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.

Abstract A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

Numerical study of convection observed during the Winter Monsoon Experiment using a mesoscale two-dimensional model [presentation]

The increasing severity of droughts/floods and worsening air quality from increasing aerosols in Asia monsoon regions are the two gravest threats facing over 60% of the world population living in Asian monsoon regions. These dual threats have fueled a large body of research in the last decade on the roles of aerosols in impacting Asian monsoon weather and climate. This paper provides a comprehensive review of studies on Asian aerosols, monsoons, and their interactions. The Asian monsoon region is a primary source of emissions of diverse species of aerosols from both anthropogenic and natural origins. The distributions of aerosol loading are strongly influenced by distinct weather and climatic regimes, which are, in turn, modulated by aerosol effects. On a continental scale, aerosols reduce surface insolation and weaken the land-ocean thermal contrast, thus inhibiting the development of monsoons. Locally, aerosol radiative effects alter the thermodynamic stability and convective potential of the lower atmosphere leading to reduced temperatures, increased atmospheric stability, and weakened wind and atmospheric circulations. The atmospheric thermodynamic state, which determines the formation of clouds, convection, and precipitation, may also be altered by aerosols serving as cloud condensation nuclei or ice nuclei. Absorbing aerosols such as black carbon and desert dust in Asian monsoon regions may also induce dynamical feedback processes, leading to a strengthening of the early monsoon and affecting the subsequent evolution of the monsoon. Many mechanisms have been put forth regarding how aerosols modulate the amplitude, frequency, intensity, and phase of different monsoon climate variables. A wide range of theoretical, observational, and modeling findings on the Asian monsoon, aerosols, and their interactions are synthesized. A new paradigm is proposed on investigating aerosol-monsoon interactions, in which natural aerosols such as desert dust, black carbon from biomass burning, and biogenic aerosols from vegetation are considered integral components of an intrinsic aerosol-monsoon climate system, subject to external forcing of global warming, anthropogenic aerosols, and land use and change. Future research on aerosol-monsoon interactions calls for an integrated approach and international collaborations based on long-term sustained observations, process measurements, and improved models, as well as using observations to constrain model simulations and projections.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015RG000500

Aerosol and monsoon climate interactions over Asia

The Moderate-Resolution Imaging Spectroradiometer (MODIS) level-2 (L2) cloud product (earth science data set names MOD06 and MYD06 for Terra and Aqua MODIS, respectively) provides pixel-level retrievals of cloud top properties (day and night pressure, temperature, and height) and cloud optical properties (optical thickness, effective particle radius, and water path for both liquid water and ice cloud thermodynamic phases—daytime only) Collection 6 (C6) reprocessing of the product was completed in May 2014 and March 2015 for MODIS Aqua and Terra, respectively Here we provide an overview of major C6 optical property algorithm changes relative to the previous Collection 5 (C5) product Notable C6 optical and microphysical algorithm changes include: 1) new ice cloud optical property models and a more extensive cloud radiative transfer code lookup table (LUT) approach; 2) improvement in the skill of the shortwave-derived cloud thermodynamic phase; 3) separate cloud effective radius retrieval data sets for each spectral combination used in previous collections; 4) separate retrievals for partly cloudy pixels and those associated with cloud edges; 5) failure metrics that provide diagnostic information for pixels having observations that fall outside the LUT solution space; and 6) enhanced pixel-level retrieval uncertainty calculations The C6 algorithm changes can collectively result in significant changes relative to C5, though the magnitude depends on the data set and the pixel’s retrieval location in the cloud parameter space Example L2 granule and level-3 gridded data set differences between the two collections are shown While the emphasis is on the suite of cloud optical property data sets, other MODIS cloud data sets are discussed when relevant

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The MODIS Cloud Optical and Microphysical Products: Collection 6 Updates and Examples From Terra and Aqua

Atmospheric reanalyses depend on a mix of observations and model forecasts. In data-sparse regions such as the Arctic, the reanalysis solution is more dependent on the model structure, assumptions, and data assimilation methods than in data-rich regions. Applications such as the forcing of ice–ocean models are sensitive to the errors in reanalyses. Seven reanalysis datasets for the Arctic region are compared over the 30-yr period 1981–2010: National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research Reanalysis 1 (NCEP-R1) and NCEP–U.S. Department of Energy Reanalysis 2 (NCEP-R2), Climate Forecast System Reanalysis (CFSR), Twentieth-Century Reanalysis (20CR), Modern-Era Retrospective Analysis for Research and Applications (MERRA), ECMWF Interim Re-Analysis (ERA-Interim), and Japanese 25-year Reanalysis Project (JRA-25). Emphasis is placed on variables not observed directly including surface fluxes and precipitation and their trends. The monthly averaged surface tem...

