The rapid glaciation of tropical cumulus clouds has been an enigma and has been debated in the literature for over 60 years. Possible mechanisms responsible for the rapid freezing have been postulated, but until now direct evidence has been lacking. Recent high-speed photography of electrostatically suspended supercooled drops in the laboratory has shown that freezing events produce small secondary ice particles. Aircraft observations from the Ice in Clouds Experiment-Tropical (ICE-T), strongly suggest that the drop-freezing secondary ice production mechanism is operating in strong, tropical cumulus updraft cores. The result is the production of small ice particles colliding with large supercooled drops (hundreds of microns up to millimeters in diameter), producing a cascading process that results in rapid glaciation of water drops in the updraft. The process was analyzed from data collected using state-of-the-art cloud particle probes during 54 Learjet penetrations of strong cumulus updraft cores over open ocean in a temperature range from 58 to 2208C. Repeated Learjet penetrations of an updraft core containing 3-5 g m 23 supercooled liquid showed an order-of-magnitude decrease in liquid mass concentration 3 min later at an elevation 1-1.5 km higher in the cloud. The aircraft observations were simulated using a one-dimensional cloud model with explicit bin microphysics. The model was initialized with drop and ice particle size distributions observed prior to rapid glaciation. Simulations show that the model can explain the observed rapid glaciation by the drop-freezing secondary ice production process and subsequent riming, which results when large supercooled drops collide with ice particles.

Abstract. This study presents airborne in situ and satellite remote sensing climatologies of cirrus clouds and humidity. The climatologies serve as a guide to the properties of cirrus clouds, with the new in situ database providing detailed insights into boreal midlatitudes and the tropics, while the satellite-borne data set offers a global overview. To this end, an extensive, quality-checked data archive, the Cirrus Guide II in situ database, is created from airborne in situ measurements during 150 flights in 24 campaigns. The archive contains meteorological parameters, ice water content (IWC), ice crystal number concentration (Nice), ice crystal mean mass radius (Rice), relative humidity with respect to ice (RHice), and water vapor mixing ratio (H2O) for each of the flights. Depending on the parameter, the database has been extended by about a factor of 5–10 compared to earlier studies. As one result of our investigation, we show that the medians of Nice, Rice, and RHice have distinct patterns in the IWC–T parameter space. Lookup tables of these variables as functions of IWC and T can be used to improve global model cirrus representation and remote sensing retrieval methods. Another outcome of our investigation is that across all latitudes, the thicker liquid-origin cirrus predominate at lower altitudes, while at higher altitudes the thinner in situ-origin cirrus prevail. Further, examination of the radiative characteristics of in situ-origin and liquid-origin cirrus shows that the in situ-origin cirrus only slightly warm the atmosphere, while liquid-origin cirrus have a strong cooling effect. An important step in completing the Cirrus Guide II is the provision of the global cirrus Nice climatology, derived by means of the retrieval algorithm DARDAR-Nice from 10 years of cirrus remote sensing observations from satellite. The in situ measurement database has been used to evaluate and improve the satellite observations. We found that the global median Nice from satellite observations is almost 2 times higher than the in situ median and increases slightly with decreasing temperature. Nice medians of the most frequently occurring cirrus sorted by geographical regions are highest in the tropics, followed by austral and boreal midlatitudes, Antarctica, and the Arctic. Since the satellite climatologies enclose the entire spatial and temporal Nice occurrence, we could deduce that half of the cirrus are located in the lowest, warmest (224–242 K) cirrus layer and contain a significant amount of liquid-origin cirrus. A specific highlight of the study is the in situ observations of cirrus and humidity in the Asian monsoon anticyclone and the comparison to the surrounding tropics. In the convectively very active Asian monsoon, peak values of Nice and IWC of 30 cm−3 and 1000 ppmv are detected around the cold point tropopause (CPT). Above the CPT, ice particles that are convectively injected can locally add a significant amount of water available for exchange with the stratosphere. We found IWCs of up to 8 ppmv in the Asian monsoon in comparison to only 2 ppmv in the surrounding tropics. Also, the highest RHice values (120 %–150 %) inside of clouds and in clear sky are observed around and above the CPT. We attribute this to the high H2O mixing ratios (typically 3–5 ppmv) observed in the Asian monsoon compared to 1.5 to 3 ppmv found in the tropics. Above the CPT, supersaturations of 10 %–20 % are observed in regions of weak convective activity and up to about 50 % in the Asian monsoon. This implies that the water available for transport into the stratosphere might be higher than the expected saturation value.

