Abstract

On 21 January 2009, the warm front of an extensive low pressure system affected U.K. weather. In this work, macroscopic and microphysical characteristics of this warm front are investigated using in situ (optical array probes, temperatures sensors, and radiosondes) and S-band polarimetric radar data from the Aerosol Properties, Processes and Influences on the Earth’s Climate–Clouds project. The warm front was associated with a warm conveyor belt, a zone of wind speeds of up to 26 m s⁻¹, which played a key role in the formation of extensive mixed-phase cloud mass by ascending significant liquid water (LWC; ~0.22 g m⁻³) at a level ~3 km and creating an ideal environment at temperatures ~ −5°C for ice multiplication. Then, “generating cells,” which formed in the unstable and sheared layer above the warm conveyor belt, influenced the structure of the stratiform cloud layer, dividing it into two types of elongated and slanted ice fall streaks: one depicted by large Z _DR values and the other by large Z _H values. The different polarimetric characteristics of these ice fall streaks reveal their different microphysical properties, such as the ice habit, concentration, and size. We investigate their evolution, which was affected by the warm conveyor belt, and their impact on the surface precipitation.

Abstract

Simple parameterizations of droplet effective radius in stratiform and convective clouds are presented for use in global climate models. Datasets from subtropical marine stratocumulus, continental and maritime convective clouds, and hill cap clouds in middle latitudes and a small amount of data from stratocumulus clouds in middle latitudes have been examined. The results suggest strongly that a simple relationship exists between droplet effective radius and liquid water content in layer clouds with the droplet effective radius proportional to the cube root of the liquid water content. The constant of proportionality is different over oceans and continents. In current global climate models liquid water content is not a predicted variable in convective clouds, and the data strongly suggest that a fixed value of droplet effective radius between 9 and 10 μm should be used for continental clouds more than 500 m deep and 16 μm for maritime cumulus more than 1.5 km deep.

Abstract

Three case studies in frontal clouds from the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project are described to understand the microphysical development of the mixed phase regions of these clouds. The cases are a kata-type cold front, a wintertime warm front, and a summertime occluded frontal system. The clouds were observed by radar, satellite, and in situ microphysics measurements from the U.K. Facility for Airborne Atmospheric Measurements (FAAM) research aircraft. The kata cold front cloud was shallow with a cloud-top temperature of approximately −13°C. Cloud-top heterogeneous ice nucleation was found to be consistent with predictions by a primary ice nucleation scheme. The other case studies had high cloud tops (< −40°C) and despite no direct cloud-top measurements in these regions, homogeneous ice nucleation would be expected. The maximum ice crystal concentrations and ice water contents in all clouds were observed at temperatures around −5°C. Graupel was not observed, hence, secondary ice was produced by riming on snow falling through regions of supercooled liquid water. Within these regions substantial concentrations (10–150 L⁻¹) of supercooled drizzle were observed. The freezing of these drops increases the riming rate due to the increase in rimer surface area. Increasing rime accretion has been shown to lead to higher ice splinter production rates. Despite differences in the cloud structure, the maximum ice crystal number concentration in all three clouds was ~100 L⁻¹. Ice water contents were similar in the warm and occluded frontal cases, where median values in both cases reached ~0.2–0.3 g m⁻³, but lower in the cold front case.

During November and December 2005, two consortia of mainly European groups conducted an aircraft campaign in Darwin, Australia, to measure the composition of the tropical upper-troposphere and tropopause regions, between 12 and 20 km, in order to investigate the transport and transformation in deep convection of water vapor, aerosols, and trace chemicals. The campaign used two high-altitude aircraft—the Russian M55 Geophysica and the Australian Grob 520 Egrett, which can reach 20 and 15 km, respectively—complemented by upward-pointing lidar measurements from the DLR Falcon and low-level aerosol and chemical measurements from the U.K. Dornier-228. The meteorology during the campaign was characterized mainly by premonsoon conditions—isolated afternoon thunderstorms with more organized convective systems in the evening and overnight. At the beginning of November pronounced pollution resulting from widespread biomass burning was measured by the Dornier, giving way gradually to cleaner conditions by December, thus affording the opportunity to study the influence of aerosols on convection. The Egrett was used mainly to sample in and around the outflow from isolated thunderstorms, with a couple of survey missions near the end. The Geophysica–Falcon pair spent about 40% of their flight hours on survey legs, prioritizing remote sensing of water vapor, cirrus, and trace gases, and the remainder on close encounters with storm systems, prioritizing in situ measurements. Two joint missions with all four aircraft were conducted: on 16 November, during the polluted period, sampling a detached anvil from a single-cell storm, and on 30 November, around a much larger multicellular storm.

