| 1. |
- Eliasson, Salomon, et al.
(författare)
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Systematic and random errors between collocated satellite ice water path observations
- 2013
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Ingår i: Journal of Geophysical Research - Atmospheres. - : John Wiley & Sons. - 2169-897X .- 2169-8996. ; 118:6, s. 2629-2642
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Tidskriftsartikel (refereegranskat)abstract
- There remains large disagreement between IWP in observational datasets, largely because the sensors observe different parts of the ice particle size distribution. A detailed comparison of retrieved IWP from satellite observations in the Tropics ({plus minus}30{degree sign} latitude) in 2007 is made using collocated measurements. The DARDAR IWP dataset, based on combined Radar/Lidar measurements, is used as a reference as it provides arguably the best estimate of the total column IWP. For each dataset, usable IWP dynamic ranges are inferred from this comparison. IWP retrievals based on solar reflectance measurements, MODIS, and AVHRR-based CMSAF, and PATMOS-x, were found to be correlated with DARDAR over a large IWP range (~20-7000 g/m-2;). The random errors of the collocated datasets have a close to log-normal distribution, and the combined random error of MODIS and DARDAR is less than a factor of 2, which also sets the upper limit for MODIS alone. In the same way the upper limit for the random error of all considered datasets is determined. Datasets based on passive microwave measurements,MSPPS, MiRS, and CMO, are largely correlated with DARDAR for IWP values larger than approximately 700 g/m². The combined uncertainty between these datasets and DARDAR in this range is slightly less MODIS-DARDAR, but the systematic bias is nearly an order of magnitude.
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| 2. |
- Holl, Gerrit, et al.
(författare)
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Collocating satellite-based radar and radiometer measurements : methodology and usage examples
- 2011
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Ingår i: Proceedings of the ESA Living Planet Symposium. - : European Space Agency, ESA.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Collocations between two satellite sensors are occasions where both sensors observe the same place at roughly the same time. We study collocations between the Microwave Humidity Sounder (MHS) on-board NOAA-18 and the Cloud Profiling Radar (CPR) on-board CloudSat. We present some statistical properties of the collocations. For 2007, we find approximately two and a half million MHS measurements with CPR pixels close to their centrepoints. We present some possible applications. We use the collocations to validate an operational Ice Water Path (IWP) product from MHS measurements, produced by the National Environment Satellite, Data and Information System (NESDIS) in the Microwave Surface and Precipitation Products System (MSPPS). IWP values from the CloudSat CPR are found to be significantly larger than those from the MSPPS. Finally, we use the collocations to train an Artificial Neural Network and describe how we can use it to develop a new MHS-based IWP product. The collocations described in the article are available for public use.
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| 4. |
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| 5. |
- Holl, Gerrit, et al.
(författare)
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Collocating satellite-based radar and radiometer measurements – methodology and usage examples
- 2010
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Ingår i: Atmospheric Measurement Techniques. - : Copernicus GmbH. - 1867-1381 .- 1867-8548. ; 3:3, s. 693-708
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Tidskriftsartikel (refereegranskat)abstract
- Collocations between two satellite sensors are occasions where both sensors observe the same place at roughly the same time. We study collocations between the Microwave Humidity Sounder (MHS) on-board NOAA-18 and the Cloud Profiling Radar (CPR) on-board CloudSat. First, a simple method is presented to obtain those collocations and this method is compared with a more complicated approach found in literature. We present the statistical properties of the collocations, with particular attention to the effects of the differences in footprint size. For 2007, we find approximately two and a half million MHS measurements with CPR pixels close to their centrepoints. Most of those collocations contain at least ten CloudSat pixels and image relatively homogeneous scenes. In the second part, we present three possible applications for the collocations. Firstly, we use the collocations to validate an operational Ice Water Path (IWP) product from MHS measurements, produced by the National Environment Satellite, Data and Information System (NESDIS) in the Microwave Surface and Precipitation Products System (MSPPS). IWP values from the CloudSat CPR are found to be significantly larger than those from the MSPPS. Secondly, we compare the relation between IWP and MHS channel 5 (190.311 GHz) brightness temperature for two datasets: the collocated dataset, and an artificial dataset. We find a larger variability in the collocated dataset. Finally, we use the collocations to train an Artificial Neural Network and describe how we can use it to develop a new MHS-based IWP product. We also study the effect of adding measurements from the High Resolution Infrared Radiation Sounder (HIRS), channels 8 (11.11 μm) and 11 (8.33 μm). This shows a small improvement in the retrieval quality. The collocations described in the article are available for public use.
