scholarly journals Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

2016 ◽  
Vol 9 (8) ◽  
pp. 4151-4165 ◽  
Author(s):  
Felipe A. Mejia ◽  
Ben Kurtz ◽  
Keenan Murray ◽  
Laura M. Hinkelman ◽  
Manajit Sengupta ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Optical Depth ◽  
Zenith Angle ◽  
Microwave Radiometer ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Cloud Detection ◽  
Cloud Optical Depth ◽  
Angle Scattering ◽  
Atmospheric Radiation Measurement

Abstract. A method for retrieving cloud optical depth (τc) using a UCSD developed ground-based sky imager (USI) is presented. The radiance red–blue ratio (RRBR) method is motivated from the analysis of simulated images of various τc produced by a radiative transfer model (RTM). From these images the basic parameters affecting the radiance and red–blue ratio (RBR) of a pixel are identified as the solar zenith angle (θ0), τc, solar pixel angle/scattering angle (ϑs), and pixel zenith angle/view angle (ϑz). The effects of these parameters are described and the functions for radiance, Iλτc, θ0, ϑs, ϑz, and RBRτc, θ0, ϑs, ϑz are retrieved from the RTM results. RBR, which is commonly used for cloud detection in sky images, provides non-unique solutions for τc, where RBR increases with τc up to about τc = 1 (depending on other parameters) and then decreases. Therefore, the RRBR algorithm uses the measured Iλmeasϑs, ϑz, in addition to RBRmeasϑs, ϑz, to obtain a unique solution for τc. The RRBR method is applied to images of liquid water clouds taken by a USI at the Oklahoma Atmospheric Radiation Measurement (ARM) program site over the course of 220 days and compared against measurements from a microwave radiometer (MWR) and output from the Min et al. (2003) method for overcast skies. τc values ranged from 0 to 80 with values over 80, being capped and registered as 80. A τc RMSE of 2.5 between the Min et al. (2003) method and the USI are observed. The MWR and USI  have an RMSE of 2.2, which is well within the uncertainty of the MWR. The procedure developed here provides a foundation to test and develop other cloud detection algorithms.

scholarly journals Coupling sky images with three-dimensional radiative transfer models: a new method to estimate cloud optical depth

2015 ◽  
Vol 8 (10) ◽  
pp. 11285-11321 ◽  
Author(s):  
F. A. Mejia ◽  
B. Kurtz ◽  
K. Murray ◽  
L. M. Hinkelman ◽  
M. Sengupta ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Optical Depth ◽  
Zenith Angle ◽  
Microwave Radiometer ◽  
Three Dimensional ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Cloud Detection ◽  
Cloud Optical Depth ◽  
Direct Normal Irradiance

Abstract. A method for retrieving cloud optical depth (τc) using a ground-based sky imager (USI) is presented. The Radiance Red-Blue Ratio (RRBR) method is motivated from the analysis of simulated images of various τc produced by a 3-D Radiative Transfer Model (3DRTM). From these images the basic parameters affecting the radiance and RBR of a pixel are identified as the solar zenith angle (θ0), τc, solar pixel angle/scattering angle (ϑs), and pixel zenith angle/view angle (ϑz). The effects of these parameters are described and the functions for radiance, Iλ(τc, θ0, ϑs, ϑz) and the red-blue ratio, RBR(τc, θ0, ϑs, ϑz) are retrieved from the 3DRTM results. RBR, which is commonly used for cloud detection in sky images, provides non-unique solutions for τc, where RBR increases with τc up to about τc = 1 (depending on other parameters) and then decreases. Therefore, the RRBR algorithm uses the measured Iλmeas(ϑs, ϑz), in addition to RBRmeas(ϑs, ϑz) to obtain a unique solution for τc. The RRBR method is applied to images taken by a USI at the Oklahoma Atmospheric Radiation Measurement program (ARM) site over the course of 220 days and validated against measurements from a microwave radiometer (MWR); output from the Min method for overcast skies, and τc retrieved by Beer's law from direct normal irradiance (DNI) measurements. A τc RMSE of 5.6 between the Min method and the USI are observed. The MWR and USI have an RMSE of 2.3 which is well within the uncertainty of the MWR. An RMSE of 0.95 between the USI and DNI retrieved τc is observed. The procedure developed here provides a foundation to test and develop other cloud detection algorithms.

