Transition from Fog to Stratus over the Northwest Pacific Ocean: Large-eddy Simulation

Over the midlatitude northwest Pacific Ocean, summer fog frequents the Kuroshio-Oyashio front as a result of the warm advection by the prevailing southerly to southwesterly winds, and stratus clouds are prevalent downstream of the fog regime in the subpolar northwest Pacific. The present study tracks a boundary-layer air column along a typical northeastward trajectory along which fog on the sea surface temperature (SST) front makes its transition to stratus clouds. A turbulence-closure large-eddy simulation model can capture the evolution of the air column forced by the time-varying SST along the trajectory. Results show that the surface cooling effects across the SST front and the longwave radiative cooling (LRC) at the cloud top dominate the evolution of the boundary layer and the related turbulent processes. The sharp SST decrease across the SST front cools the surface layer, leading to condensation through shear-induced turbulence. Once the fog forms, the LRC at the fog top cools the boundary layer strongly through thermal turbulent mixing. The buoyancy-induced turbulence near the fog top entrains the warm and dry air from the free atmosphere into the boundary layer, reducing surface humidity and ultimately lifting the cloud base away from the sea surface to form stratus clouds. Sensitivity simulations also suggest that neither the latent heat flux from ocean nor and the diurnal solar variation is essential for the summer fog-to-stratus transition over the northwestern Pacific.

Benefit of microwave radiometer and cloud radar observations for data assimilation and fog process studies during the SOFOG3D experiment

<p>Fog forecasts still remain quite inaccurate due to the complexity, non linearities and fine scale of the main physical processes driving the fog lifecycle. Additionally to the complex modelling of fog processes, current numerical weather prediction models are known to suffer from a lack of operational observations in the atmospheric boundary layer and more generally during cloudy-sky conditions. Continuous observations of both thermodynamics and microphysics during the fog lifecycle are thus essential to develop future operational networks with the aim of validating current physical parameterizations and improving the model initial state through data assimilation techniques. In this context, an international network of 8 ground-based microwave radiometers (MWRs) has been deployed at a regional-scale on a 300 x 300 km domain during the SOFOG3D (SOuth FOGs 3D experiment for fog processes study) that has been conducted from October 2019 to April 2020. The MWR network has been extended with ceilometers at all MWR sites and additional microphysical observations from the 95 GHz cloud radar BASTA at two major sites as well as wind measurements from a Doppler lidar deployed at the super-site. After an overview of the SOFOG3D objectives and experimental set-up, preliminary results exploiting mainly the MWR network and cloud radar observations will be presented. Firstly, the capability of MWRs to provide temperature and humidity retrievals within fog and stratus clouds will be evaluated and discussed against radiosoundings launched during intensive observation periods (IOPs). Secondly, first retrievals of liquid water content profiles within fog and stratus clouds derived from the synergy between MWRs and the BASTA cloud radar will be presented. To that end, a one dimensional variational approach (1D-Var) directly assimilating MWR brightness temperatures and cloud-radar reflectivities has been developed. 1D-Var retrievals will be validated through a dataset of simulated observations and real fog cases of the SOFOG3D experiment. The capability of MWR and cloud radar observations to improve the initial state of the AROME model during fog conditions will be discussed with a focus on selected case studies. Finally, the usefulness of ground-based remote sensing networks to improve our understanding of fog processes and to validate physical parameterizations will be illustrated using the operational AROME model and the AROME Ensemble Prediction System</p>

Influence of unbroken clouds stochastic structure on the solar radiation transfer with results of Monte Carlo simulation

The paper is focused on construction of computational models of stratus clouds using remote sensing data and on Monte Carlo analysis of optical radiation transfer peculiarities caused by stochastic structure of clouds.

Stratus 17 Seventeenth Setting of the Stratus Ocean Reference Station Cruise on Board RV Cabo de Hornos April 3 - 16, 2018 Valparaiso - Valparaiso, Chile

The Ocean Reference Station at 20°S, 85°W under the stratus clouds west of northern Chile is being maintained to provide ongoing climate-quality records of surface meteorology, air-sea fluxes of heat, freshwater, and momentum, and of upper ocean temperature, salinity, and velocity variability. The Stratus Ocean Reference Station (ORS Stratus) is supported by the National Oceanic and Atmospheric Administration’s (NOAA) Climate Observation Program. It is recovered and redeployed annually, with past cruises that have come between October and May. This cruise was conducted on the Chilean research vessel Cabo de Hornos. During the 2018 cruise on the Cabo de Hornos to the ORS Stratus site, the primary activities were the recovery of the previous (Stratus 16) WHOI surface mooring, deployment of the new Stratus 17 WHOI surface mooring, in-situ calibration of the buoy meteorological sensors by comparison with instrumentation installed on the ship, CTD casts near the moorings. The Stratus 17 had parted from its anchor site on January 4 2018, so its recovery was done in two separate operations: first the drifting buoy with mooring line under it, then the bottom part still attached to the anchor. Surface drifters and ARGO floats were also launched along the track.

Stratus 16 Sixteenth Setting of the Stratus Ocean Reference Station Cruise on Board RV Ronald H. Brown May 5 - 20, 2017 Rodman, Panama - Arica, Chile

The Ocean Reference Station at 20°S, 85°W under the stratus clouds west of northern Chile is being maintained to provide ongoing climate-quality records of surface meteorology, air-sea fluxes of heat, freshwater, and momentum, and of upper ocean temperature, salinity, and velocity variability. The Stratus Ocean Reference Station (ORS Stratus) is supported by the National Oceanic and Atmospheric Administration’s (NOAA) Climate Observation Program. It is recovered and redeployed annually, with past cruises that have come between October and May. This cruise was conducted on the NOAA research vessel Ronald H. Brown. During the 2017 cruise on the Ronald H. Brown to the ORS Stratus site, the primary activities were the recovery of the previous (Stratus 15) WHOI surface mooring, deployment of the new Stratus 16 WHOI surface mooring, in-situ calibration of the buoy meteorological sensors by comparison with instrumentation installed on the ship, CTD casts near the moorings. Surface drifters and ARGO floats were also launched along the track.