In situ data collected by three research aircraft in four geographical locations are analyzed to determine the relationship between cloud-base temperature, drop size distribution, and the development of supercooled water drops and ice in strong updraft cores of convective clouds. Data were collected in towering cumulus and feeder cells in the Caribbean, over the Gulf of Mexico, over land near the Gulf Coast, over land in the southeastern United States, and the high plains in Colorado and Wyoming. Convective clouds in the Caribbean, over the Gulf of Mexico and its coast, and over the southeastern United States all develop millimeter-diameter supercooled drops in updraft cores. Clouds over the high plains do not generate supercooled large drops, and rarely are drops >70 μm observed in updraft cores. Commensurate with the production of supercooled large drops, ice is generated and rapidly glaciates updraft cores through a hypothesized secondary ice process that is based on laboratory observations of large drops freezing and emitting tiny ice particles. Clouds over the high plains do not experience the secondary ice process and significant concentrations of supercooled liquid in the form of small drops are carried much higher (up to −35.5°C) in the updraft cores. An empirical relationship that estimates the maximum level to which supercooled liquid water will be transported, based on cloud-base drop size distribution and temperature, is developed. Implications have applications for modeling the transport of water vapor and particles into the upper troposphere and hygroscopic seeding of cumulus clouds.

Volcanic ash is often neglected in climate simulations because ash particles are assumed to have a short atmospheric lifetime, and to not participate in sulfur chemistry. After the Mt. Kelut eruption in 2014, stratospheric ash-rich aerosols were observed for months. Here we show that the persistence of super-micron ash is consistent with a density near 0.5 g cm−3, close to pumice. Ash-rich particles dominate the volcanic cloud optical properties for the first 60 days. We also find that the initial SO2 lifetime is determined by SO2 uptake on ash, rather than by reaction with OH as commonly assumed. About 43% more volcanic sulfur is removed from the stratosphere in 2 months with the SO2 heterogeneous chemistry on ash particles than without. This research suggests the need for re-evaluation of factors controlling SO2 lifetime in climate model simulations, and of the impact of volcanic ash on stratospheric chemistry and radiation.

We have developed semi‐independent methods for determining CH2O scavenging efficiencies (SEs) during strong midlatitude convection over the western, south‐central Great Plains, and southeastern regions of the United States during the 2012 Deep Convective Clouds and Chemistry (DC3) Study. The Weather Research and Forecasting model coupled with chemistry (WRF‐Chem) was employed to simulate one DC3 case to provide an independent approach of estimating SEs and the opportunity to study CH2O retention in ice when liquid drops freeze. Measurements of CH2O in storm inflow and outflow were acquired on board the NASA DC‐8 and the NSF/National Center for Atmospheric Research Gulfstream V (GV) aircraft employing cross‐calibrated infrared absorption spectrometers. This study also relied heavily on the nonreactive tracers i‐/n‐butane and i‐/n‐pentane measured on both aircraft in determining lateral entrainment rates during convection as well as their ratios to ensure that inflow and outflow air masses did not have different origins. Of the five storm cases studied, the various tracer measurements showed that the inflow and outflow from four storms were coherently related. The combined average of the various approaches from these storms yield remarkably consistent CH2O scavenging efficiency percentages of: 54% ± 3% for 29 May; 54% ± 6% for 6 June; 58% ± 13% for 11 June; and 41 ± 4% for 22 June. The WRF‐Chem SE result of 53% for 29 May was achieved only when assuming complete CH2O degassing from ice. Further analysis indicated that proper selection of corresponding inflow and outflow time segments is more important than the particular mixing model employed.