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| 6. |
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| 7. |
- Holl, Gerrit
(författare)
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Microwave and infrared remote sensing of ice clouds : measurements and radiative transfer simulations
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis considers the combination of multiple instruments for remote sensing of the Earth atmosphere from space. The primary focus is on remote sensing of atmospheric ice. Ice clouds are important for the Earth’s radiation budget, but their properties are difficult to measure and therefore poorly known. A better quantification of ice clouds is needed to improve global climate models. This thesis introduces the reader to the subject and describes how to combine measurements and radiative transfer simulations in an attempt to improve our understanding. A major part of this work is the development of a toolkit to find co-incident measurements, or collocations, between any pair of down-looking satellite sensors. Firstly, this toolkit is used to collocate passive microwave and thermal infrared sensors on meteorological satellites with the Cloud Profiling Radar on CloudSat. With the resulting collocated dataset, the Ice Water Path (IWP) signal in passive thermal radiation is studied and an improved IWP retrieval is presented. The toolkit is also used to better characterise the bias between different copies of passive microwave radiometers on-board polar-orbiting operational satellites. For the Atmospheric Radiative Transfer Simulator (ARTS), version 2, an optimised frequency grid for infrared broadband simulations is shown to be applicable for cloudy simulations. This frequency grid can and will be used to study the IWP signal in thermal infrared radiances. An outlook on a comparison between collocations and simulations is presented in the thesis.
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| 8. |
- Holl, Gerrit, et al.
(författare)
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Optimised frequency grids for infrared radiative transfer simulations in cloudy conditions
- 2012
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 113:16, s. 2124-2134
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Tidskriftsartikel (refereegranskat)abstract
- This paper shows that radiometer channel radiances for cloudy atmospheric conditions can be simulated with an optimised frequency grid derived under clear-sky conditions. A new clear-sky optimised grid is derived for AVHRR channel . For HIRS channel 11 and AVHRR channel 5, radiative transfer simulations using an optimised frequency grid are compared with simulations using a reference grid, where the optimised grid has roughly 100–1000 times less frequencies than the full grid. The root mean square error between the optimised and the reference simulation is found to be less than 0.3 K for both comparisons, with the magnitude of the bias less than 0.03 K. The simulations have been carried out with the radiative transfer model Atmospheric Radiative Transfer Simulator (ARTS), version 2, using a backward Monte Carlo module for the treatment of clouds. With this module, the optimised simulations are more than 10 times faster than the reference simulations. Although the number of photons is the same, the smaller number of frequencies reduces the overhead for preparing the optical properties for each frequency. With deterministic scattering solvers, the relative decrease in runtime would be even more. The results allow for new radiative transfer applications, such as the development of new retrievals, because it becomes much quicker to carry out a large number of simulations. The conclusions are applicable to any downlooking infrared radiometer.
