| 1. |
- Eliasson, S., 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. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-897X. ; 118:6, s. 2629-2642
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Tidskriftsartikel (refereegranskat)abstract
- There remains large disagreement between ice-water path (IWP) in observational data sets, 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 (+/- 30 degrees latitude) in 2007 was made using collocated measurements. The radio detection and ranging(radar)/light detection and ranging (lidar) (DARDAR) IWP data set, based on combined radar/lidar measurements, is used as a reference because it provides arguably the best estimate of the total column IWP. For each data set, usable IWP dynamic ranges are inferred from this comparison. IWP retrievals based on solar reflectance measurements, in the moderate resolution imaging spectroradiometer (MODIS), advanced very high resolution radiometer-based Climate Monitoring Satellite Applications Facility (CMSAF), and Pathfinder Atmospheres-Extended (PATMOS-x) datasets, were found to be correlated with DARDAR over a lar!
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| 2. |
- Eriksson, Patrick, 1964, et al.
(författare)
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Diurnal variations of humidity and ice water content in the tropical upper troposphere
- 2010
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 10:23, s. 11519-11533
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Tidskriftsartikel (refereegranskat)abstract
- Observational results of diurnal variations of humidity from Odin-SMR and AURA-MLS, and cloud ice mass from Odin-SMR and CloudSat are presented for the first time. Comparisons show that the retrievals of humidity and cloud ice from these two satellite combinations are in good agreement. The retrieved data are combined from four almost evenly distributed times of the day allowing mean values, amplitudes and phases of the diurnal variations around 200 hpa to be estimated. This analysis is applied to six climatologically distinct regions, five located in the tropics and one over the subtropical northern Pacific Ocean. The strongest diurnal cycles are found over tropical land regions, where the amplitude is ∼7 RHi for humidity and ∼50% for ice mass. The greatest ice mass for these regions is found during the afternoon, and the humidity maximum is observed to lag this peak by ∼6 h. Over tropical ocean regions the variations are smaller and the maxima in both ice mass and humidity are found during the early morning. Observed results are compared with output from three climate models (ECHAM, EC-EARTH and CAM3). Direct measurement-model comparisons were not possible because the measured and modelled cloud ice masses represent different quantities. To make a meaningful comparison, the amount of snow had to be estimated from diagnostic parameters of the models. There is a high probability that the models underestimate the average ice mass (outside the 1-σ uncertainty). The models also show clear deficiencies when it comes to amplitude and phase of the regional variations, but to varying degrees. © 2010 Author(s).
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| 3. |
- Eriksson, Patrick, 1964, et al.
(författare)
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Overview and sample applications of SMILES and Odin-SMR retrievals of upper tropospheric humidity and cloud ice mass
- 2014
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 14:23, s. 12613-12629
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Tidskriftsartikel (refereegranskat)abstract
- Retrievals of cloud ice mass and humidity from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) and the Odin-SMR (Sub-Millimetre Radiometer) limb sounder are presented and example applications of the data are given. SMILES data give an unprecedented view of the diurnal variation of cloud ice mass. Mean regional diurnal cycles are reported and compared to some global climate models. Some improvements in the models regarding diurnal timing and relative amplitude were noted, but the models' mean ice mass around 250 hPa is still low compared to the observations. The influence of the ENSO (El Niño-Southern Oscillation) state on the upper troposphere is demonstrated using 12 years of Odin-SMR data. The same retrieval scheme is applied for both sensors, and gives low systematic differences between the two data sets. A special feature of this Bayesian retrieval scheme, of Monte Carlo integration type, is that values are produced for all measurements but for some atmospheric states retrieved values only reflect a priori assumptions. However, this "all-weather" capability allows a direct statistical comparison to model data, in contrast to many other satellite data sets. Another strength of the retrievals is the detailed treatment of "beam filling" that otherwise would cause large systematic biases for these passive cloud ice mass retrievals. The main retrieval inputs are spectra around 635/525 GHz from tangent altitudes below 8/9 km for SMILES/Odin-SMR, respectively. For both sensors, the data cover the upper troposphere between 30° S and 30° N. Humidity is reported as both relative humidity and volume mixing ratio. The vertical coverage of SMILES is restricted to a single layer, while Odin-SMR gives some profiling capability between 300 and 150 hPa. Ice mass is given as the partial ice water path above 260 hPa, but for Odin-SMR ice water content, estimates are also provided. Besides a smaller contrast between most dry and wet cases, the agreement with Aura MLS (Microwave Limb Sounder) humidity data is good. In terms of tropical mean humidity, all three data sets agree within 3.5 %RHi. Mean ice mass is about a factor of 2 lower compared to CloudSat. This deviation is caused by the fact that different particle size distributions are assumed, combined with saturation and a priori influences in the SMILES and Odin-SMR data.
