Back in March, another CLASS4GL paper was published by Denissen et al. (2021) in the Nature partner journal Climate and Atmospheric Sciences. In this research, sensible heat and humidity input from the surface to the atmospheric boundary layer was estimated from weather balloon observations using the CLASS4GL framework. The atmospheric boundary layer model was initialized with morning temperature and specific humidity observations from balloon soundings. Afterwards, the model simulations were matched to the respective afternoon soundings by scaling the soil moisture and consequently the surface heat flux partitioning between sensible heat and humidity. As a result, the coupling between soil moisture and the atmospheric boundary layer is achieved on a much larger scale than state-of-the-art observational products, such as FLUXNET/AMERIFLUX.

The estimated fluxes are then used to distinguish water-limited (evaporation is controlled by available soil moisture) from energy-limited (evaporation controlled by available surface energy) sounding days. We find significantly different behavior of the atmospheric boundary layer between these two regimes, associating dry conditions with on average warmer (+3K), higher (+400m) and drier (+1kPa) afternoon atmospheric boundary layers than wet conditions.

This work is a result of a collaboration between the Meteorology and Air Quality Group and the Hydrology and Quantitative Water Management Group at Wageningen University, the Hydro-Climate Extremes Lab at Ghent University and the Hydrosphere-Biosphere-Climate Interactions Group at the Max Planck Institute for Biogeochemistry.

All the details can be found in the original reference: Denissen, J.M.C., Orth, R., Wouters, H. et al. Soil moisture signature in global weather balloon soundings. npj Clim Atmos Sci4, 13 (2021). https://doi.org/10.1038/s41612-021-00167-w

Other recent CLASS4GL activities:

• A revision of the study on the effect of soil dryness on human heat stress conducted by Hendrik Wouters (Ghent University; Flemish Institute for Technological Research) is under review.
• Douwe De Vestele (Ghent Universtiy) investigates in his master thesis the role of land surface and soil moisture in flash drought termination.
• João Geirinhas (University of Lisbon) has been analyzing the drivers of recent severe heatwaves in Brasil, including the effect of drought conditions. His work was submitted recently for peer-review.

Twice a day, weather balloons are released simultaneously from hundreds of meteorological institutes and airports around the world. They monitor the global weather for more than a century, in which they measure the properties of the air layers —including temperature, humidity, wind pressure, cloudiness— up to 30km above the Earth’s surface. The measurements are collected in central databases, and provide key information for the daily weather forecast. The daily global party of meteorological measurements is at its downturn since the golden era in the 70s and 80s (right figure). After all, the balloon measurements are costly and labor intensive, and they are increasingly competing against other emerging resources from aircrafts and satellite imagery.

Nonetheless, weather balloons remain an exclusive global network of unequivocal measurements of the upper atmosphere with all its details. The launches are reliable at times when other sources of upper air data break down such as the coronavirus-linked decline in aviation data affecting weather forecasts. Balloon soundings also provide the persistent information about severe weather events, including thunderstorms, droughts and heatwaves. They are continuously integrated in weather and climate reconstructions, and used to verify and improve computer simulations of the atmosphere.

There is more. The soundings enable us to identify the mechanisms behind the onset, evolution, movement, and demise of extreme weather directly from the measured air properties. In this way, the emerging CLASS4GL.eu community continues to exploit the global archive of weather balloon soundings, and several studies are being conducted (see below). Since weather phenomena are highly variable, it is our experience that the abundance of soundings and launch sites distributed over different regions are indispensable to identify drivers of extreme weather.

Heatwaves, droughts and extreme precipitation are on the rise! The decay of the global weather balloon network impedes research on their responsible mechanisms under global warming. It also hampers hazardous weather monitoring for warning operations, and the development of climate mitigation scenarios and adaption strategies. Fortunately, new network rollouts in data scarce regions like Africa can turn the tide, but also novel technologies to automate the launches or to make them more controllable and sustainable, like drones. In either case, we need to perpetuate and extend the global weather balloon network as one of the most genuine legacies of the weather and climate community.

Let’s make weather balloons great again!

HW

—

Recent CLASS4GL applications:

• The first CLASS4GL application is included in a study led by Brecht Martens (Ghent University) that was published recently in the journal Geoscientific Model Development. The study concerns the validation of ‘surface fluxes’ from the reconstruction of the past climate ERA5. These surface fluxes comprise the heat exchange and moist exchanges between the Earth’s surface and the atmosphere, which largely influence outdoor temperature and humidity. The study found that the surface fluxes from ERA5 could better describe the evolution of upper air layers as observed from the weather balloons compared to its predecessor ERA-Interim. These results are one amongst the many benchmarks of the ERA5 surface fluxes against multiple types of observations.
  Martens, B., Schumacher, D. L., Wouters, H., Muñoz-Sabater, J., Verhoest, N. E. C., and Miralles, D. G.: Evaluating the land-surface energy partitioning in ERA5, Geosci. Model Dev., 13, 4159–4181, https://doi.org/10.5194/gmd-13-4159-2020, 2020.
• In a study led by Jasper Denissen (Max Planck Institute for Biogeochemistry; Wageningen University & Research), it is investigated whether moisture levels of the surface can be detected in the weather balloon observations. The study demonstrates in a unique way how weather balloons can be used to detect the influence of land conditions —particularly drought— on the atmosphere and vice versa. It was found to enhance our understanding of the interactions between the land conditions and the atmosphere conditions. The study is currently in revision for npj Atmosphere and Climate Sciences.
• A study conducted by Hendrik Wouters (Ghent University; Flemish Institute for Technological Research) has investigated the effect of soil dryness on human heat stress and has been sent to review.
• A study is being conducted by Vicky Meulenberg (Wageningen University & Research) and investigates the soil moisture influence on convective triggering —the atmospheric tipping point that precedes the presence of clouds and precipitation— and its temporal variability in different regions. CLASS4GL has been used to identify weather regimes at which either drier or moister land are advantageous for convective triggering.
  

• The two master theses using CLASS4GL have been successfully defended:
  1. “Soil moisture influence on convective triggering and its temporal dynamics in different climate regions” by Vicky Meulenberg (Wageningen University & Research): CLASS4GL has been used to identify dry and wet soil advantage regimes for convective triggering in contrasting climate regions.
  2. “Heatwaves in urban areas: analysis of the main drivers” by Joke De Meester (Ghent University): CLASS4GL was upgraded with an urban land parameterization and applied using Microwave Temperature Profile data to study the urban boundary layer effects of climate adaptation measures for Moscow mega-city (pdf).
  Congratulations to the graduates!
• PhD candidate Jasper Denissen is deploying CLASS4GL to quantify critical soil moisture levels (transition point between energy-limited regime and water-limited regime for Earth’s surface evaporation) from weather balloon observations. He is doing this work for the project ‘Multivariate Analysis of Land-Atmosphere Interactions in a Changing Climate’ at the Max Planck Institute of Biogeochemistry.
• A global assessment using CLASS4GL of the impact of land dryness on the health threatening nature of hot spells was presented during the DRY–2–DRY workshop at Ghent University. More news on this study will appear soon.