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| 9. |
- Holl, Gerrit
(författare)
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Remote sensing of ice clouds : synergistic measurements and radiative transfer simulations
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis primarily considers the spaceborne remote sensing of ice clouds and frozen precipitation. Ice clouds are important for hydrology and for the Earth’s radiation budget, but many properties are difficult to measure, in particular using spaceborne instruments. A better quantification of ice clouds is needed to improve global climate models. This thesis presents steps toward such an improvement.The first part of the thesis introduces topics related to the research presented in the second part, but presents no new scientific results. It gives a brief introduction to the history of atmospheric remote sensing and describes how the different parts of the electromagnetic spectrum can be used actively or passively. Then, it describes why ice clouds are important and what microphysical, optical, and macrophysical properties are used to describe atmospheric ice. Next, it briefly introduces the relevant topics in atmospheric radiative transfer. The first part concludes with a description of various approaches to retrievals, with a particular focus on those applied in this thesis.The second part of the thesis describes new results. The bulk of the new results is described in five peer-reviewed publications, that are appended verbatim.A major part of the work builds on the development of a toolkit to easily find co-incident measurements, or collocations, between any pair of satellite sensors. Four appended articles rely on this toolkit.The first appended article uses the toolkit to obtain collocations between passive microwave and infrared on operational meteorological satellites with the Cloud Profiling Radar on CloudSat. It presents three examples. Firstly, from the collocated dataset and a dataset of synthetic profiles, the article compares the statistical relations between an official CloudSat Ice Water Path (IWP) product and microwave radiances. Secondly, it shows a point-by-point comparison between the same CloudSat IWP product, and an IWP product based on passive microwave. A more sophisticated set of systematic comparisons, including more satellites and sensors, is presented in a dedicated paper. Finally, the first paper provides a first preview of how the collocations can be used to train a new IWP retrieval from passive operational measurements. This too is the topic of a dedicated paper, where solar, terrestrial infrared, and microwave radiances are combined to obtain an improved IWP product from passive operational sensors, by training with an active combined radar-lidar product from CloudSat-CALIPSO.The second appended article also relies on the collocations toolkit. Here, collocations between different copies of identical or very similar microwave sounders are used to assess how the inter-satellite bias depends on radiance and latitude.The remaining two studies described in the thesis do not use existing measurements, but are based on radiative transfer modelling. One attached paper verifies that optimised frequency grids obtained in clear-sky simulations for terrestrial infrared instrument studies, can be applied directly for cloudy simulations. This result is relevant for future studies. Finally, the thesis includes a short study with retrieval simulations for a new sub-millimetre instrument concept.
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| 10. |
- Holl, Gerrit, et al.
(författare)
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SPARE-ICE : Synergistic ice water path from passive operational sensors
- 2014
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996. ; 119:3, s. 1504-1523
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Tidskriftsartikel (refereegranskat)abstract
- This article presents SPARE-ICE, the Synergistic Passive Atmospheric Retrieval Experiment-ICE. SPARE-ICE is the first Ice Water Path (IWP) product combining infrared and microwave radiances. By using only passive operational sensors, the SPARE-ICE retrieval can be used to process data from at least the NOAA 15 to 19 and MetOp satellites, obtaining time series from 1998 onward. The retrieval is developed using collocations between passive operational sensors (solar, terrestrial infrared, microwave), the CloudSat radar, and the CALIPSO lidar. The collocations form a retrieval database matching measurements from passive sensors against the existing active combined radar-lidar product 2C-ICE. With this retrieval database, we train a pair of artificial neural networks to detect clouds and retrieve IWP. When considering solar, terrestrial infrared, and microwave-based measurements, we show that any combination of two techniques performs better than either single-technique retrieval. We choose not to include solar reflectances in SPARE-ICE, because the improvement is small, and so that SPARE-ICE can be retrieved both daytime and nighttime. The median fractional error between SPARE-ICE and 2C-ICE is around a factor 2, a figure similar to the random error between 2C-ICE ice water content (IWC) and in situ measurements. A comparison of SPARE-ICE with Moderate Resolution Imaging Spectroradiometer (MODIS), Pathfinder Atmospheric Extended (PATMOS-X), and Microwave Surface and Precipitation Products System (MSPPS) indicates that SPARE-ICE appears to perform well even in difficult conditions. SPARE-ICE is available for public use.
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