SwePub
Sök i SwePub databas

  Utökad sökning

Träfflista för sökning "WFRF:(Ekström Mattias 1977 ) "

Sökning: WFRF:(Ekström Mattias 1977 )

  • Resultat 1-10 av 11
Sortera/gruppera träfflistan
   
NumreringReferensOmslagsbildHitta
1.
  • Ekström, Mattias, 1977, et al. (författare)
  • Altitude resolved ice-fraction in the uppermost tropical troposphere
  • 2008
  • Ingår i: Geophysical Research Letters. - 1944-8007 .- 0094-8276. ; 35:2, s. L13822-
  • Tidskriftsartikel (refereegranskat)abstract
    • The exact nature of the processes responsible for the moistening of the upper tropical troposphere is still uncertain. Altitude resolved measurements of water vapor from Aura/Microwave Limb Sounder (MLS) and cloud ice from CloudSat are used to investigate the ratio of ice mass to total water. Horizontal and vertical ice-fraction distributions in the pressure range 100-316 hPa over the tropical region are presented. They reveal that the ice-fraction is generally low, less than 10% around 316 hPa in general and outside regions of deep convection in the altitude levels above. On the other hand, the ice-fraction can be significant at higher altitudes in large regions above deep convection, reaching values of ∼90%. Below the tropical tropopause layer (TTL) ice and water vapor distributions have similar spatial patterns indicating that water in both phases is transported up to the upper troposphere by the same processes. Over regions of strong deep convection where ice is transported into the TTL, the dissimilar patterns of ice and water vapor could be interpreted as that the cloud ice gives a limited final moistening effect. Copyright 2008 by the American Geophysical Union.
  •  
2.
  • Ekström, Mattias, 1977, et al. (författare)
  • Comparison of satellite limb-sounding humidity climatologies of the uppermost tropical troposphere
  • 2008
  • Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 8:2, s. 309-320
  • Tidskriftsartikel (refereegranskat)abstract
    • Humidity climatologies of the tropical uppermost troposphere from satellite limb emission measurements have been compared. Four instruments are considered; UARS-MLS, Odin-SMR, and Aura-MLS operating in the microwave region, and MIPAS in the infrared region. A reference for the comparison is obtained by MOZAIC in-situ measurements.The upper tropospheric humidity products were compared on basis of their empirical probability density functions and seasonally averaged horizontal fields at two altitude layers, 12 and 15 km. The probability density functions of the microwave datasets were found to be in very good agreement with each other, and were also consistent with MOZAIC. The average seasonal humidities differ with less than 10%RHi between the instruments, indicating that stated measurement accuracies of 20–30% are conservative estimates. The systematic uncertainty in Odin-SMR data due to cloud correction was also independently estimated to be 10%RHi. MIPAS humidity profiles were found to suffer from cloud contamination, with only 30% of the measurements reaching into the upper troposphere, but under clear-sky conditions there is a good agreement between MIPAS, Odin-SMR and Aura-MLS.Odin-SMR and the two MLS datasets can be treated as independent, being based on different underlying spectroscopy and technology. The good agreement between the microwave limb-sounders, and MOZAIC, is therefore an important step towards understanding the upper tropospheric humidity. The found accuracy of 10%RHi is approaching the level required to validate climate modelling of the upper troposphere humidity. The comparison of microwave and infrared also stresses that microwave limb-sounding is necessary for a complete view of the upper troposphere.
  •  
3.
  •  
4.
  • Ekström, Mattias, 1977 (författare)
  • Satellite measurements of upper tropospheric water
  • 2008
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Water vapour and clouds in the upper troposphere have strong influence on the Earth's climate system. The present uncertainties regarding the distribution and feedback mechanisms of water to a changing climate are today among the greatest limitations for accurate climate predictions.The poor knowledge of upper tropospheric water results from the inability of earlier measurement systems to give a complete view of the upper troposphere.In-situ instruments are restricted in spatial coverage and satellite observations are needed to provide global coverage. Traditional infrared and optical satellite observations are mainly sensitive to the middle troposphere and applicable only in relatively cloud-free conditions.In this thesis satellite microwave limb-sounding is shown to be an advantageous technique to observe the upper troposphere with high vertical resolution and capability to penetrate thin cirrus clouds. Present measurement systems have the potential to give accurate measurements of both upper tropospheric humidity and ice water content.Retrieval methods have been developed for the sub-millimeter limb-sounding observations of Odin-SMR. The retrieved quantities are the all-weather upper tropospheric humidity at 12 and 15 km in altitude and a partial ice water path above 12 km.The humidity results agree within 10% with data from other microwave limb-sounders and high quality in-situ measurements. A comparison with infrared measurements emphasizes the necessity of microwave observations to measure in presence of clouds. The cloud ice retrieval shows a general agreement with other microwave data sets. The comparison highlights the difficulty of cloud ice retrieval since important assumptions on cloud microphysics and 3D inhomogeneities are presently unavoidable.Microwave measurements from Aura-MLS and CloudSat are used to investigate the relative amounts of water vapour and ice in the upper troposphere. Ice is found to dominate the total water budget at 14 km over regions of deep convection, but appears to give a small contribution to the moistening of the upper troposphere.
