Do you know what cloud beards are? If not, keep reading and you will find out.
First of all, you have to know that our 35 GHz (Ka-Band) cloud radar, located at the Barbados cloud observatory (BCO), has a large antenna (2 m in diameter), which leads to a very high instrument sensitivity (for the radar nerds: -57 dBZ at 5 km). Therefore, it is way more sensitive than the former radars located at the BCO. This has the advantage that we can better sample cloud areas with smaller droplets and/or lower droplet number concentrations.
The layer below the clouds is called the sub-cloud layer and extends up to ~700 m. In this layer, the air is usually subsaturated, which makes it unlikely for cloud droplets to form. In the attached figure (made with measurements from yesterday), the top of the sub-cloud layer is marked by the black line, which represents the lifting condensation level (LCL). It is very obvious that the radar detects a lot of weak radar reflectivity signals in this sub-cloud layer, and because we didn’t know in the beginning what it is, we called this phenomenon „cloud beards“.
When I started looking closer at these beards, I was thinking that this signal is an instrumental artifact or it is caused by some other objects, like big dust particles or insects. But after comparing these signals with a Raman lidar, I figured out that this signal is real, and it is caused by spherical shaped particles. Another comparison with the ground measured horizontal wind speed revealed that there is a strong correlation between the horizontal wind speed and the magnitude of the weak signals in the sub-cloud layer.
Combining all of these facts leads to the conclusion that sea salt particles from the Atlantic ocean cause the weak radar reflectivity signals in the sub-cloud layer. Sea salt particles are highly hygroscopic and absorb water from the atmosphere in subsaturated conditions. Considering that the relative humidity on Barbados is usually higher than 70%, it is very simple for sea salt particles to absorb water and grow to spherical droplets. These hygroscopically grown sea salt particles appear to be more dominant below clouds than in between clouds because they are carried upward towards the cloud base by convection.
In a study, which we recently submitted, we called these hygroscopically grown sea salt particles „haze-echoes“ instead of „cloud beards“, because the radar echoes looked more like a haze in the sub-cloud layer. These haze-echoes are visible almost every day (in 73% of the days) with the radar, but are not visible to the naked eye. Combining the radar and Raman lidar measurements, we were able to retrieve a size distribution of the hygroscopic grown sea salt particles, which showed a good agreement with other instruments (e.g an optical particle sizer from Ragged point) for the larger particle diameters.
All in all, I think it is fascinating that we can use a high sensitivity radar instrument to detect sea salt particles. In future studies that might be helpful to get a better understanding of the contribution of hygroscopically grown sea salt particles to shallow cumulus clouds.
If you want to see more about this phenomenon, keep an eye out for our paper:
Klingebiel, M., V. P. Ghate, A. K. Naumann, F. Ditas, M. L. Pöhlker, C. Pöhlker, K. Kandler, H. Konow and B. Stevens: Remote sensing of sea salt aerosol below trade wind clouds, J. Atmos. Sci, submitted and under review.