmospheric boundary layer, cloud and precipitation processes:

  • Diurnal dynamics of wheat evapotranspiration derived from ground-based thermal imagery20
  • Mixing layer height retrieval with Doppler lidar and ceilometer21
  • Innovative approaches using high-frequency cloud radar22 and triple-frequency radar to retrieve shape, size and mass of snowfall23
  • Use higher moments of the radar Doppler spectrum to study ice clouds24 and drizzle.
  • Snowfall observations using low-power FM-CW K-band radar (Micro Rain Radar)25,26,27
Cooperation: Metek GmbH (GER), Selex ES Germany (GER), Radiometer Physics GmbH (GER), University of Leicester (UK), KU Leuven (BE), McGill University (CA)
Sensor Synergy

In many cases, a single remote sensing instrument does not deliver enough information on the atmospheric quantity to be derived. Thus, a major focus of the group is in combining different sensors and measurement configurations to fully exploit the information content of our observations. Research has concentrated on:

  • The combination of microwave and infrared radiometer, lidar and cloud radar28
  • Statistical retrieval algorithms using multiple sensor techniques29,30,31,32
  • Variational retrievals: the Integrated Profiling Technique (IPT)33,34,35
  • Information content studies of passive36,37,38 and active sensors39
  • Integration of ground-based and satellite observations40,41
  • Water vapor tomography42
  • Supporting KU Leuven in establishing a cloud observatory in East-Antarctica43
Cooperation: NOAA SSL (USA), KNMI (NL), TU Delft (NL), University of Reading (UK), KU Leuven (BE)
Observatories and Field Experiments

In order to be able to carry out investigations on the inter-play between boundary layer processes, clouds and precipitation through observations, the group is leading since 2008 the long-term observations at JOYCE(Jülich ObservatorY for Cloud Evolution44). We have also been responsible for GOP: the General Observation Period45 within the DFG-funded German Priority Program QPF Quantitative precipitation forecast, including the deployment of ARM mobile facility in the Black Forest, 200746. Cooperation exists with other long-term atmospheric observatories.

  • UFS Environmental research stations Schneefernerhaus, Germany since 2005 (Löhnert et al., 2011)
  • CESAR Cabauw Experimental Site for Atmospheric Research, Cabauw, The Netherlands, since 2001
Also, the group has been involved in a variety of past field campaign experiments:
  • FLUXPAT: Integrative characterization of patterns in the atmospheric boundary layer48 and heat and moisture budgets from airborne measurements49
  • NARVAL: Next-generation Aircraft Remote sensing for VALidation studies Barbados and Iceland, winter 2013/14
  • HOPE: HD(CP)2 Observational Prototype Experiment, Apr-May 2013
  • RHUBC-II: Radiative Heating in Underexplored Bands Campaign, Chile, Aug-Oct 200950
  • GCPEx, Egbert, Ontario, Canada, winter 2011/12
  • COPS, Black Forest, Jun-Aug 200751,52,53
  • AquaRadar, Southern Germany, May-Aug 2006
  • AMMA: African Monsoon Multidisciplinary Analysis, Benin, 200654
  • TWP-ICE: Tropical Warm Pool – International Cloud Experiment, Darwin, Australia, Jan-Feb 2006
  • LAUNCH, Lindenberg, Germany, Sep/Oct 200555
  • VAPIC Water vapor profiling experiment, SIRTA IPSL, France, May/Jun 2004
  • BBC2, Cabauw, NL, May 2003
  • BBC BALTEX BRIDGE Campaign56, Cabauw, NL, Aug/Sep 2001

Satellite Missions

A further research topic is in preparing and developing observation strategies for future satellite missions, specifically in the millimeter range:

  • Simulation study of precipitating clouds from geostationary orbits with passive microwaves57,58,59
  • Propagation corrections for satellite communications and Bepi Colombo mission60
  • Application studies for Earth Explorer Mission EarthCare to be launched in 2019
  • Observing Ice Clouds in the Sub-millimeter Spectral Range61
  • Satellite hyper-spectral microwave radiometer feasibility studies

Cooperation: LERMA Paris, Chalmers University, University of Hamburg, Meteo France, ESA


Improved modeling

Observations can be used to assess and improve the performance of different types of models: radiative transfer, land surface processes, large eddy simulations (LES), numerical weather prediction (NWP) and climate models.

Process studies

Process understanding can be improved by combining models on different scales with observations of land surface exchange parameters, water vapor, clouds and precipitation.

  • Modeling of atmospheric absorption characteristics in the microwave region62
  • Parameterization of ice microphysics in atmospheric63 and snow microphysical scattering properties in radiative transfer64
  • Modeling of land surface exchange processes over heterogeneous surfaces in the Rur catchment and comparison with airborne and surface observations65
  • Derivation of heat and moisture budgets from airborne measurements and high resolution model simulations66
  • Investigation of cloud overlap assumptions using cloud radar67
  • Analysis of the diurnal cycle of the inter-tropical discontinuity over West Africa by remote sensing and mesoscale modelling68
Long-term model evauation

In order to understand whether models can reproduce the reality under all conditions long-time series of model reanalysis and forecasts need to be thoroughly tested with observation for a great variety of atmospheric situations. In this way model shortcoming for certain regimes can be identified and subsequently improved.

  • Variability of atmospheric water vapor including its diurnal cycle69
  • Cloud liquid water path within the BALTEX Cloud Liquid Water Network CLIWA-NET70
  • Dependence of forecast skill of water vapor and cloud parameters on circulation weather type71
  • Precipitation microphysics using multi-dimensional remote sensing observations72 and in dependence of regimes73
  • BBC Case studies for the WMO Cloud Modelling workshop, Hamburg 200474
  • Satellite observations for model evaluation75 with focus on the global distribution of ice clouds and snowfall76
  • Benefit of a high-resolution reanalyses with 6 km resolution77
Cooperation: Deutscher Wetterdienst, KU Leuven, FU Berlin, University of Hamburg