Surface Atmospheric Observations Research Articles

BalkanMed real time severe weather service: Progress and prospects in Bulgaria

The main objective of the “BalkanMed real time severe weather service” (BeRTISS, 2017–2019) project is to establish a pilot transnational severe weather service by exploiting Global Navigation Satellite Systems (GNSS) tropospheric products to enhance the safety, the quality of life and environmental protection in the Balkan-Mediterranean region. In Bulgaria, severe weather events, like intense precipitation, hail and thunderstorms, are common in the summer months and are associated with large economic losses for example in agriculture. The Bulgarian Hail Suppression Agency and the Sofia University are partners in the BeRTISS project with final aim to develop the Bulgarian Integrated NowCAsting tool (BINCA). BINCA uses data from the recently deployed ground-based GNSS network of 12 stations in Bulgaria. The GNSS data is delivered to Sofia University GNSS Analysis Center (SUGAC) and processed in near real time. To derive GNSS Integrated Water Vapour from GNSS the Weather Research and Forecast (WRF) model is computed on daily basis. The GNSS IWV, weather radar as well as surface atmospheric observations and WRF model simulations covering Bulgaria are integrated in the BINCA web platform. The platform is publicly accessible and facilitates the operational tasks in hail suppression in Bulgaria but also other operational and public services. This manuscripts presents the BeRTISS service elements and their implementation in Bulgaria.

High-Resolution Underway Upper Ocean and Surface Atmospheric Observations in Drake Passage: Synergistic Measurements for Climate Science

Recent and ongoing studies are highlighted that analyze the in situ underway upper ocean and surface atmospheric observations from frequently-repeated transects by the Antarctic Research and Supply Vessel Laurence M. Gould (LMG) in Drake Passage. High-resolution measurements of upper ocean temperature, salinity and velocity, along with concurrent shipboard meteorological, surface water CO2 and nutrient sampling have been routinely acquired aboard the LMG since the late 1990s. There are significant benefits and synergy of air-sea observations when they are measured at similar time and space scales from the same platform. The multi-year measurements have been used to examine seasonal and spatial variability in upper ocean heat content, Antarctic Circumpolar Current transport variability, eddy heat and momentum fluxes, frontal variability, validation of satellite and model-based air-sea fluxes, and the upper ocean response to climate variability. At present the LMG provides the only year-round shipboard air-sea measurements in the Southern Ocean. Collectively the measurements in Drake Passage have provided much insight as to the characteristics, mechanisms and impacts of the processes and changes that are occurring within the Southern Ocean.

Sun, sea and ozone destruction

Halogens are known to decrease the levels of stratospheric ozone. The latest measurements show that something similar occurs in the lower atmosphere over tropical oceans — and probably above most other oceans, too. Tropospheric ozone is an important greenhouse gas in addition to its influence on air quality, on the photochemical processing of atmospheric chemicals and on ecosystem viability. Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation and a full understanding of the factors controlling the tropospheric ozone budget is required. The tropical marine boundary layer is the most important global region for loss of ozone, because of its high water vapour content, high solar radiation levels and large geographical extent. Surface atmospheric observations in this region are extremely sparse, hence the importance of a new year-round dataset from Cape Verde Observatory in the tropical North Atlantic Ocean. The observations reveal that the photochemical destruction rate of ozone in the tropical marine boundary layer is about 50% greater than predicted by current global models, and that this destruction is caused by halogen chemistry.

Air–Sea Exchange in Hurricanes: Synthesis of Observations from the Coupled Boundary Layer Air–Sea Transfer Experiment

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements. The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery. As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.