Estimation of Mixed-Phase Cloud Optical Depth and Position Using In Situ Radiation and Cloud Microphysical Measurements Obtained from a Tethered-Balloon Platform

2013 ◽  
Vol 70 (1) ◽  
pp. 317-329 ◽  
Author(s):  
M. Sikand ◽  
J. Koskulics ◽  
K. Stamnes ◽  
B. Hamre ◽  
J. J. Stamnes ◽  
...  
Keyword(s):  
Optical Depth ◽  
Rayleigh Scattering ◽  
High Arctic ◽  
Mixed Phase ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Liquid Droplets ◽  
Tethered Balloon ◽  
Cloud Particle ◽  
Cloud Optical Depth

Abstract Microphysical and radiative measurements in boundary layer mixed-phase clouds (MPCs), consisting of ice crystals and liquid droplets, have been analyzed. These cloud measurements were collected during a May–June 2008 tethered-balloon campaign in Ny-Ålesund, Norway, located at 78.9°N, 11.9°E in the High Arctic. The instruments deployed on the tethered-balloon platform included a radiometer, a cloud particle imager (CPI), and a meteorological package. To analyze the data, a radiative transfer model (RTM) was constructed with two cloud layers—consistent with the CPI data—embedded in a background Rayleigh scattering atmosphere. The mean intensities estimated from the radiometer measurements on the balloon were used in conjunction with the RTM to quantify the vertical structure of the MPC system, while the downward irradiances measured by an upward-looking ground-based radiometer were used to constrain the total cloud optical depth. The time series of radiometer and CPI data obtained while profiling the cloud system was used to estimate the time evolution of the liquid water and ice particle optical depths as well as the vertical location of the two cloud layers.

scholarly journals RTTOV-gb – Adapting the fast radiative transfer model RTTOV for the assimilation of ground-based microwave radiometer observations

2016 ◽  
Author(s):  
Francesco De Angelis ◽  
Domenico Cimini ◽  
James Hocking ◽  
Pauline Martinet ◽  
Stefan Kneifel
Keyword(s):  
Radiative Transfer ◽  
Liquid Water ◽  
Weather Prediction ◽  
Microwave Radiometer ◽  
Software Tool ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
60 Ghz ◽  
Variational Assimilation ◽  
Temperature And Humidity

Abstract. Ground-based microwave radiometers (MWR) offer a new capability to provide continuous observations of the atmospheric thermodynamic state in the planetary boundary layer. Thus, they are potential candidates to supplement radiosonde network and satellite data to improve numerical weather prediction (NWP) models through a variational assimilation of their data. However in order to assimilate MWR observations a fast radiative transfer model is required and such a model is not currently available. This is necessary for going from the model state vector space to the observation space at every observation point. The fast radiative transfer model RTTOV is well accepted in the NWP community, though it was developed to simulate satellite observations only. In this work, the RTTOV code has been modified to allow for simulations of ground-based upward looking microwave sensors. In addition, the Tangent Linear, Adjoint, and K-modules of RTTOV have been adapted to provide Jacobians (i.e. the sensitivity of observations to the atmospheric thermodynamical state) for ground-based geometry. These modules are necessary for the fast minimization of the cost function in a variational assimilation scheme. The proposed ground-based version of RTTOV, called RTTOV-gb, has been validated against accurate and less time-efficient line-by-line radiative transfer models. In the frequency range commonly used for temperature and humidity profiling (22–60 GHz), root-mean-square brightness temperature differences are smaller than typical MWR uncertainties (~ 0.5 K) at all channels used in this analysis. Brightness temperatures (TB) computed with RTTOV-gb from radiosonde profiles have been compared with nearly simultaneous and colocated ground-based MWR observations. Differences between simulated and measured TB are below 0.5 K for all channels except for the water vapor band, where most of the uncertainty comes from instrumental errors. The Jacobians calculated with the K-module of RTTOV-gb have been compared with those calculated with the brute force technique and those from the line-by-line model ARTS. Jacobians are found to be almost identical, except for liquid water content Jacobians for which a 10 % difference between ARTS and RTTOV-gb at transparent channels around 450 hPa is attributed to differences in liquid water absorption models. Finally, RTTOV-gb has been applied as the forward model operator within a 1-Dimensional Variational (1D-Var) software tool in an Observing-System Simulation Experiment (OSSE). For both temperature and humidity profiles, the 1D-Var with RTTOV-gb improves the retrievals with respect to NWP model in the first few kilometers from the ground.

scholarly journals Optical instruments synergy in determination of optical depth of thin clouds

