Search

During a number of transmission experiments over littoral waters, quantitative measurements of atmospheric refraction phenomena were carried out to determine the range performance of optical-IR sensors. Examples of distortion and intensity gain generated by spatial variations of the atmospheric refractive index are shown. A high-precision ray-tracing model has been developed for better understanding of the phenomena and to satisfy the requirements for accuracy of the meteorological data used in refraction models. The output of the model includes the propagation function, the intensity gain, and details of the ray curvature and of the optical phase behavior along the path between the target and the observer. Examples of measured transmission data and their interpretation are presented. OCIS codes: 010.0010, 010.1300, 010.1310, 010.4030.

The European Space Agency (ESA) Earth Explorer Atmospheric Dynamics Mission Aeolus is the first satellite mission providing wind profile information on a global scale, and its wind products have been released on 12 May 2020. Here we verify and intercompare the wind observations from ESA's satellite mission Aeolus and the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth generation atmospheric reanalyses (ERA5) with radiosonde (RS) observations over China, to allow a fitting application of Aeolus winds. Aeolus provides wind observations in aerosol-free (referred to as Rayleigh-clear winds) and cloudy atmospheres (Mie-cloudy winds). In terms of Aeolus and RS winds, the correlation coefficient (R) and mean difference of Rayleigh-clear (Mie-cloudy) vs RS winds are 0.94 (0.97) and -0.24 ± 7.01 (0.18 ± 4.42) m/s, respectively. The vertical profiles of wind speed differences between Aeolus and RS winds are similar to each other during ascending and descending orbits. The comparison of ECMWF winds relative to Aeolus winds provides the R and mean difference of Rayleigh-clear (Mie-cloudy) winds, which are 0.95 (0.97) and -0.16 ± 6.78 (-0.21 ± 3.91) m/s, respectively. The Rayleigh-clear and Mie-cloudy winds are very consistent with the ECMWF winds, likely due to the assimilation of Aeolus wind observations into the ECMWF analysis. Moreover, we find that among the results of comparing Aeolus with RS and ECMWF winds, a small difference between Rayleigh-clear winds relative to RS winds is appeared in the height range of 2-3 km during descending orbits. This result may be due to the high vertical velocity during the descending orbits. The mean differences between Rayleigh-clear (Mie-cloudy) winds and RS winds during the ascending and descending orbit phase are -0.07 ± 0.69 (-0.72 ± 1.48) and 0.3 ± 1.25 (0.1 ± 1.32) m/s. These small deviations indicate that the performance of Aeolus wind products may be unaffected by the orbit phase or HLOS wind conditions. In addition, the R and mean difference between ERA5 and RS zonal wind components are 0.97 and -0.46 ± 3.12 m/s, respectively. Overall, the Aeolus winds over China are similar to the RS and ECMWF winds. The findings give us sufficient confidence and information to apply Aeolus wind products in numerical weather prediction in China and in climate change research. [ABSTRACT FROM AUTHOR]

A pulsed hybrid C[O.sub.2] transversely excited atmosphere (TEA) laser has been used in a bistatic laser rangefinder-velocimeter system with heterodyne detection. Several techniques have been applied to improve the performance of the system. These include the stabilization of the hybrid C[O.sub.2] TEA-laser and the stabilization of the frequency offset of the local oscillator (better than [+ or -] 74 kHz peak to peak), phase-front matching at the detector surfac resulting in a heterodyne beat efficiency of 0.4-0.6, and chirp correction. Wit this system, targets at distances of up to 25 km can be detected with an accuracy of 15m. The velocity of the targets can be estimated with an accuracy of approximately [+ or -] 0.5 m/s.

In homogeneous atmospheres, backscatter and extinction coefficients are commonly determined by the inversion of lidar signals by using the slope method, i.e., from a linear least-squares fit to the logarithm on the range-compensated lidar return. We investigate the accuracy of this method. A quantitative analysis is presented of the influence of white noise and atmospheric extinction on the accuracy of the slope method and on the maximum range of lidar systems. To meet this objective, we simulate lidar signals with extinction coefficients ranging from 10 to the -3 km to the -1 to 10 km to the -1 with different signal-to-noise ratios. It is shown that the backscatter coefficient can be determined by using the slope method with an accuracy of better than approximately 10% if the extinction coefficient is smaller than 1 km to the -1 and the signal-to-noise ratio is better than approximately 1000. The accuracy in the calculated extinction coefficient is only better than approximately 10% if the extinction is larger than 1 km to the -1 and the signal-to-noise ratio is better than approximately 2000. If the atmospheric extinction coefficient is smaller than 0.1 km to the -1, then it is not possible to invert the extinction from lidar measurements with an accuracy of 10% or better unless the signal-to-noise ratio is unrealistically high.