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| 4. |
- Johnston, Marston Sheldon, 1971, et al.
(författare)
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Diagnosing the average spatio-temporal impact of convective systems – Part 1: A methodology for evaluating climate models
- 2013
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 13:23, s. 12043-12058
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Tidskriftsartikel (refereegranskat)abstract
- An earlier method to determine the mean response of upper-tropospheric water to localised deep convective systems (DC systems) is improved and applied to the EC-Earth climate model. Following Zelinka and Hartmann (2009), several fields related to moist processes and radiation from various satellites are composited with respect to the local maxima in rain rate to determine their spatio-temporal evolution with deep convection in the central Pacific Ocean. Major improvements to the earlier study are the isolation of DC systems in time so as to prevent multiple sampling of the same event, and a revised definition of the mean background state that allows for better characterisation of the DC-system-induced anomalies. The observed DC systems in this study propagate westward at similar to 4 ms(-1). Both the upper-tropospheric relative humidity and the outgoing longwave radiation are substantially perturbed over a broad horizontal extent and for periods > 30 h. The cloud fraction anomaly is fairly constant with height but small maximum can be seen around 200 hPa. The cloud ice water content anomaly is mostly confined to pressures greater than 150 hPa and reaches its maximum around 450 hPa, a few hours after the peak convection. Consistent with the large increase in upper-tropospheric cloud ice water content, albedo increases dramatically and persists about 30 h after peak convection. Applying the compositing technique to EC-Earth allows an assessment of the model representation of DC systems. The model captures the large-scale responses, most notably for outgoing longwave radiation, but there are a number of important differences. DC systems appear to propagate east-ward in the model, suggesting a strong link to Kelvin waves instead of equatorial Rossby waves. The diurnal cycle in the model is more pronounced and appears to trigger new convection further to the west each time. Finally, the modelled ice water content anomaly peaks at pressures greater than 500 hPa and in the upper troposphere between 250 hPa and 500 hPa, there is less ice than the observations and it does not persist as long after peak convection. The modelled upper-tropospheric cloud fraction anomaly, however, is of a comparable magnitude and exhibits a similar longevity as the observations.
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| 5. |
- Johnston, Marston Sheldon, 1971, et al.
(författare)
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Diagnosing the average spatio-temporal impact of convective systems - Part 2: A model intercomparison using satellite data
- 2014
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 14:16, s. 8701-8721
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Tidskriftsartikel (refereegranskat)abstract
- The representation of the effect of tropical deep convective (DC) systems on upper-tropospheric moist processes and outgoing longwave radiation is evaluated in the EC-Earth3, ECHAM6, and CAM5 (Community Atmosphere Model) climate models using satellite-retrieved data. A composite technique is applied to thousands of deep convective systems that are identified using local rain rate maxima in order to focus on the temporal evolution of the deep convective processes in the model and satellite-retrieved data. The models tend to over-predict the occurrence of rain rates that are less than approximate to 3 mm h(-1) compared to Tropical Rainfall Measurement Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA). While the diurnal distribution of oceanic rain rate maxima in the models is similar to the satellite-retrieved data, the land-based maxima are out of phase. Despite having a larger climatological mean uppertropospheric relative humidity, models closely capture the satellite-derived moistening of the upper troposphere following the peak rain rate in the deep convective systems. Simulated cloud fractions near the tropopause are larger than in the satellite data, but the ice water contents are smaller compared with the satellite-retrieved ice data. The models capture the evolution of ocean-based deep convective systems fairly well, but the land-based systems show significant discrepancies. Over land, the diurnal cycle of rain is too intense, with deep convective systems occurring at the same position on subsequent days, while the satellite-retrieved data vary more in timing and geographical location. Finally, simulated outgoing longwave radiation anomalies associated with deep convection are in reasonable agreement with the satellite data, as well as with each other. Given the fact that there are strong disagreements with, for example, cloud ice water content, and cloud fraction, between the models, this study supports the hypothesis that such agreement with satellite-retrieved data is achieved in the three models due to different representations of deep convection processes and compensating errors.
