Search

High ice water content (HIWC) regions with small ice crystals, where ice water contents (IWCs) are greater than 1.5 g m-3 and median mass diameters (MMDs) less than about 300 micrometers, occur above tropical mesoscale convective systems (MCSs) and can have detrimental impacts on aircraft engines. Data collected by the French Falcon aircraft and the National Research Council of Canada Convair-580 during the 2014 and 2015 High Altitude Ice Crystals and High Ice Water Content (HAIC/HIWC) projects are revisited here along with coordinated modeling studies to investigate processes that can produce such HIWCs. In particular, data collected from 2014 in the vicinity of Darwin Australia and from 2015 in the vicinity of Cayenne French Guyana are used to determine how bulk microphysical properties (e.g., number concentration, IWC, median volume diameter) and characteristics of ice crystal size distributions (i.e., multimodal nature, parameters fit to gamma distributions for each mode) vary with environmental conditions such as temperature, vertical velocity, MCS age, distance from MCS core, and surface characteristics. It is determined that temperature and vertical velocity are the biggest controls of small ice crystals, but younger cells, stronger convective strengths and closer proximity to convective cores also increase the relative importance of small crystals.Numerical simulations conducted using the Weather Research and Forecasting model with four different bulk microphysics schemes generally reproduce the observed temperature, dew-point, and wind structure. However, comparison of regime-specific observations against properties simulated over Cayenne using a variety of existing parameterization schemes show that although the coverage and evolution of convection is well predicted, simulations overestimate the intensity and spatial extent of observed airborne X-band radar reflectivity and do not well depict the peak of observed size distributions with maximum dimensions between 0.1 and 1 mm. To explore formation mechanisms for large numbers of small ice crystals, a series of simulations varying the representation of secondary ice production (SIP) processes were conducted. Simulations including one of three SIP mechanisms separately (i.e., the Hallett–Mossop mechanism, fragmentation during ice–ice collisions, and shattering of freezing droplets) did not replicate the observed ratio of number concentration divided by IWC. However, the simulation including all three SIP processes produced HIWC regions consistent with observations in terms of number concentration and radar reflectivity, which was not replicated using the original P3 two-ice category configuration that only included the Hallett-Mossop mechanism. In summary, observations and simulations show primary ice production plays a key role in generating HIWC regions at temperatures < -40 Celsius, shattering of freezing droplets dominates ice particle production in HIWC regions between -15 and 0 Celsius during the early stage of convection, and fragmentation during ice–ice collisions dominates between -15 and 0 Celsius during the later stage of convection and between -40 and -20 Celsius over the whole convection period. This study thus shows the dominant role of SIP processes in the formation of numerous small crystals in HIWC regions. Implications for future measurement and modeling needs are discussed.

Ice particle terminal fall velocity (Vt) is fundamental for determining microphysical processes, yet remains extremely challenging to measure. Current theoretical best estimates of Vt are functions of Reynolds number. The Reynolds number is related to the Best number, which is a function of ice particle mass, area ratio (Ar) and maximum dimension (Dmax). These estimates are not conducive for use in most models since model parameterizations often take the form Vt=αDmaxβ, where (α,β) depend on habit and Dmax. A previously developed framework is used to determine surfaces of equally plausible (α,β) coefficients whereby ice particle size/shape distributions are combined with Vt best estimates to determine mass- (VM) or reflectivity-weighted (VZ) velocities that closely match parameterized VM,SD or VZ,SD calculated using the (α,β) coefficients using two approaches. The first uses surfaces of equally plausible (a,b) coefficients describing mass (M)-dimension relationships (i.e., M=aDmaxb) to calculate mass- or reflectivity-weighted velocity from size/shape distributions that are then used to determine (α,β) coefficients. The second investigates how uncertainties in Ar, Dmax, and size distribution N(D) affect VM or VZ. For seven of nine flight legs flown 20/23 May 2011 during MC3E, uncertainty from natural parameter variability – namely the variability in ice particle parameters in similar meteorological conditions – exceeds uncertainties arising from different Ar assumptions or Dmax estimates. The combined uncertainty between Ar, Dmax and N(D) produced smaller variability in (α,β) compared to varying M(D), demonstrating M(D) must be accurately quantified for model fall velocities. Primary sources of uncertainty vary considerably depending on environmental conditions.

