Advanced Baseline Imager Cloud-Top Trajectories and Properties of Electrified Snowfall Flash Initiation

Using gridded and interpolated Derived Motion Winds from the Advanced Baseline Imager (ABI), a Lagrangian cloud-feature tracking technique was developed to create, and document trajectories associated with electrified snowfall and changes in cloud characteristics leading up to the initiation of lightning, respectively. This study implemented the thundersnow detection algorithm and defined thundersnow initiation (TSI) as the first group in a flash detected by the Geostationary Lightning Mapper when snow was occurring. Ten ABI channels and four multispectral (e.g., red-green-blue–RGB) composites were analyzed to investigate characteristics that lead up to TSI for 16,644 thundersnow (TSSN) flashes. From the 10.3 μm channel, TSI trajectories were associated with a median decrease of 12.2 K in brightness temperature (TB) one hour prior to TSI. Decreases in the reflectance component of the 3.9 μm channel indicated that TSI trajectories were associated with ice crystal collisions and/or particle settling at cloud top. The Nighttime Microphysics, Day Cloud Phase Distinction, Differential Water Vapor, and Airmass RGBs were examined to evaluate the microphysical and environmental changes prior to TSI. For daytime TSI trajectories, the predominant colors associated with the Day Cloud Phase Distinction RGB transitioned from cyan to yellow/green, physically representing cloud growth and glaciation at cloud top. Gold/orange hues in the Differential Water Vapor RGB indicated that some trajectories were associated with dry upper-level air masses prior to TSI. The analysis of ABI characteristics prior to TSI, and subsequently relating those characteristics to physical processes, inherently increases the fundamental understanding and ability to forecast TSI; thus, providing additional lead-time into changes in surface conditions (i.e., snowfall rates).

Investigation of Thundercloud Features in Different Regions

A comparison of thundercloud characteristics in different regions of the world was conducted. The clouds studied developed in India, China and in two regions of Russia. Several field projects were discussed. Cloud characteristics were measured by weather radars, the SEVERI instrument installed on board of the Meteosat satellite, and lightning detection systems. The statistical characteristics of the clouds were tabulated from radar scans and correlated with lightning observations. Thunderclouds in India differ significantly from those observed in other regions. The relationships among lightning strike frequency, supercooled cloud volume, and precipitation intensity were analyzed. In most cases, high correlation was observed between lightning strike frequency and supercooled volume.

Day and Night Clouds Detection Using a Thermal-Infrared All-Sky-View Camera

The formation and evolution of clouds are associated with their thermodynamical and microphysical progress. Previous studies have been conducted to collect images using ground-based cloud observation equipment to provide important cloud characteristics information. However, most of this equipment cannot perform continuous observations during the day and night, and their field of view (FOV) is also limited. To address these issues, this work proposes a day and night clouds detection approach integrated into a self-made thermal-infrared (TIR) all-sky-view camera. The TIR camera consists of a high-resolution thermal microbolometer array and a fish-eye lens with a FOV larger than 160°. In addition, a detection scheme was designed to directly subtract the contamination of the atmospheric TIR emission from the entire infrared image of such a large FOV, which was used for cloud recognition. The performance of this scheme was validated by comparing the cloud fractions retrieved from the infrared channel with those from the visible channel and manual observation. The results indicated that the current instrument could obtain accurate cloud fraction from the observed infrared image, and the TIR all-sky-view camera developed in this work exhibits good feasibility for long-term and continuous cloud observation.

Evaluating Convective Initiation in High-Resolution Numerical Weather Prediction Models Using GOES-16 Infrared Brightness Temperatures

The evolution of model-based cloud-top brightness temperatures (BT) associated with convective initiation (CI) is assessed for three bulk cloud microphysics schemes in the Weather Research and Forecasting Model. Using a composite-based analysis, cloud objects derived from high-resolution (500 m) model simulations are compared to 5-min GOES-16 imagery for a case study day located near the Alabama–Mississippi border. Observed and simulated cloud characteristics for clouds reaching CI are examined by utilizing infrared BTs commonly used in satellite-based CI nowcasting methods. The results demonstrate the ability of object-based verification methods with satellite observations to evaluate the evolution of model cloud characteristics, and the BT comparison provides insight into a known issue of model simulations producing too many convective cells reaching CI. The timing of CI from the different microphysical schemes is dependent on the production of ice in the upper levels of the cloud, which typically occurs near the time of maximum cloud growth. In particular, large differences in precipitation formation drive differences in the amount of cloud water able to reach upper layers of the cloud, which impacts cloud-top glaciation. Larger cloud mixing ratios are found in clouds with sustained growth leading to more cloud water lofted to the upper levels of the cloud and the formation of ice. Clouds unable to sustain growth lack the necessary cloud water needed to form ice and grow into cumulonimbus. Clouds with slower growth rates display similar BT trends as clouds exhibiting growth, which suggests that forecasting CI using geostationary satellites might require additional information beyond those derived at cloud top.

