Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

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Climate Change 2007: The Physical Science Basis.

. A modal aerosol module (MAM) has been developed for the Community Atmosphere Model version 5 (CAM5), the atmospheric component of the Community Earth System Model version 1 (CESM1). MAM is capable of simulating the aerosol size distribution and both internal and external mixing between aerosol components, treating numerous complicated aerosol processes and aerosol physical, chemical and optical properties in a physically-based manner. Two MAM versions were developed: a more complete version with seven lognormal modes (MAM7), and a version with three lognormal modes (MAM3) for the purpose of long-term (decades to centuries) simulations. In this paper a description and evaluation of the aerosol module and its two representations are provided. Sensitivity of the aerosol lifecycle to simplifications in the representation of aerosol is discussed. Simulated sulfate and secondary organic aerosol (SOA) mass concentrations are remarkably similar between MAM3 and MAM7. Differences in primary organic matter (POM) and black carbon (BC) concentrations between MAM3 and MAM7 are also small (mostly within 10%). The mineral dust global burden differs by 10% and sea salt burden by 30–40% between MAM3 and MAM7, mainly due to the different size ranges for dust and sea salt modes and different standard deviations of the log-normal size distribution for sea salt modes between MAM3 and MAM7. The model is able to qualitatively capture the observed geographical and temporal variations of aerosol mass and number concentrations, size distributions, and aerosol optical properties. However, there are noticeable biases; e.g., simulated BC concentrations are significantly lower than measurements in the Arctic. There is a low bias in modeled aerosol optical depth on the global scale, especially in the developing countries. These biases in aerosol simulations clearly indicate the need for improvements of aerosol processes (e.g., emission fluxes of anthropogenic aerosols and precursor gases in developing countries, boundary layer nucleation) and properties (e.g., primary aerosol emission size, POM hygroscopicity). In addition, the critical role of cloud properties (e.g., liquid water content, cloud fraction) responsible for the wet scavenging of aerosol is highlighted.

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Toward a Minimal Representation of Aerosols in Climate Models: Description and Evaluation in the Community Atmosphere Model CAM5

[1] A process-based treatment of ice supersaturation and ice nucleation is implemented in the National Center for Atmospheric Research Community Atmosphere Model (CAM). The new scheme is designed to allow (1) supersaturation with respect to ice, (2) ice nucleation by aerosol particles, and (3) ice cloud cover consistent with ice microphysics. The scheme is implemented with a two-moment microphysics code and is used to evaluate ice cloud nucleation mechanisms and supersaturation in CAM. The new model is able to reproduce field observations of ice mass and mixed phase cloud occurrence better than previous versions. The model is able to reproduce observed patterns and frequency of ice supersaturation. Simulations indicate homogeneous freezing of sulfate and heterogeneous freezing on dust are both important ice nucleation mechanisms, in different regions. Simulated cloud forcing and climate is sensitive to different formulations of the ice microphysics. Arctic surface radiative fluxes are sensitive to the parameterization of ice clouds. These results indicate that ice clouds are potentially an important part of understanding cloud forcing and potential cloud feedbacks, particularly in the Arctic.

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Global simulations of ice nucleation and ice supersaturation with an improved cloud scheme in the Community Atmosphere Model

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...

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Rain in Shallow Cumulus Over the Ocean: The RICO Campaign

The Colorado State University cloud model is applied to the simulation of orogrophic cloud snowfall. A model of ice crystal aggregation processes and primary nucleation and secondary ice particle production of crystals is described. Sensitivity experiments demonstrated that aggregation plays an important role in controlling the fields of cloud liquid water content, ice crystal concentrations, and surface precipitation amounts. The sensitivity experiments also support observations that the air mass is often quite clean in upper levels of stable orographic clouds. Introducing a reduction of available nuclei that can be activated by deposition/sorption processes brought concentrations to within observed values. This study clearly emphasizes the need for a great deal more fundamental research in the physics of aggregation processes and primary and secondary nucleation of ice crystals.

Numerical Simulation of the Effects of Varying Ice Crystal Nucleation Rates and Aggregation Processes on Orographic Snowfall

The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the twodimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-m pixel resolution at aircraft speeds up to 250 m s 1 . An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (100 m) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles. Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops 150 m when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to 150 m and collected at airspeeds 100 m s 1 by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.