2018 ◽  
Vol 176 ◽  
pp. 08008
Author(s):  
Daniela Viviana Vlăduţescu ◽  
Stephen E. Schwartz ◽  
Dong Huang
Keyword(s):  
Optical Depth ◽  
Rayleigh Scattering ◽  
Climate Models ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Doppler Lidar ◽  
Cloud Optical Depth ◽  
Cloud Structure ◽  
Optical Instruments

Optically thin clouds have a strong radiative effect and need to be represented accurately in climate models. Cloud optical depth of thin clouds was retrieved using high resolution digital photography, lidar, and a radiative transfer model. The Doppler Lidar was operated at 1.5 μm, minimizing return from Rayleigh scattering, emphasizing return from aerosols and clouds. This approach examined cloud structure on scales 3 to 5 orders of magnitude finer than satellite products, opening new avenues for examination of cloud structure and evolution.

scholarly journals Using a Parameterization of a Radiative Transfer Model to Build High-Resolution Maps of Typical Clear-Sky UV Index in Catalonia, Spain

2005 ◽  
Vol 44 (6) ◽  
pp. 789-803 ◽  
Author(s):  
Jordi Badosa ◽  
Josep-Abel González ◽  
Josep Calbó ◽  
Michiel van Weele ◽  
Richard L. McKenzie
Keyword(s):  
High Resolution ◽  
Radiative Transfer ◽  
Zenith Angle ◽  
Solar Zenith Angle ◽  
Absolute Error ◽  
Single Scattering ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Uv Index ◽  
Wide Range

Abstract To perform a climatic analysis of the annual UV index (UVI) variations in Catalonia, Spain (northeast of the Iberian Peninsula), a new simple parameterization scheme is presented based on a multilayer radiative transfer model. The parameterization performs fast UVI calculations for a wide range of cloudless and snow-free situations and can be applied anywhere. The following parameters are considered: solar zenith angle, total ozone column, altitude, aerosol optical depth, and single-scattering albedo. A sensitivity analysis is presented to justify this choice with special attention to aerosol information. Comparisons with the base model show good agreement, most of all for the most common cases, giving an absolute error within ±0.2 in the UVI for a wide range of cases considered. Two tests are done to show the performance of the parameterization against UVI measurements. One uses data from a high-quality spectroradiometer from Lauder, New Zealand [45.04°S, 169.684°E, 370 m above mean sea level (MSL)], where there is a low presence of aerosols. The other uses data from a Robertson–Berger-type meter from Girona, Spain (41.97°N, 2.82°E, 100 m MSL), where there is more aerosol load and where it has been possible to study the effect of aerosol information on the model versus measurement comparison. The parameterization is applied to a climatic analysis of the annual UVI variation in Catalonia, showing the contributions of solar zenith angle, ozone, and aerosols. High-resolution seasonal maps of typical UV index values in Catalonia are presented.

scholarly journals A Parametric Radiative Forcing Model for Contrail Cirrus

2012 ◽  
Vol 51 (7) ◽  
pp. 1391-1406 ◽  
Author(s):  
U. Schumann ◽  
B. Mayer ◽  
K. Graf ◽  
H. Mannstein
Keyword(s):  
Analytical Model ◽  
Optical Depth ◽  
Zenith Angle ◽  
Radiative Forcing ◽  
Solar Zenith Angle ◽  
Life Cycles ◽  
Radiative Transfer Model ◽  
Model Parameters ◽  
Transfer Model ◽  
Wide Range

AbstractA new parameterized analytical model is presented to compute the instantaneous radiative forcing (RF) at the top of the atmosphere (TOA) produced by an additional thin contrail cirrus layer (called “contrail” below). The model calculates the RF using as input the outgoing longwave radiation and reflected solar radiation values at TOA for a contrail-free atmosphere, so that the model is applicable for both cloud-free and cloudy ambient atmospheres. Additional input includes the contrail temperature, contrail optical depth (at 550 nm), effective particle radius, particle habit, solar zenith angle, and the optical depth of cirrus above the contrail layer. The model parameters (5 for longwave and 10 for shortwave) are determined from least squares fits to calculations from the “libRadtran” radiative transfer model over a wide range of atmospheric and surface conditions. The correlation coefficient between model and calculations is larger than 98%. The analytical model is compared with published results, including a 1-yr simulation of global RF, and is found to agree well with previous studies. The fast analytical model is part of a larger modeling system to simulate contrail life cycles (“CoCiP”) and can allow for the rapid simulation of contrail cirrus RF over a wide range of meteorological conditions and for a given size-dependent habit mixture. Ambient clouds are shown to have large local impact on the net RF of contrails. Net RF of contrails may both increase and decrease and even change sign in the presence of higher-level cirrus, depending on solar zenith angle.

