Deep convective clouds with sustained supercooled liquid water down to -37.5 °C

Abstract

In cirrus and orographic wave clouds, highly supercooled water has been observed in small quantities (less than 0.15 g m(-3)). This high degree of supercooling was attributed to the small droplet size and the lack of ice nuclei at the heights of these clouds. For deep convective clouds, which have much larger droplets near their tops and which take in aerosols from near the ground, no such measurements have hitherto been reported. However, satellite data suggest that highly supercooled water (down to -38 degrees C) frequently occurs in vigorous continental convective storms. Here we report in situ measurements in deep convective clouds from an aircraft, showing that most of the condensed water remains liquid down to -37.5 degrees C. The droplets reach a median volume diameter of 17 microm and amount to 1.8 gm(-3), one order of magnitude more than previously reported. At slightly colder temperatures only ice was found, suggesting homogeneous freezing. Because of the poor knowledge of mixed-phase cloud processes, the simulation of clouds using numerical models is difficult at present. Our observations will help to understand these cloud processes, such as rainfall, hail, and cloud electrification, together with their implications for the climate system.

Comparison between CARIBIC Aerosol Samples Analysed by Accelerator-Based Methods and Optical Particle Counter Measurements

“…The measurements were undertaken during flights from Frankfurt in Germany to northern South America (24 flights), western North America (14), the Indian subcontinent (8) and eastern Asia (2), thus spanning a large region from 120 • W to 120 • E and 10 to 75 • N. The average flight altitude was 10 900 m with a span of 9500 to 11 900 m. For those samples collected in clouds the average air temperature was below 230 K, implying that the clouds were dominated of ice particles (Koop et al, 2000;Rosenfeld and Woodley, 2000). The meteorological modelling along the CARIBIC flight paths indicates that each sample affected by clouds encountered hydrometeors that in most cases consisted to 100 % of ice, the lowest fraction of ice being 99 % (http://www.knmi.nl/samenw/campaign_ support/CARIBIC/).…”

On the role of aerosols, humidity, and vertical wind shear in the transition of shallow-to-deep convection at the Green Ocean Amazon 2014/5 site

“…These results also suggest that CCN tend to have higher concentrations during BSH conditions. This is potentially due to the drier environment: high aerosol concentrations owing to drier conditions can form large numbers of small CCN (Rosenfeld and Woodley, 2000) due to slower coagulation and coalescence; less wet deposition would also occur due to the smaller probability of precipitation.…”

Characteristics of cloud liquid water path from SEVIRI onboard the Meteosat Second Generation 2 satellite for several cloud types

“…First of all, measurements of SEVIRI always provide a snapshot of the current state of the atmosphere and therefore contain also clouds that are still in the process of glaciation, which occurs at timescales of the order of a few minutes (Ansmann et al, 2009) or even up to tens of minutes depending on certain atmospheric conditions such as ice nuclei concentration or updraft velocities (Korolev and Isaac, 2003). From the experimental side, supercooled liquid water can be found in clouds down to temperatures of −37.5 • C, as was experimentally proven by Rosenfeld and Woodley (2000). They conducted in situ aircraft measurements in deep convective clouds and found that most of the condensed water remained liquid until −37.5 • C. The amounts of detected liquid water content were not negligible with values between 0.4 and 4.0 g m −2 measured during several passages through the same cloud fields.…”

“…Freezing times were about 7 min. Rosenfeld and Woodley (2000) suggest that in those cases heterogeneous freezing plays a minor role and homogeneous freezing is the main glaciating mechanism. In a further study by Khain et al (2001), the mechanisms leading to these supercooled cloud water droplets are simulated with the bin microphysics Hebrew University cloud model (HUCM).…”