| 11. |
- 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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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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° 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 large IWP range (~20–7000 g m-2). The random errors of the collocated data sets have a close to lognormal 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 data sets is determined. Data sets based on passive microwave measurements, microwave surface and precipitation products system (MSPPS), microwave integrated retrieval system (MiRS), and collocated microwave only (CMO), are largely correlated with DARDAR for IWP values larger than approximately 700 g m-2. The combined uncertainty between these data sets and DARDAR in this range is slightly less MODIS-DARDAR, but the systematic bias is nearly an order of magnitude.
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| 12. |
- 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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| 13. |
- 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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| 14. |
- 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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| 15. |
- 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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| 16. |
- 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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| 17. |
- Moradi, Isaac, et al.
(författare)
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Comparing upper tropospheric humidity data from microwave satellite instruments and tropical radiosondes
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115:24
-
Tidskriftsartikel (refereegranskat)abstract
- Atmospheric humidity plays an important role in the Earth's climate. Microwave satellite data provide valuable humidity observations in the upper troposphere with global coverage. In this study, we compare upper tropospheric humidity (UTH) retrieved from the Advanced Microwave Sounding Unit (AMSU-B) and the Microwave Humidity Sounder (MHS) against radiosonde data measured at four of the central facilities of the Atmospheric Radiation Measurement (ARM) program. The Atmospheric Radiative Transfer Simulator (ARTS) was used to simulate satellite brightness temperatures from the radiosonde profiles. Strong ice clouds were filtered out, as their influence on microwave measurements leads to incorrect UTH values. Day and night radiosonde profiles were analyzed separately, to take into account the radiosonde radiation bias. The comparison between radiosonde and satellite is most meaningful for data in cloud free, night time conditions, and with a time difference of less than 2 hours. We found good agreement between the two data sets. The satellite data are slightly moister than the radiosonde data, with a mean difference of 1-2.3 %RH, depending on the radiosonde site. Monthly gridded data were also compared, and showed slightly larger mean difference of up to 3.3 %RH, which can be explained by sampling issues.
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| 18. |
- Vázquez-Martín, Sandra, et al.
(författare)
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Mass of different snow crystal shapes derived from fall speed measurements
- 2021
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus Publications. - 1680-7316 .- 1680-7324. ; 21:24, s. 18669-18688
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Tidskriftsartikel (refereegranskat)abstract
- Meteorological forecast and climate models require good knowledge of the microphysical properties of hydrometeors and the atmospheric snow and ice crystals in clouds. For instance, their size, cross-sectional area, shape, mass, and fall speed. Especially shape is an important parameter in that it strongly affects the scattering properties of ice particles, and consequently their response to remote sensing techniques. The fall speed and mass of ice particles are other important parameters both for numerical forecast models and for the representation of snow and ice clouds in climate models. In the case of fall speed, it is responsible for the rate of removal of ice from these models. The particle mass is a key quantity that connects the cloud microphysical properties to radiative properties. Using an empirical relationship between the dimensionless Reynolds and Best numbers, fall speed and mass can be derived from each other if particle size and cross-sectional area are also known.In this work, ground-based in-situ measurements of snow particle microphysical properties are used to analyse mass as a function of shape and the other properties particle size, cross-sectional area, and fall speed. The measurements for this study were done in Kiruna, Sweden during snowfall seasons of 2014 to 2019 and using the ground-based in-situ instrument Dual Ice Crystal Imager (D-ICI), which takes high-resolution side- and top-view images of natural hydrometeors. From these images, particle size (maximum dimension), cross-sectional area, and fall speed of individual particles are determined. The particles are shape classified according to the scheme presented in our previous work, in which particles sort into 15 different shape groups depending on their shape and morphology. Particle masses of individual ice particles are estimated from measured particle size, cross-sectional area, and fall speed. The selected dataset covers sizes from about 0.1 mm to 3.2 mm, fall speeds from 0.1 m s−1 to 1.6 m s−1, and masses from close to 0.2 μg to 320 μg. In our previous work, the fall speed relationships between particle size and cross-sectional area were studied. In this work, the same dataset is used to determine the particle mass, and consequently, the mass relationships between particle size, cross-sectional area, and fall speed are studied for these 15 shape groups. Furthermore, the mass relationships presented in this study are compared with the previous studies.For certain crystal habits, in particular columnar shapes, the maximum dimension is unsuitable for determining Reynolds number. Using a selection of columns, for which the simple geometry allows the verification of an empirical Best-number-to-Reynolds-number relationship, we show that Reynolds number and fall speed are more closely related to the diameter of the basal facet than the maximum dimension. The agreement with the empirical relationship is further improved using a modified Best number, a function of an area ratio based on the falling particle seen in the vertical direction.