Abstract

In situ measurements associated with the passage of a kata cold front over the United Kingdom on 29 November 2011 are used to initialize a Lagrangian parcel model for the purpose of calculating rates of diabatic heating and cooling associated with the phase changes of water within the cloud system. The parcel model calculations are performed with both bin-resolved and bulk treatments of microphysical processes. The in situ data from this case study reveal droplet number concentrations up to 100 cm⁻³, with planar ice crystals detected at cloud top, as well as columnar crystals produced by rime splinter ejection within the prefrontal warm sector. The results show that in terms of magnitude, the most significant rates of diabatic heating and cooling are produced by condensation growth of liquid water within the convective updrafts at the leading edge of the front. The peak temperature tendencies associated with condensation are typically found to be at least an order of magnitude larger than those associated with the ice phase, although the cooling effect from sublimation and melting occurs over a wide region. The parcel model framework is used in conjunction with the observations to assess the suitability of existing bulk microphysical treatments, of the kind used in operational weather forecast models. It is found that the assumption of spherical ice crystals (with diameters equal to the maximum dimension of those sampled), along with the use of negative exponential functions to describe ice particle size distributions, can lead to an overestimation of local diabatic heating and cooling rates by a factor of 2 or more.

Abstract

The European Union (EU)-funded project Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) investigates the relationship between weather, climate, and air pollution in southern West Africa—an area with rapid population growth, urbanization, and an increase in anthropogenic aerosol emissions. The air over this region contains a unique mixture of natural and anthropogenic gases, liquid droplets, and particles, emitted in an environment in which multilayer clouds frequently form. These exert a large influence on the local weather and climate, mainly owing to their impact on radiation, the surface energy balance, and thus the diurnal cycle of the atmospheric boundary layer.

In June and July 2016, DACCIWA organized a major international field campaign in Ivory Coast, Ghana, Togo, Benin, and Nigeria. Three supersites in Kumasi, Savè, and Ile-Ife conducted permanent measurements and 15 intensive observation periods. Three European aircraft together flew 50 research flights between 27 June and 16 July 2016, for a total of 155 h. DACCIWA scientists launched weather balloons several times a day across the region (772 in total), measured urban emissions, and evaluated health data. The main objective was to build robust statistics of atmospheric composition, dynamics, and low-level cloud properties in various chemical landscapes to investigate their mutual interactions.

This article presents an overview of the DACCIWA field campaign activities as well as some first research highlights. The rich data obtained during the campaign will be made available to the scientific community and help to advance scientific understanding, modeling, and monitoring of the atmosphere over southern West Africa.

Abstract

The Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project aims to improve forecasts of high-impact weather in extratropical cyclones through field measurements, high-resolution numerical modeling, and improved design of ensemble forecasting and data assimilation systems. This article introduces DIAMET and presents some of the first results. Four field campaigns were conducted by the project, one of which, in late 2011, coincided with an exceptionally stormy period marked by an unusually strong, zonal North Atlantic jet stream and a succession of severe windstorms in northwest Europe. As a result, December 2011 had the highest monthly North Atlantic Oscillation index (2.52) of any December in the last 60 years. Detailed observations of several of these storms were gathered using the U.K.’s BAe 146 research aircraft and extensive ground-based measurements. As an example of the results obtained during the campaign, observations are presented of Extratropical Cyclone Friedhelm on 8 December 2011, when surface winds with gusts exceeding 30 m s^–1 crossed central Scotland, leading to widespread disruption to transportation and electricity supply. Friedhelm deepened 44 hPa in 24 h and developed a pronounced bent-back front wrapping around the storm center. The strongest winds at 850 hPa and the surface occurred in the southern quadrant of the storm, and detailed measurements showed these to be most intense in clear air between bands of showers. High-resolution ensemble forecasts from the Met Office showed similar features, with the strongest winds aligned in linear swaths between the bands, suggesting that there is potential for improved skill in forecasts of damaging winds.