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| 6. |
- Johnston, Marston Sheldon, 1971
(författare)
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Evaluating Tropical Upper-tropospheric Water in Climate Models Using Satellite Data
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Measuring and simulating moist processes in the tropical upper troposphere are difficult tasks. Humidity in this region of the atmosphere is mainly supplied by deep convection and, problems with simulated convection areknown to be a major contributor to uncertainties in climate model projections. Observations within this region of the atmosphere are hampered by the lowabsolute humidity as well as by the presence of clouds.This thesis examines the seasonal changes in and the effects tropicaldeep convection have on upper-tropospheric water, in addition to its effecton outgoing longwave radiation (OLR). Multiple satellite observations areassessed and used to evaluate the climate models EC-Earth, CAM5, andECHAM6. The data are analysed using two main methods: longterm averagesand compositing. Compositing represents an improvement over climatologiesbecause it brings the comparison closer to the processes associated with deepconvection. The compositing method is adapted from Zelinka and Hartmann[2009], improved, and applied for the first time to climate models.Upper-tropospheric humidity (UTH) undergoes large seasonal and regionalchanges in the tropics. Over land areas, convection is more intense, producinggreater amounts of water at higher heights, and having a greater effect on theOLR. Corresponding model simulations capture the large-scale and seasonalchanges, however there are significant inconsistencies when compared withthe observations, especially over land regions. Simulated mean UTH in areaswhere DC systems develop are consistently higher than observed over bothland and ocean. However, the direct response of UTH to DC systems is foundto be similar to the observations. Modeled cloud fractions near the tropopauseare tend to be overestimated, whereas ice water content is often too low. Theobserved OLR can, regionally, differ from the simulated results by as muchas 20 W m −1 . Moreover, above and around deep convection systems, thelocal decrease of OLR is throughout underestimated. Further, the modelsall demonstrate a lack of spatial variability indicated by a diurnal repetitionof convection at the same location over land. These results obtained by thecomposite method reveal details that could not have been obtained using atraditional climatology based comparison.
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| 7. |
- Johnston, Marston Sheldon, 1971
(författare)
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Satellite measurements and climate modelling of water in the tropical upper troposphere
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The International Panel on Climate Change (2007) has identified key areas of uncertainty with regards to future climate change, e.g., the upper troposphere. A portion of the uncertain- ties come from climate models and this makes model accuracy very important. Improvement in models can be achieved through evaluations that highlight problem areas. Satellites provide quality observations that can increase the level of scientific understanding of processes that are, even now, poorly understood. But satellites suffer from errors and therefore also need to be assessed.An introduction to the basics of the atmospheric component of a climate model and a cursive overview of satellite remote sensing, are presented. The model, EC Earth, is introduced where emphasis is placed on model parametrizations and cloud microphysics.The representation of tropical upper tropospheric (500–100hPa) humidity and clouds is evaluated in the climate model EC Earth. This is done with the aid of several satellite datasets using a model-to-retrieval method and involves a novel use of multiple measurement techniques to measure upper tropospheric variables. The effects of these variables on the outgoing long- wave radiation are also compared with satellite observations. The study shows that relative humidity in the model agrees well with observations from the Microwave Limb Sounder and the Atmospheric Infrared Sounder satellites, though the observations contain large uncertain- ties. EC Earth produces too much cloud above 200 hPa with often twice as much as the best estimates of cloud fraction (CALIPSO). The model’s precipitating ice and suspended ice are combined and compared to CloudSat’s ice water content (IWC). Above 300hPa, the model IWC is lower than the observations while below the model overestimates IWC. The impact on the outgoing longwave radiation, over the Tropics, by the model cirrus is underestimated by ∼9 W m−2 while, regionally, this bias can be ∼20 W m−2.
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| 8. |
- Johnston, Marston Sheldon, 1971, et al.
(författare)
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The representation of tropical upper tropospheric water in EC Earth V2
- 2012
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 39:11, s. 2713-2731
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Tidskriftsartikel (refereegranskat)abstract
- Tropical upper tropospheric humidity, clouds, and ice water content, as well as outgoing longwave radiation (OLR), are evaluated in the climate model EC Earth with the aid of satellite retrievals. The Atmospheric Infrared Sounder and Microwave Limb Sounder together provide good coverage of relative humidity. EC Earth's relative humidity is in fair agreement with these observations. CloudSat and CALIPSO data are combined to provide cloud fractions estimates throughout the altitude region considered (500-100 hPa). EC Earth is found to overestimate the degree of cloud cover above 200 hPa and underestimate it below. Precipitating and non-precipitating EC Earth ice definitions are combined to form a complete ice water content. EC Earth's ice water content is below the uncertainty range of CloudSat above 250 hPa, but can be twice as high as CloudSat's estimate in the melting layer. CERES data show that the model underestimates the impact of clouds on OLR, on average with about 9 W m(-2). Regionally, EC Earth's outgoing longwave radiation can be similar to 20 W m(-2) higher than the observation. A comparison to ERA-Interim provides further perspectives on the model's performance. Limitations of the satellite observations are emphasised and their uncertainties are, throughout, considered in the analysis. Evaluating multiple model variables in parallel is a more ambitious approach than is customary.
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