3:00 PM - 3:15 PM
[ACG52-18] Morphology and environmental characteristics of Arctic mixed-phase clouds during MOSAiC
Keywords:mixed-phase clouds, morphology, MOSAiC
In the Arctic, where rapid warming is occurring, clouds are widely distributed in space and time, affecting the energy budget of the land surface. In addition, mixed-phase clouds occur at high frequency. However, mixed-phase conditions are difficult to predict because clouds tend to dissipate due to the difference in saturated vapor pressure between the liquid and solid phases. In this study, we focus on persistent mixed-phase clouds (P-MPCs) in the Arctic, and among them, we investigate cloud physics, cloud morphology, environment, and their relationships, focusing on the single-layer low-level clouds from the viewpoint of energy budget with the surface.
Cloud microphysics, atmospheric profiles, cloud base height, surface temperature, and Doppler radar data during MOSAiC were used. From the profiles, we extract mixed-phase cloud events that persisted for more than 6 hours in a low-level single-layer. From the gradient Richardson number profile, the height of the coupling is calculated from the boundary layer and temperature profiles. Furthermore, an index combining the height of that boundary layer and Obukhov length is used to discriminate between cellular and rolled forms.
Based on the above index and threshold values, two cases were extracted: a rolled case (October : 18 hours duration) and a cellular case (November : 72 hours duration). These two cases suggest that (1) the ratio of ice particles to total particles is larger in the rolled case than in the cellular case and (2) horizontal winds in clouds are stronger in the rolled case. In the two cases mentioned earlier, the roll case also had peaks in the region where the radar reflectivity was greater than -20 dBZ, and the mean Doppler velocity had contrasting upward and downward distributions. The combination of these results with those of (1) indicates that this roll case has stronger convection than the cell case, which allows the ice particles to grow larger. In addition, the convection is very strong in this case compared to other P-MPCs, indicating that this is a unique case.
To confirm the result of (2)in other cases, we normalized the vertical profiles of horizontal wind at the time of P-MPCs by the cloud top height and performed an EOF analysis. The results showed that the loading factor of the second principal component was more often positive in the roll cases, with significant differences between the cell and roll cases. This second principal component is considered to correspond to low-level jets. The high percentage of cases that were stable near the surface at this time suggests that low-level jets bring in relatively cold air.
Regarding the seasonality of the MPCs, the lifetimes of the MPCs were particularly long around autumn, while the lifetimes were relatively short in summer when the cloud tops were low. In terms of the temperature inversion layer over the entire period, the percentage of the inversion layer observed decreases from winter to summer, and its peak altitude increases. On the other hand, the percentage of clouds above the first layer of inversion from the surface tends to decrease, while the percentage of clouds below it tends to increase. The latter effect appears to exceed that of the former, resulting in lower cloud top heights in summer.
In addition, the radar reflection factor and mean Doppler velocity characteristics of P-MPCs indicate that cloud grains are smaller, and convection is more pronounced in summer than in other seasons. This suggests that not only cloud-top height but also cloud structure and aerosol characteristic differs significantly depending on the season.
Cloud microphysics, atmospheric profiles, cloud base height, surface temperature, and Doppler radar data during MOSAiC were used. From the profiles, we extract mixed-phase cloud events that persisted for more than 6 hours in a low-level single-layer. From the gradient Richardson number profile, the height of the coupling is calculated from the boundary layer and temperature profiles. Furthermore, an index combining the height of that boundary layer and Obukhov length is used to discriminate between cellular and rolled forms.
Based on the above index and threshold values, two cases were extracted: a rolled case (October : 18 hours duration) and a cellular case (November : 72 hours duration). These two cases suggest that (1) the ratio of ice particles to total particles is larger in the rolled case than in the cellular case and (2) horizontal winds in clouds are stronger in the rolled case. In the two cases mentioned earlier, the roll case also had peaks in the region where the radar reflectivity was greater than -20 dBZ, and the mean Doppler velocity had contrasting upward and downward distributions. The combination of these results with those of (1) indicates that this roll case has stronger convection than the cell case, which allows the ice particles to grow larger. In addition, the convection is very strong in this case compared to other P-MPCs, indicating that this is a unique case.
To confirm the result of (2)in other cases, we normalized the vertical profiles of horizontal wind at the time of P-MPCs by the cloud top height and performed an EOF analysis. The results showed that the loading factor of the second principal component was more often positive in the roll cases, with significant differences between the cell and roll cases. This second principal component is considered to correspond to low-level jets. The high percentage of cases that were stable near the surface at this time suggests that low-level jets bring in relatively cold air.
Regarding the seasonality of the MPCs, the lifetimes of the MPCs were particularly long around autumn, while the lifetimes were relatively short in summer when the cloud tops were low. In terms of the temperature inversion layer over the entire period, the percentage of the inversion layer observed decreases from winter to summer, and its peak altitude increases. On the other hand, the percentage of clouds above the first layer of inversion from the surface tends to decrease, while the percentage of clouds below it tends to increase. The latter effect appears to exceed that of the former, resulting in lower cloud top heights in summer.
In addition, the radar reflection factor and mean Doppler velocity characteristics of P-MPCs indicate that cloud grains are smaller, and convection is more pronounced in summer than in other seasons. This suggests that not only cloud-top height but also cloud structure and aerosol characteristic differs significantly depending on the season.