SOKUR K., PALAMARCHUK L. CONDITIONS FOR THE FORMATION OF SURFACE RUNOFF, WHICH IS FORMED AS A RESULT OF HEAVY AND DANGEROUS PRECIPITATION WITHIN URBANIZED AREAS

The research investigates atmospheric precipitation, which according to Ukrainian national regulation has reached the criteria of heavy (≥ 50 mm at ≤ 12 hours) and dangerous (15 – 49 mm at ≤ 12 hours). A total of 98 cases of heavy precipitation (2005 – 2018), and 14 cases of dangerous precipitation (2017 – 2018) were analyzed. The research focuses on the formation conditions and volumes of surface runoff that forms on various types of underlying surface. To obtain a statistically valid classification, a cluster analysis of heavy and dangerous precipitation was carried out. The analysis allowed to distinguish three blocks or clusters, one of which corresponds to the type “mix” of heavy and slight precipitations during the development of frontal stratus clouds with so-called “submerged or flooded” convection, the second cluster corresponds to the type “heavy precipitations”, the third cluster is similar to the first cluster, but is marked by a decrease in the intensity of processes. The degree of connection between the intensity of heavy precipitation and the height of the clouds top was investigated. As a result, it was found that there is an inverse relation between the values. The physical features of the processes of cloud and precipitation formation was considered. The exceptional role of convective clouds in individual frontal massifs and also in the form of “submerged or flooded convection” in the massifs of stratus clouds in the formation of significant volumes of rainwater on the underlying surface was noted. The structure of the temporal changes in precipitation intensity was established: the maximum intensity values, the time of their occurrence, the availability and number of amplification waves and their temporal parameters. On the basis of the obtained indicators, the quantity of the surface runoff for determined processes and for amplification periods were calculated. As a result, it was found that the surface runoff, which formed in the built-up areas, exceeds the surface runoff from the moderately built-up area with almost no artificial pavement by 100 – 300 %.

Observations on aerosol optical properties and scavenging during cloud events

Long term statistics of atmospheric aerosol and especially cloud scavenging were studied at the Puijo measurement station in Kuopio, Finland, during October 2010–November 2014. Aerosol size distributions, scattering coefficients at three different wavelengths (450 nm, 550 nm, and 700 nm), and absorption coefficient at wavelength 637 nm were measured with a special inlet system to sample interstitial and total aerosol in clouds. On average, accumulation mode particle concentration was found to be temperature dependent with lowest average concentrations of 200 cm−3 around 0 °C increasing to more than 800 cm−3 for temperatures higher than 20 °C. From the in-cloud measurements, both scattering and absorbing material scavenging efficiencies were observed to have slightly increasing temperature dependence. At 0 °C the efficiencies of scattering and absorbing matter were 0.85 and 0.55 with slopes of 0.005 °C−1 and 0.003 °C−1, respectively. Additionally, scavenging efficiencies were studied as a function of the diameter at which half of the particles are activated into cloud droplets. This analysis indicated that the is a higher fraction of absorbing material, typically black carbon, in smaller sizes so that at least 20–30 % of interstitial particles within clouds consist of absorbing material. In addition, the PM1-inlet revealed that approximately 20 % of absorbing material was observed to reside in particles with ambient diameter larger than ~ 1 µm at relative humidity below 90 %. Similarly, 40 % of scattering material was seen to be in particles larger than 1 µm. Altogether, this dataset provides information on size dependent aerosol composition that can be applied in evaluating how well large-scale aerosol models reproduce aerosol composition, especially with respect to scavenging in stratus clouds.

The fog/low stratus clouds in the Arctic: detection with multispectral satellite imagery

<p>            The safe operation of aviation and shipping, particularly in areas of insufficient coverage of automatic meteorological stations in the Arctic requires accurate interpretation of satellite images. Operational detection of fog and low stratus clouds and recognizing of them on the background of snow and ice cover and cloudiness of the upper layer is very important challenge. </p><p>           The verified images obtained by Aqua and Terra satellites with a scanning radiometer MODIS, which operates in 36 spectral bands, with wavelengths from 0.4 µm to 14.4 µm, were collected.  With the Beam VISAT 5.0 software, which was designed to work with satellite data in raster format, thematic digital techniques of satellite multispectral information, based on difference in the values of the integral brightness of the images, both in optical and far-infrared ranges of the spectrum, have been developed.  These techniques, models of additive color synthesis, improve the quality of interpretation of fogs and low stratus clouds in terms of the complex structure of cloudiness and underlying surface in polar regions. Developed RGB combinations, which are based on the selected MODIS bands are:</p><ol><li>RGB (1.6 µm; 0.8 µm; 0.6 µm)</li> <li>RGB (0.8 µm; 3.9-8.7 µm; 10.8 µm)</li> <li>RGB (0.8 µm; 1.6 µm; 3.9-8.7 µm)</li> <li>RGB ((0-12)-(0-11) µm, (0-11)-(0-3.8) µm, (0-11) µm)</li> </ol><p>          Analysis of the obtained images has shown that the developed models of color synthesis help to distinguish the fog/low stratus clouds under different conditions of cloudiness and underlying surface accurately.</p><p>Key words: remote sensing, satellite imagery, additive color synthesis, fog, low stratus clouds, polar regions</p>