The Nordic Centre of Excellence CRAICC (CRyosphere-Atmosphere Interactions in a Changing Arctic Climate), funded by NordForsk in the years 2011-2016, was the largest joint Nordic research and innovation initiative to date, aiming to strengthen research and innovation regarding climate change issues in the Nordic Region. CRAICC gathered more than 100 scientists from all Nordic countries in a virtual Centre with the objectives to identify and quantify the major processes controlling Arctic warming and related feedback mechanisms, to outline strategies to mitigate Arctic warming and to develop Nordic Earth System modelling with a focus on the short-lived climate forcers (SLCF), including natural and anthropogenic aerosols. The outcome of CRAICC is reflected in more than 150 peer-reviewed scientific publications, most of which are in the CRAICC special-issue of the journal Atmospheric Chemistry and Physics. This manuscript presents an overview on the main scientific topics investigated in the Centre and provides the reader a state-of-the-art comprehensive summary of what has been achieved in CRAICC with links to the particular publications for further detail. Facing the vast amount of outcomes we are not claiming to cover all results from CRAICC in this manuscript but concentrate here on the main results which are related to the feedback loops in the climate change-cryosphere interaction scheme affecting the Arctic amplification. [ABSTRACT FROM AUTHOR]

The data flow involved in a long-term continuous solar spectral UV irradiance monitoring program is investigated and structured to provide an overall view on the multiphase process from data acquisition to the final products. The program employing Brewer spectrophotometers as measuring instruments is maintained by the Finnish Meteorological Institute (FMI) ever since the 1990s at two sites in Finland: Sodankylä (67◦ N) and Jokioinen (61◦ N). It is built upon rigorous operation routines, processing procedures, and tools for quality control (QC) and quality analysis (QA) under continuous development and evaluation. Three distinct levels of data emerge, each after certain phase in the data flow: Level 0 denoting raw data, Level 1 meaning calibrated data processed in near-real time, and Level 2 comprising of postprocessed data corrected for all distinguishable errors and known inaccuracies. The final products disseminated to the users are demonstrated to result from a process with a multitude of separate steps, each required in the production of high-quality data on solar UV radiation at the Earth’s surface. [ABSTRACT FROM AUTHOR]

Affordable high quality charge-coupled device (CCD) video cameras and image processing software are powerful tools for bubble measurements. Because of the wide variation between bubble populations, different bubble measurement systems (BMSs) are required depending upon the application. Two BMSs are described: a mini-BMS designed to observe the background bubble population from breaking waves, and a large-BMS designed to noninvasively determine the time-resolved bubble distribution inside dense bubble plumes and near the interface, as are details of the analysis techniques. Using the two systems in conjunction with each other allowed size distributions over the range 15–5000-μm radius to be obtained. The BMSs were designed for application to breaking-wave bubble plumes in the field or laboratory. Distributions measured by both BMSs in aerator-generated plumes agreed very well for the overlapping size range. Also presented are observations of bubble plumes produced by breaking waves in a large wind-wave flume, and calibration experiments showing the effect on measured bubble size due to blur induced by slow shutter speeds. [ABSTRACT FROM AUTHOR]

The SEMARA approach, an integration of the Simplified and Robust Surface Reflectance Estimation (SREM) and Simplified Aerosol Retrieval Algorithm (SARA) methods, was used to retrieve aerosol optical depth (AOD) at 550 nm from a Landsat 8 Operational Land Imager (OLI) at 30 m spatial resolution, a Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) at 500 m resolution, and a Visible Infrared Imaging Radiometer Suite (VIIRS) at 750 m resolution over bright urban surfaces in Beijing. The SEMARA approach coupled (1) the SREM method that is used to estimate the surface reflectance, which does not require information about water vapor, ozone, and aerosol, and (2) the SARA algorithm, which uses the surface reflectance estimated by SREM and AOD measurements obtained from the Aerosol Robotic NETwork (AERONET) site (or other high-quality AOD) as the input to estimate AOD without prior information on the aerosol optical and microphysical properties usually obtained from a look-up table constructed from long-term AERONET data. In the present study, AOD measurements were obtained from the Beijing AERONET site. The SEMARA AOD retrievals were validated against AOD measurements obtained from two other AERONET sites located at urban locations in Beijing, i.e., Beijing_RADI and Beijing_CAMS, over bright surfaces. The accuracy and uncertainties/errors in the AOD retrievals were assessed using Pearson's correlation coefficient (r), root mean squared error (RMSE), relative mean bias (RMB), and expected error (EE = ± 0.05 ± 20%). EE is the envelope encompassing both absolute and relative errors and contains 68% (±1σ) of the good quality retrievals based on global validation. Here, the EE of the MODIS Dark Target algorithm at 3 km resolution is used to report the good quality SEMARA AOD retrievals. The validation results show that AOD from SEMARA correlates well with AERONET AOD measurements with high correlation coefficients (r) of 0.988, 0.980, and 0.981; small RMSE of 0.08, 0.09, and 0.08; and small RMB of 4.33%, 1.28%, and −0.54%. High percentages of retrievals, i.e., 85.71%, 91.53%, and 90.16%, were within the EE for Landsat 8 OLI, MODIS, and VIIRS, respectively. The results suggest that the SEMARA approach is capable of retrieving AOD over urban areas with high accuracy and small errors using high to medium spatial resolution satellite remote sensing data. This approach can be used for aerosol monitoring over bright urban surfaces such as in Beijing, which is frequently affected by severe dust storms and haze pollution, to evaluate their effects on public health. [ABSTRACT FROM AUTHOR]

