Wednesday, 23 August 2023
Handout (2.1 MB)
Riming, i.e. freezing of supercooled liquid water droplets on ice crystals, is an important process for ice particle growth and precipitation formation in clouds. The most robust and established method to detect riming in radar observations is based on the unusually fast fall speed (2 to 3 m/s) of rimed particles, compared to other types of hydrometeors above the melting layer.
In this study, we use observations from a Ka band cloud radar located in Jülich, Germany, to investigate riming. Based on more than a decade of vertically pointing radar observations, we exploit an automated algorithm to detect riming in the atmospheric column above the radar and estimate the rime mass fraction. We confirm previously observed riming characteristics such as the temperature range from 0 ºC to -12 ºC, where riming is most likely to occur. We then split the data into individual riming events. This separation of individual riming events allows us to tackle open questions such as: How long does a typical riming event last? At what heights does riming occur and what is the vertical extent of riming events? By incorporating additional information about the wind field, we can obtain a first-order approximation of the spatial dimension of the zones where riming is observed. Our results for 500 riming events indicate that they extend about 15 km horizontally on average. For individual cases, the analysis of vertically pointing radar observations is complemented by RHI scans. By correcting the RHI Doppler velocity for the influence of the large-scale horizontal wind, riming regions can also be identified in the RHI scans, giving an additional perspective on the spatial structure of riming events. This information is important for quantifying the spatial resolution required to detect fingerprints of riming in weather model simulations and operational weather radar data. If such riming events can be detected and quantified on a regional scale, our results could also contribute to better locating atmospheric zones of hazardous aircraft icing.
In this study, we use observations from a Ka band cloud radar located in Jülich, Germany, to investigate riming. Based on more than a decade of vertically pointing radar observations, we exploit an automated algorithm to detect riming in the atmospheric column above the radar and estimate the rime mass fraction. We confirm previously observed riming characteristics such as the temperature range from 0 ºC to -12 ºC, where riming is most likely to occur. We then split the data into individual riming events. This separation of individual riming events allows us to tackle open questions such as: How long does a typical riming event last? At what heights does riming occur and what is the vertical extent of riming events? By incorporating additional information about the wind field, we can obtain a first-order approximation of the spatial dimension of the zones where riming is observed. Our results for 500 riming events indicate that they extend about 15 km horizontally on average. For individual cases, the analysis of vertically pointing radar observations is complemented by RHI scans. By correcting the RHI Doppler velocity for the influence of the large-scale horizontal wind, riming regions can also be identified in the RHI scans, giving an additional perspective on the spatial structure of riming events. This information is important for quantifying the spatial resolution required to detect fingerprints of riming in weather model simulations and operational weather radar data. If such riming events can be detected and quantified on a regional scale, our results could also contribute to better locating atmospheric zones of hazardous aircraft icing.
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