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| 19. |
- Vázquez Martín, Sandra
(författare)
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Microphysical Properties of Snow Crystals Using Ground-Based In-Situ Instrumentation : Hunting Snowflakes
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding what happens to hydrometeors, such as atmospheric snow particles (ice crystals, snow crystals, and snowflakes) in clouds is crucial for improving meteorolog-ical forecast and climate models. Consequently, improved predictions of the precipitation amount reaching the ground (snowfall) require accurate knowledge of the microphysical properties of ice crystals, such as their size, cross-sectional area, shape, fall speed, and mass. In particular, the shape is an important parameter. It strongly influences the scattering properties of these ice particles. Snowfall has long been monitored by ground-based instruments, but instruments that can simultaneously measure all microphysical properties are still scarce. Accurate knowledge of microphysical properties is essential to achieve more realistic parameterizations in atmospheric models. Also, this knowledge is required for increasing accuracy of different remote sensing applications such as cloud and precipitation retrievals from passive and active measurements from satellites. Questions of particular interest are whether microphysical properties of precipitating snow particles show notably different characteristics depending on location, for instance at high-latitudes and what parame-terizations best describe these microphysical properties. How particle shape affects other properties, such as fall speed and mass, is also important. The particle shape is an important parameter, not only for the investigation of growth processes but also because of its importance for optical remote sensing retrievals of cloud properties and snow albedo. Therefore, studying snow microphysical properties and how they depend on particle shape is crucial to ensure accurate cloud parameterizations in climate and forecast models, and to the understanding of precipitation in cold climates.In this thesis ground-based in-situ measurements carried out in Kiruna, Sweden, are presented. Natural snow, ice crystals, and other hydrometeors covering particle sizes from 0.05 to 4 mm have been classified. Measurements have been taken during the snow-fall season from the beginning of November to the middle of May from 2014 to 2019. A ground-based in-situ instrument, Dual Ice Crystal Imager (D-ICI), which takes high-resolution side- and top-view images of hydrometeors was used. Particle size (maximum dimension), cross-sectional area, area ratio, aspect ratio, fall speed and mass of individual particles have been determined. A novel shape classification, where each particle shape is sorted into different shape groups, has been proposed, comprising a total of 135 unique shapes, including 34 new snow crystal shapes found in Kiruna. The main contributions of this thesis will enhance the improvement in the under-standing of precipitation in a cold climate. An updated snow crystal shape classification is presented and a different shape classification method is proposed. The new snow mea-surements and parameterizations studied in this work for different snow crystal shapes will be useful for climate and forecast models. These parameterizations include rela-tionships between particle size, cross-sectional area, fall speed and mass as a function of shape. The measured data shows a wide spread; however, binning the data according to size or cross-sectional area has improved correlations leading to more reliable parameteri-zations of fall speed versus size or cross-sectional area. Vertically orientated particles fall faster on average, but most particles for which orientation can be defined fall horizontally. The particle mass has been determined from measured particle size, cross-sectional area, and fall speed. When binning the data, the fall speed vs mass, mass vs particle size, and mass vs cross-sectional area relationships also show a high correlation. The relationships presented in this thesis have been compared with the results shown in previous studies.
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| 20. |
- Vázquez-Martín, Sandra, et al.
(författare)
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Shape Dependence of Falling Snow Crystals’ Microphysical Properties Using an Updated Shape Classification
- 2020
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Ingår i: Applied Sciences. - : MDPI. - 2076-3417. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- We present ground-based in situ snow measurements in Kiruna, Sweden, using the ground-based in situ instrument Dual Ice Crystal Imager (D-ICI). D-ICI records dual high-resolution images from above and from the side of falling natural snow crystals and other hydrometeors with particle sizes ranging from 50 µm to 4 mm. The images are from multiple snowfall seasons during the winters of 2014/2015 to 2018/2019, which span from the beginning of November to the middle of May. From our images, the microphysical properties of individual particles, such as particle size, cross-sectional area, area ratio, aspect ratio, and shape, can be determined. We present an updated classification scheme, which comprises a total of 135 unique shapes, including 34 new snow crystal shapes. This is useful for other studies that are using previous shape classification schemes, in particular the widely used Magono–Lee classification. To facilitate the study of the shape dependence of the microphysical properties, we further sort these individual particle shapes into 15 different shape groups. Relationships between the microphysical properties are determined for each of these shape groups.
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