The 2D-S (Stereo) Probe: Design and Preliminary Tests of a New Airborne, High-Speed, High-Resolution Particle Imaging Probe

A Learjet research aircraft was used to collect microphysical data, including cloud particle imager (CPI) measurements of ice particle size and shape, in 22 midlatitude cirrus clouds. The dataset was collected while the aircraft flew 104 horizontal legs, totaling over 15 000 km in clouds. Cloud temperatures ranged from 28° to 61°C. The measurements show that cirrus particle size distributions are mostly bimodal, displaying a maximum in number concentration, area, and mass near 30 m and another smaller maximum near 200–300 m. CPI images show that particles with rosette shapes, which include mixed-habit rosettes and platelike polycrystals, constitute over 50% of the surface area and mass of ice particles 50 m in cirrus clouds. Approximately 40% of the remaining mass of ice particles 50 m are found in irregular shapes, with a few percent each in columns and spheroidal shapes. Plates account for 1% of the total mass. Particles 50 m account for 99% of the total number concentration, 69% of the shortwave extinction, and 40% of the mass in midlatitude cirrus. Plots and average equations for area versus particle size are shown for various particle habits, and can be used in studies involving radiative transfer. The average particle concentration in midlatitude cirrus is on the order o f1c m 3 with occasional 10-km averages exceeding 5 cm 3 . There is a strong similarity of microphysical properties of ice particles between wave clouds and cirrus clouds, suggesting that, like wave clouds, cirrus ice particles first experience conversion to liquid water and/or solution drops before freezing.

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In Situ Observations of the Microphysical Properties of Wave, Cirrus, and Anvil Clouds. Part II: Cirrus Clouds

The ability of airborne instruments to measure temperature in cloud is studied using theoretical analyses and experimental data. Theoretical predictions of the effects of sensor wetting are reviewed and modified, and are then compared to measurements. Two airborne immersion thermometers, the NCAR “reverse-flow” thermometer and the Rosemount 102 thermometer, are compared to each other and to a new radiometric thermometer. The comparisons show that out of cloud all three thermometers agree well with each other. However, there is clear evidence that the immersion thermometers become wet in some clouds and measure erroneously low temperatures as a result. The evidence, particularly from measurements in unmixed parcels, supports the validity of the measurements from the radiometric thermometer both inside and outside clouds. Supporting evidence that the immersion sensors are susceptible to wetting is provided from tests in a wind tunnel and from measurements using a conductivity sensor placed at the l...

Performance of Some Airborne Thermometers in Clouds

The general characteristics of the clouds that were included in the HIPLEX-1 experiment are reviewed, and the results for the response variables are interpreted in light of other measurements from the instrumented aircraft. In most seeded clouds, the HIPLEX-1 experimental hypothesis corresponded with the observed precipitation development for only the first ∼8 min after seeding. The failure to obtain a stronger statistical result is attributed to the inherent inefficiency of the small cumulus congestus selected as experimental units. This inefficiency was only partly due to low ice concentrations; a more significant cause of the low precipitation efficiency was the limited lifetime and low liquid water content of these clouds. Some calculations which indicate that these clouds could not support a rapid enough accretional growth process to lead to precipitation after seeding are discussed. Other reasons for the successes and failures of the experiment are discussed.

Physical Interpretation of Results from the HIPLEX-1 Experiment

A tethered-balloon system capable of making microphysical and radiative measurements in clouds is describedandexamplesofmeasurementsinboundarylayerstratuscloudsintheArcticandattheSouthPoleare presented. A 43-m 3 helium-filled balloon lofts an instrument package that is powered by two copper conductors in the tether. The instrument package can support several instruments, including, but not limited to, a cloud particle imager; a forward-scattering spectrometer probe; temperature, pressure, humidity, and wind sensors; ice nuclei filters; and a 4-p radiometer that measures actinic flux at 500 and 800 nm. The balloon can stay aloft for an extended period of time (in excess of 24 h) and conduct vertical profiles up to about 1‐2 km, contingent upon payload weight, wind speed, and surface elevation. Examples of measurements in mixed

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R. Paul Lawson

Papers

The 2D-S (Stereo) Probe: Design and Preliminary Tests of a New Airborne, High-Speed, High-Resolution Particle Imaging Probe

In Situ Observations of the Microphysical Properties of Wave, Cirrus, and Anvil Clouds. Part II: Cirrus Clouds

Performance of Some Airborne Thermometers in Clouds

Physical Interpretation of Results from the HIPLEX-1 Experiment

Deployment of a Tethered-Balloon System for Microphysics and Radiative Measurements in Mixed-Phase Clouds at Ny-Ålesund and South Pole