Biomass burning (BB) and anthropogenic aerosols and their influence on clouds represent one of the poorly quantified uncertainties in radiative forcing of the climate system. Cumulus clouds, common over maritime regions impacted by BB or anthropogenic aerosols, are important regulators of the global radiative energy budget and global hydrologic cycle. In 2019, a NASA-funded field campaign, the Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) based in South East Asia sampled three distinct regions around the Philippines, the West Pacific, the South China Sea, and the Sulu Sea. The aerosols and clouds located in these three areas were sampled by instruments mounted on the NASA P3 aircraft, quantifying the microphysical properties of clouds and the chemical composition of aerosols. Our analysis focuses on analyzing the statistical properties of cloud-pass statistics as a means to compare the relationships between cloud properties and their aerosol environments.During CAMP2Ex the NASA P3 penetrated 1698 clouds just above cloud base. The cloud pass diameters ranged from 0.11 km to 4.5 km with most clouds in the range of 0.2 - 0.3 km. Updraft strengths ranged from 0.1 to 3.0 m/s. The cloud droplet concentrations ranged from 171.4 to 1971.6 cm-3 with the smallest number concentration found in marine cumulus outside the region of BB plumes and aerosol plumes originating from anthropogenic and natural sources of the Asian continent. The highest particle concentrations were within BB plumes. In biomass burning regions, organic aerosol ranged from 32.1 to 53.4 µg/m3, sulfate aerosols ranged from 4.9 to 7.6 µg/m3, and black carbon ranged from 90.1 to 181.3 µg/m3. In comparison, anthropogenic aerosol regions, where sulfate aerosol was dominant, had sulfate ranging from 1.5 - 15.0 µg/m3 and organics from 0.1 - 3.3 µg/m3. The Manila plume recorded a range of sulfate aerosols of 1.2 - 10.2 µ/m3, nitrate 0.8 - 3.4 µg/m3, and ammonium 0.7 - 4.7 µg/m3 and black carbon 20 - 615 ng/m3. These aerosol source regions were compared to open-ocean marine aerosol with chemical masses less than 0.1 µg/m3 for all species measured by the on board aerosol mass spectrometer. The relationship between cloud number concentrations (Nd), effective radii (re), liquid water content (LWC), and cloud drop size distributions just above cloud base within updrafts exceeding 0.4 m/s are related to aerosol chemical composition in four aerosol regimes, biomass burning, industrial anthropogenic aerosol over the South China Sea, the Manila plume around Metro Manila, and open ocean marine aerosol.

The roles of ice particle size distributions (SDs) and particle shapes in cirrus cloud solar radiative transfer are investigated by analyzing SDs obtained from optical array probe measurements (particle sizes larger than 20–40 μm) during intensive field observations of the International Cirrus Experiment, the European Cloud and Radiation Experiment, the First ISCCP Regional Experiment, and the Central Equatorial Pacific Experiment. It is found that the cloud volume extinction coefficient is more strongly correlated with the total number density than with the effective particle size. Distribution-averaged mean single scattering properties are calculated for hexagonal columns, hexagonal plates, and polycrystals at a nonabsorbing (0.5 μm), moderately absorbing (1.6 μm), and strongly absorbing (3.0 μm) wavelength. At 0.5 μm (1.6 μm) (3.0 μm), the spread in the resulting mean asymmetry parameters due to different SDs is smaller than (comparable to) (smaller than) the difference caused by applying different particle shapes to these distributions. From a broadband solar radiative transfer point of view it appears more important to use the correct particle shapes than to average over the correct size distributions.