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.

[1] Spectral (bin) microphysics was coupled to the Weather Research Forecast model to investigate the effect of aerosols (i.e., air pollution) on precipitation in the Sierra Nevada Mountains. Two-dimensional simulations were produced using either maritime (''clean-air'') or continental (''dirty-air'') aerosols. The simulation with clean air produced more precipitation on the upwind mountain slope than the simulation with continental aerosols. After 3 hours of simulation time, the simulation with maritime aerosols produced about 30% more precipitation over the length of the mountain slope than the simulation with continental aerosols. Sensitivity tests demonstrated the importance of relative humidity and vertical velocity on cloud microphysical structure and precipitation amount. Greater differences in precipitation amounts between simulations with clean and dirty air were obtained when ice microphysical processes were included in the model simulations.

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[1] A new conceptual model that facilitates the inference of the vigor of severe convective storms, producing tornadoes and large hail, by using satellite-retrieved vertical profiles of cloud top temperature (T)-particle effective radius (r e ) relations is presented and tested. The driving force of these severe weather phenomena is the high updraft speed, which can sustain the growth of large hailstones and provide the upward motion that is necessary to evacuate the violently converging air of a tornado. Stronger updrafts are revealed by the delayed growth of r e to greater heights and lower T, because there is less time for the cloud and raindrops to grow by coalescence. The strong updrafts also delay the development of a mixed phase cloud and its eventual glaciation to colder temperatures. Analysis of case studies making use of these and related criteria show that they can be used to identify clouds that possess a significant risk of large hail and tornadoes. Although the strength and direction of the wind shear are major modulating factors, it appears that they are manifested in the updraft intensity and cloud shapes and hence in the T-r e profiles. It is observed that the severe storm T-r e signature is an extensive property of the clouds that develop ahead in space and time of the actual hail or tornadic storm, suggesting that the probabilities of large hail and tornadoes can be obtained at substantial lead times. Analysis of geostationary satellite time series indicates lead times of up to 2 h.

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Recent publications suggest that anthropogenic aerosols suppress orographic precipitation in California and elsewhere. A field campaign (SUPRECIP: Suppression of Precipitation) was conducted to investigate this hypothesized aerosol effect. The campaign consisted of in situ aircraft measurements of the polluting aerosols, the composition of the clouds ingesting them, and the way the precipitation‐forming processes are affected. SUPRECIP was conducted during February and March of 2005 and February and March of 2006. The flights documented the aerosols and orographic clouds flowing into the central Sierra Nevada from the upwind densely populated industrialized/urbanized areas and contrasted them with the aerosols and clouds downwind of the sparsely populated areas in the northern Sierra Nevada. SUPRECIP found that the aerosols transported from the coastal regions are augmented greatly by local sources in the Central Valley resulting in high concentrations of aerosols in the eastern parts of the Central Valley and the Sierra foothills. This pattern is consistent with the detected patterns of suppressed orographic precipitation, occurring primarily in the southern and central Sierra Nevada, but not in the north. The precipitation suppression occurs mainly in the orographic clouds that are triggered from the boundary layer over the foothills and propagate over the mountains. The elevated orographic clouds that form at the crest are minimally affected. The clouds are affected mainly during the second half of the day and the subsequent evening, when solar heating mixes the boundary layer up to cloud bases. Local, yet unidentified nonurban sources are suspected to play a major role.

It is shown here that hygroscopic seeding requires two orders of magnitude more hygroscopic agent than can be delivered by flare technology for producing raindrop embryos in concentrations to detect by cloud physics aircraft the microphysical signature of rain initiation. An alternative method of finely milled salt powder is shown to be capable of achieving this goal. During field experiments the use of a sulfur hexafluoride (SF6) gas tracer to identify the exact seeded cloud volume and to quantify dilution of the seeding agent showed that the seeding agent dilutes to the order of 10−10 of its released concentration in updrafts at a height of ≥1 km above cloud base. This means that the theoretically expected changes in the cloud drop size distribution (DSD) would not be detectable with a cloud droplet spectrometer in a measurement volume collected during the several seconds that the seeded volume is traversed by an aircraft. The actual measurements failed to identify a clear microphysical seeding signature from the burning of hygroscopic flares within the seeded convective clouds. This uncertainty with respect to hygroscopic flare–seeding experiments prompted an experimental and theoretical search for optimal hygroscopic seeding materials. This search culminated in the production of a salt powder having 2–5-μm-diameter particle sizes that are optimal according to model simulations, and can be distributed from a crop duster aircraft. Such particles act as giant cloud condensation nuclei (GCCN). Any potential broadening of the DSD at cloud base by the competition effect (i.e., when the seeded aerosols compete with the natural ambient aerosols for water vapor) occurs when the seeding agent has not been substantially diluted, and hence affects only a very small cloud volume that dilutes quickly. Therefore, the main expected effect of the GCCN is probably to serve as raindrop embryos. The salt powder–seeding method is more productive by two orders of magnitude than the hygroscopic flares in producing GCCN that can initiate rain in clouds with naturally suppressed warm rain processes, because of a combination of change in the particle size distribution and the greater seeding rate that is practical with the powder. Experimental seeding of salt powder in conjunction with the simultaneous release of an SF6 gas tracer produced strong seeding signatures, indicating that the methodology works as hypothesized. The efficacy of the accelerated warm rain processes in altering rainfall amounts may vary under different conditions, and requires additional research that involves both observations and simulations.

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