LM
Lydia Mak
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1
The vertical profiles of the wind speed and direction in atmospheric boundary layers are strongly controlled by turbulence. Most global weather forecast and climate models parameterize the vertical transport of horizontal momentum by turbulent eddies by means of a downgradient eddy diffusion approach, in which the same stability-dependent eddy viscosity profile is applied to both horizontal wind components. In this study we diagnose eddy viscosity profiles from large-eddy simulations of five convective boundary layers with wind shear. Each simulation was forced by the same geostrophic wind of 7.5 (Formula presented.), but with different surface heat fluxes in the range between 0.03 and 0.18 (Formula presented.). We find that the eddy viscosity profiles for the two horizontal wind components differ significantly, in particular, we diagnose negative eddy viscosities, indicating vertical turbulent transport that is counter the mean gradient. This suggests that a purely downgradient diffusion approach for turbulent momentum fluxes is inadequate. A modified solution of the Ekman spiral demonstrates that different eddy viscosity profiles for the two horizontal wind components lead to a different wind profile. To improve parameterizations that apply a downgradient diffusion approach for momentum, correction terms to allow for non-local, boundary-layer scale transport should be incorporated.
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The vertical profiles of the wind speed and direction in atmospheric boundary layers are strongly controlled by turbulence. Most global weather forecast and climate models parameterize the vertical transport of horizontal momentum by turbulent eddies by means of a downgradient eddy diffusion approach, in which the same stability-dependent eddy viscosity profile is applied to both horizontal wind components. In this study we diagnose eddy viscosity profiles from large-eddy simulations of five convective boundary layers with wind shear. Each simulation was forced by the same geostrophic wind of 7.5 (Formula presented.), but with different surface heat fluxes in the range between 0.03 and 0.18 (Formula presented.). We find that the eddy viscosity profiles for the two horizontal wind components differ significantly, in particular, we diagnose negative eddy viscosities, indicating vertical turbulent transport that is counter the mean gradient. This suggests that a purely downgradient diffusion approach for turbulent momentum fluxes is inadequate. A modified solution of the Ekman spiral demonstrates that different eddy viscosity profiles for the two horizontal wind components lead to a different wind profile. To improve parameterizations that apply a downgradient diffusion approach for momentum, correction terms to allow for non-local, boundary-layer scale transport should be incorporated.
Peering into the heart of thunderstorm clouds
Insights from cloud radar and spectral polarimetry
Lightning is a natural phenomenon that can be dangerous to humans. It is therefore challenging to study thunderstorm clouds using direct observations since it can be dangerous to fly into them. In this study, a cloud radar at 35 GHz with 45° elevation is used to study the properties and dynamics of thunderstorm clouds. It is based on a thunderstorm case on 18 June 2021 from 16:10 to 17:45 UTC near Cabauw, the Netherlands. The observed thunderstorm was associated with severe weather conditions over The Netherlands, attributed to the remnants of storm “Bill”. The time and location of individual lightning strikes are determined using the BLIDS system, operated by Siemens, which is based on the time-of-arrival principle. Concurrently, spectral polarimetry in the millimetre band – an innovative technique not previously applied in thunderstorm cloud studies – is employed to elucidate the behaviour of various particle types within a radar resolution volume. Spectral polarimetric radar variables are also used to look for vertical alignment of ice crystals that is expected due to electric torque. Due to challenges posed by non-Rayleigh scattering, scattering simulations are carried out to aid the interpretation of spectral polarimetric variables. It is shown that the start of the Mie regime in the Doppler spectrum can be clearly identified by the use of the spectral differential phase. Furthermore, variations in the location of the first Mie minimum across different spectral polarimetric variables may be attributed to different sensitivities of these variables to particle shape and ice fraction. From the results, there is a high chance that supercooled liquid water and conical graupel are present in the investigated thunderstorm clouds. There is also a possibility of ice crystals arranged in chains at the cloud top. Ice crystals become vertically aligned a few seconds before lightning and return to their usual horizontal alignment afterwards. However, this phenomenon has been witnessed in only a few cases of cloud-to-cloud lightning, specifically when the lightning strike is in close proximity to the radar's line of sight or when the lightning is strong. Doppler analyses show that updrafts are found near the core of the thunderstorm cloud, while downdrafts are observed at the edges. Strong turbulence is also observed as shown by the large Doppler spectrum width.
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Lightning is a natural phenomenon that can be dangerous to humans. It is therefore challenging to study thunderstorm clouds using direct observations since it can be dangerous to fly into them. In this study, a cloud radar at 35 GHz with 45° elevation is used to study the properties and dynamics of thunderstorm clouds. It is based on a thunderstorm case on 18 June 2021 from 16:10 to 17:45 UTC near Cabauw, the Netherlands. The observed thunderstorm was associated with severe weather conditions over The Netherlands, attributed to the remnants of storm “Bill”. The time and location of individual lightning strikes are determined using the BLIDS system, operated by Siemens, which is based on the time-of-arrival principle. Concurrently, spectral polarimetry in the millimetre band – an innovative technique not previously applied in thunderstorm cloud studies – is employed to elucidate the behaviour of various particle types within a radar resolution volume. Spectral polarimetric radar variables are also used to look for vertical alignment of ice crystals that is expected due to electric torque. Due to challenges posed by non-Rayleigh scattering, scattering simulations are carried out to aid the interpretation of spectral polarimetric variables. It is shown that the start of the Mie regime in the Doppler spectrum can be clearly identified by the use of the spectral differential phase. Furthermore, variations in the location of the first Mie minimum across different spectral polarimetric variables may be attributed to different sensitivities of these variables to particle shape and ice fraction. From the results, there is a high chance that supercooled liquid water and conical graupel are present in the investigated thunderstorm clouds. There is also a possibility of ice crystals arranged in chains at the cloud top. Ice crystals become vertically aligned a few seconds before lightning and return to their usual horizontal alignment afterwards. However, this phenomenon has been witnessed in only a few cases of cloud-to-cloud lightning, specifically when the lightning strike is in close proximity to the radar's line of sight or when the lightning is strong. Doppler analyses show that updrafts are found near the core of the thunderstorm cloud, while downdrafts are observed at the edges. Strong turbulence is also observed as shown by the large Doppler spectrum width.