Breakup of nocturnal low-level stratiform clouds during the southern West African monsoon season

<p><span>During monsoon season in southern West Africa (SWA), nocturnal stratiform low-level clouds (LLSC) frequently form over a region extending from Guinean coast to several hundred kilometers inland. The cloud deck </span><span>persists at least until sunrise next day, </span><span>affecting surface-energy budget and related processes. However, LLSC lifetime is underestimated by numerical weather prediction and climate models.</span></p><p><span>The DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions-over-West-Africa) field campaign, in June-July 2016, paved the way for studying LLSC over SWA based on high-quality-observational dataset. The first analyzes of this data highlighted that the LLSC diurnal cycle consists of four main stages: the stable, jet, stratus and convective phases. Unlike the first three, the convective phase, which starts after sunrise and ends when LLSC breaks up, has not been well documented yet.</span></p><p><span>This study analyzes the LLSC evolution during stratus and convective phases, specifically addressing the LLSC transition toward other low-cloud types during sunlight hours. It is based on comprehensive dataset acquired during twenty-two precipitation-free LLSC occurrences at Savè (Benin) during the DACCIWA fiel campaign. The cloud-characteristics are deduced from ceilometer and cloud-radar measurements. The associated atmospheric conditions are provided by surface meteorological and energy balance stations, radiosoundings and an Ultra-High-Frequency wind profiler.</span></p><p><span>The LLSC forms (beginning of the stratus phase) decoupled from surface. In thirteen cases, the LLSC remains decoupled until the convective phase (case D). Conversely, in the other nine cases, the cloud gets coupled with surface before sunrise, within the four hours after cloud formation (case C). The coupling is accompanied by cloud base lowering and near-neutral thermal stability in subcloud-layer. Almost all cases C are observed during a period with well-established monsoon-flow over SWA. But, the weak differences of thermodynamical conditions between cases C and D suggest that, contributions of both mesoscale and local processes are crucial for coupling LLSC to the surface before sunrise. In early morning, the macrophysical and thermodynamical characteristics of the LLSC in case C are slightly different from the case D, suggesting that, even during night, the coupling with surface impacts the cloud characteristics.</span></p><p><span>The LLSC evolution during convective phase depends upon the coupling at initial stage. In cases C, the evolution pattern is quite similar, the cloud base rises up under solar heating and shallow cumulus form when the cloud deck breaks up, around 11:30 UTC or later. For some of cases D, the LLSC couples with surface as the convective atmospheric boundary-layer grows and reaches the cloud base. The subsequent evolution and breakup time are then similar to case C. For most of cases D, LLSC remains decoupled from surface, and shallow cumulus form at the convective mixed layer top, under the LLSC deck. In this scenario, the LLSC breakup-time mostly occurs before 11:30 UTC. Thus, the coupling between LLSC and surface is a key factor for its evolution and maintenance after sunrise. Correct simulation of this feature may improve models performance over SWA.</span> <span>The impacts of LLSC on surface-energy budget and verical development of boundary-layer are also quantified.</span></p>

Characteristics of Toxic Gas Leakages with Change in Duration

One of the emergencies rescue crews have to face is toxic gas leakages. The characteristics of the gas leakages differ with regard to their leakage duration. Long-term releases have plume-like behaviors that can be described by utilizing mean concentrations at individual exposed locations. In contrast, ensemble statistics of individual cloud characteristics are needed for short-term releases with puff-like behaviors to ensure fully aware risk assessment. The reason is that the time evolution of the concentration of short-term gas releases can differ wildly under the same mean ambient and leakage conditions. The duration from which the release can be classified as plume-like can be found only by studying the releases of different durations, which is the main aim of this paper. To investigate gas releases of different durations, wind tunnel experiments of gas releases in an idealized urban area were conducted. The results present a new method by which concentration signals of releases can be divided into three cloud phases: the arrival, the central and the departure cloud phase. The characteristics (e.g., lengths, mean concentrations) of the individual cloud phases are explored. The results indicate that the finite-duration releases for which the central cloud phase exists have the plume-like behavior for this cloud part.