  •  
5.
  • Ekström, Mattias, 1977 (författare)
  • Sub-Millimetre Measurements of Upper Tropospheric Humidity
  • 2006
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Humidity and clouds in the upper troposphere are important factors for the Earth's radiative balance.Consequently a weakness in global climate models is that very little is known about the concentration of water vapour or the distribution and properties of clouds in this altitude region.The balloon sondes that are regularly launched around the globe show very poor agreement in the upper troposphere and available satellite data have problems connected to spatial resolution and the occurrence of clouds.The frequent cloud cover restricts satellite applications of visible and infrared to down-looking geometries, resulting in low vertical resolution and measurements most sensitive to the middle troposphere.Observations in the microwave region are less sensitive to cloud scattering and therefore limb sounding can be performed to give improved vertical resolution in the upper troposphere.The sub-millimetre radiometer onboard the Odin satellite, Odin-SMR, performs limb sounding at frequencies around 500 GHz since its launch in 2001. The observations have sensitivity down to 10 km, but tangent altitudes inside the troposphere have up to now been ignored due to cloud scattering. This effect can now be simulated by a new radiative transfer software and first retrievals of tropospheric quantities are possible.The thesis presents the steps taken towards a first retrieval of upper tropospheric humidity and ice cloud properties.A thorough investigation of the Odin-SMR calibration at high brightness temperature has been performed. The results show that the systematic calibration error is low, and that a retrieval of humidity from the tropospheric spectra is achievable.A method to retrieve upper tropospheric humidity from the lowest tangent altitudes has been realised, including a cloud correction scheme, used for spectra with apparent cloud signature. The products of the retrieval are the all-weather humidity field and ice cloud signal. Together with an accompanying method to derive ice cloud properties, the retrieved ice cloud signal can be translated into ice water content with additional altitude information.
  •  
6.
  • Eriksson, Patrick, 1964, et al. (författare)
  • Comparison between early Odin-SMR, Aura MLS and CloudSat retrievals of cloud ice mass in the upper tropical troposphere
  • 2008
  • Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 8:7, s. 1937-1948
  • Tidskriftsartikel (refereegranskat)abstract
    • Emerging microwave satellite techniques are expected to provide improved global measurements of cloud ice mass. CloudSat, Aura MLS and Odin-SMR fall into this category and early cloud ice retrievals from these instruments are compared. The comparison follows the SMR retrieval product and is made for partial ice water columns above 12 km. None of the retrievals shows a significant degree of false cloud detections, the ratio between local mean values from the instruments is fairly constant and a consistent view of the geographical distribution of cloud ice is obtained. However, important differences on the absolute levels exist, where the overall mean is 9.6, 4.2 and 3.7 g m−2 for CloudSat, SMR and MLS, respectively. Assumptions about the particle size distribution (PSD) are a consideration for all three instruments and constitute the dominating retrieval uncertainty for CloudSat. The mean for CloudSat when applying the same PSD as for MLS and SMR was estimated to 6.3 g m−2. A second main consideration for MLS and SMR are the effects caused by the poorer spatial resolution: a possible vertical misplacement of retrieved values and an impact of cloud inhomogeneities. The latter effect was found to be the dominating retrieval uncertainty for SMR, giving a possible mean value range of 2.3–8.9 g m−2. The comparison indicates a common retrieval accuracy in the order of 70%. Already this number should suffice for improved validations of cloud ice parametrisation schemes in atmospheric models, but a substantially better consistency between the datasets should be attainable through an increased understanding of main retrieval error sources.
  •  
7.
  •  
8.
  •  
9.
  •  
10.
  •  
Skapa referenser, mejla, bekava och länka
  • Resultat 1-10 av 11

Kungliga biblioteket hanterar dina personuppgifter i enlighet med EU:s dataskyddsförordning (2018), GDPR. Läs mer om hur det funkar här.
Så här hanterar KB dina uppgifter vid användning av denna tjänst.

 
pil uppåt Stäng

Kopiera och spara länken för att återkomma till aktuell vy