This study investigates spatiotemporal changes in air pollution (particulate as well as gases) during the COVID-19 lockdown period over major cities of Bangladesh. The study investigated the aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Terra and Aqua satellites, PM2.5 and PM10 from Copernicus Atmosphere Monitoring Service (CAMS), and NO2 and O3 from TROPOMI-5P, from March to June 2019–2020. Additionally, aerosol subtypes from the Cloud-Aerosol Lidar and Infrared Pathfinder (CALIPSO) were used to explore the aerosol types. The strict lockdown (26 March–30 May 2020) led to a significant reduction in AOD (up to 47%) in all major cities, while the partial lockdown (June 2020) led to increased and decreased AOD over the study area. Significant reductions in PM2.5 (37–77%) and PM10 (33–70%) were also observed throughout the country during the strict lockdown and partial lockdown. The NO2 levels decreased by 3–25% in March 2020 in the cities of Rajshahi, Chattogram, Sylhet, Khulna, Barisal, and Mymensingh, in April by 3–43% in Dhaka, Chattogram, Khulna, Barisal, Bhola, and Mymensingh, and May by 12–42% in Rajshahi, Sylhet, Mymensingh, and Rangpur. During the partial lockdown in June, NO2 decreased (9–35%) in Dhaka, Chattogram, Sylhet, Khulna, Barisal, and Rangpur compared to 2019. On the other hand, increases were observed in ozone (O3) levels, with an average increase of 3–12% throughout the country during the strict lockdown and only a slight reduction of 1–3% in O3 during the partial lockdown. In terms of aerosol types, CALIPSO observed high levels of polluted dust followed by dust, smoke, polluted continental, and clean marine-type aerosols over the country in 2019, but all types were decreased during the lockdown. The study concludes that the strict lockdown measures were able to significantly improve air quality conditions over Bangladesh due to the shutdown of industries, vehicles, and movement of people. [ABSTRACT FROM AUTHOR]

A cold vortex is an important weather system that can cause low temperatures and large amounts of rainfall. Many scientific studies have focused on the climatological features of the cold vortex along with its formation, maintenance, structure, circulation features and precipitation distribution from a synoptic perspective. However, not many studies have been conducted related to the microphysical processes in a cold vortex. In this paper, a model study is presented on the microphysical features in a cloud system associated with a cold vortex system which lasted for four days. The system formed, strengthened, split into two and dissipated while it moved towards the east and brought extensive precipitation to the influenced area. The type and amount of precipitation were not evenly distributed: liquid precipitation covered the whole area influenced by the cloud system, while solid precipitation mainly covered high latitudes north of 48° N. In this case, the cloud system was very cold and the 0 °C; level was very low, with the result that the microphysical features were dominated by cold cloud processes. The mixing ratio of liquid particles was very small, and liquid particles only covered limited areas in the clouds. Due to the low temperature, there were insufficient cloud droplets throughout the whole system for the efficient production of rain droplets by coalescence. The snow mixing ratio was largest, and ice and snow covered the whole cloud area, with very little graupel, which occurred in only a small area. The distribution of the solid particles was influenced by their growth through aggregation, the Bergeron process or accretion, resulting in many snow particles. There were insufficient liquid particles to support the formation of very large particles such as graupel. Liquid precipitation was primarily caused by the melting of snow particles precipitating out of the clouds; solid precipitation was also primarily caused by snow particles that did not melt, along with some graupel. [ABSTRACT FROM AUTHOR]