scholarly journals Detection of the Depth-Hoar Layer in the Snow-Pack of the Arctic Coastal Plain of Alaska, U.S.A., Using Satellite Data

1986 ◽  
Vol 32 (110) ◽  
pp. 87-94 ◽  
Author(s):  
D. K. Hall ◽  
A. T. C. Chang ◽  
J. L. Foster
Keyword(s):  
Radiative Transfer ◽  
Coastal Plain ◽  
Crystal Size ◽  
Microwave Radiometer ◽  
The Arctic ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Snow Pack ◽  
Arctic Coastal Plain ◽  
Crystal Diameter

AbstractThe snow-pack on the Arctic Coastal Plain of Alaska has a well-developed depth-hoar layer which forms each year at the base of the snow-pack due to upward vapor transfer resulting from a temperature gradient in the snow-pack. The thickness of the depth-hoar layer tends to increase inland where greater temperature extremes (in particular, lower minimum temperatures) permit larger temperature gradients to develop within the snow-pack. Brightness temperature (TB) data were analyzed from October through May for four winters using the 37 GHz horizontally polarized Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR). By mid-winter each year, a decrease in TB of approximately 20K was found between coastal and inland sites on the Arctic Coastal Plain of Alaska. Modeling has indicated that a thicker depth-hoar layer in the inland sites could be responsible for the lower TBs. The large grain-sizes of the depth-hoar crystals scatter the upwelling radiation moreso than do smaller crystals, and greater scattering lowers the microwave TB. Using a two-layered radiative transfer model, the crystal diameter in the top layer was assumed to be 0.50 mm. The crystals in the depth-hoar layer may be 5–10 mm in diameter but the effective crystal diameter used in the radiative-transfer model is 1.40 mm. The crystal size used in the model had to be adjusted downward, relative to the actual crystal size, because the hollow, cup-shaped depth-hoar crystals are not as effective at scattering the microwave radiation as are spherical crystals that are assumed in the model. In the model, when the thickness of the depth-hoar layer was increased from 5 cm to 10 cm, a 21K decrease in TB resulted. This is comparable to the decrease in TB observed from coastal to inland sites in the study area.

scholarly journals NWCSAF AVHRR Cloud Detection and Analysis Using Dynamic Thresholds and Radiative Transfer Modeling. Part II: Tuning and Validation

2005 ◽  
Vol 44 (1) ◽  
pp. 55-71 ◽  
Author(s):  
Adam Dybbroe ◽  
Karl-Göran Karlsson ◽  
Anke Thoss
Keyword(s):  
Radiative Transfer ◽  
Forecast Model ◽  
Radar Data ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Cloud Detection ◽  
Analysis Model ◽  
Cloud Type ◽  
Avhrr Data ◽  
Cloud Mask

Abstract Algorithms for cloud detection (cloud mask) and classification (cloud type) at high and midlatitudes using data from the Advanced Very High Resolution Radiometer (AVHRR) on board the current NOAA satellites and future polar Meteorological and Operational Weather Satellites (METOP) of the European Organisation for the Exploitation of Meteorological Satellites have been extensively validated over northern Europe and the adjacent seas. The algorithms have been described in detail in Part I and are based on a multispectral grouped threshold approach, making use of cloud-free radiative transfer model simulations. The thresholds applied in the algorithms have been validated and tuned using a database interactively built up over more than 1 yr of data from NOAA-12, -14, and -15 by experienced nephanalysts. The database contains almost 4000 rectangular (in the image data)-sized targets (typically with sides around 10 pixels), with satellite data collocated in time and space with atmospheric data from a short-range NWP forecast model, land cover characterization, elevation data, and a label identifying the given cloud or surface type as interpreted by the nephanalyst. For independent and objective validation, a large dataset of nearly 3 yr of collocated surface synoptic observation (Synop) reports, AVHRR data, and NWP model output over northern and central Europe have been collected. Furthermore, weather radar data were used to check the consistency of the cloud type. The cloud mask performs best over daytime sea and worst at twilight and night over land. As compared with Synop, the cloud cover is overestimated during night (except for completely overcast situations) and is underestimated at twilight. The algorithms have been compared with the more empirically based Swedish Meteorological and Hydrological Institute (SMHI) Cloud Analysis Model Using Digital AVHRR Data (SCANDIA), operationally run at SMHI since 1989, and results show that performance has improved significantly.

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