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Evaluation of Seven Different Atmospheric Reanalysis Products in the Arctic

Abstract. To investigate the cloud phase and macrophysical properties over the Southern Ocean (SO), the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF2) was installed on the Australian icebreaker research vessel (R/V) Aurora Australis during the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) field campaign (41 to 69∘ S, 60 to 160∘ E) from October 2017 to March 2018. To examine cloud properties over the midlatitude and polar regions, the study domain is separated into the northern (NSO) and southern (SSO) parts of the SO, with a demarcation line of 60∘ S. The total cloud fractions (CFs) were 77.9 %, 67.6 %, and 90.3 % for the entire domain, NSO and SSO, respectively, indicating that higher CFs were observed in the polar region. Low-level clouds and deep convective clouds are the two most common cloud types over the SO. A new method was developed to classify liquid, mixed-phase, and ice clouds in single-layered, low-level clouds (LOW), where mixed-phase clouds dominate with an occurrence frequency (Freq) of 54.5 %, while the Freqs of the liquid and ice clouds were 10.1 % (most drizzling) and 17.4 % (least drizzling). The meridional distributions of low-level cloud boundaries are nearly independent of latitude, whereas the cloud temperatures increased by ∼8 K, and atmospheric precipitable water vapor increased from ∼5 mm at 69∘ S to ∼18 mm at 43∘ S. The mean cloud liquid water paths over NSO were much larger than those over SSO. Most liquid clouds occurred over NSO, with very few over SSO, whereas more mixed-phase clouds occurred over SSO than over NSO. There were no significant differences for the ice cloud Freq between NSO and SSO. The ice particle sizes are comparable to cloud droplets and drizzle drops and well mixed in the cloud layer. These results will be valuable for advancing our understanding of the meridional and vertical distributions of clouds and can be used to improve model simulations over the SO.


Cloud phase and macrophysical properties over the Southern Ocean during the MARCUS field campaign

. In this study, the liquid water path (LWP) in stratiform precipitation systems is retrieved, which is a combination of rain liquid water path (RLWP) and cloud liquid water path (CLWP). The retrieval algorithm uses measurements from the vertically pointing radars (VPRs) at 35 GHz and 3 GHz operated by the U.S Department of Energy Atmospheric Radiation Measurement (ARM) and National Oceanic and Atmospheric Administration (NOAA) during the field campaign Midlatitude Continental Convective Clouds Experiment (MC3E). The measured radar reflectivity and mean Doppler velocity from both VPRs and spectrum width from the 35 GHz radar are utilized. With the aid of the cloud base detected by ceilometer, the LWP in the liquid layer is retrieved under two different situations: (I) no cloud exists below the melting base, and (II) cloud exists below the melting base. In (I), LWP is primarily contributed from raindrops only, i.e., RLWP, which is estimated by analyzing the Doppler velocity differences between two VPRs. In (II), cloud particles and raindrops coexist in the liquid layer. The CLWP is estimated using a modified attenuation-based algorithm. Two stratiform precipitation cases (20 May 2011 and 11 May 2011) during MC3E are illustrated for two situations, respectively. With a total of 14 hours of samples during MC3E, statistical results show that the occurrence of cloud particles below the melting base is low (8 %), however, the mean CLWP value can be up to 0.87 kg m −2 , which is much larger than the RLWP (0.22 kg m −2 ). When only raindrops exist below the melting base, the averaged RLWP value is larger (0.33 kg m −2 ) than the with cloud situation. The overall mean LWP below the melting base is 0.39 kg m −2 for stratiform systems during MC3E.

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Estimation of Liquid Water Path in Stratiform Precipitation Systems using Radar Measurements during MC3E

Abstract. To investigate the cloud properties over the Southern Ocean (SO), the MARCUS field campaign (41 to 69º S; 60 to 160º E) was conducted from October 2017 to March 2018, using ship-based measurements. To examine cloud properties over the mid-latitude and Polar regions, the study domain is separated into northern (NSO) and southern (SSO) parts of the SO with a demarcation line of 60° S. The total cloud fractions (CFs) were 77.9 %, 67.6 %, and 90.3 % for the entire domain, NSO and SSO, respectively, indicating that higher CFs were observed in the Polar region. Low-level clouds, deep cumulus, and shallow cumulus clouds are the three most common cloud types over the SO. For single-layered low-level clouds, mixed-phase clouds dominate with an occurrence frequency (Freq) of 54.5 %, while the Freq of the liquid and ice clouds were 10.1 % (most drizzling) and 17.4 % (least drizzling). The meridional distributions of low-level cloud boundaries are nearly independent of latitude, whereas the cloud temperatures increased ~ 8 K and atmospheric precipitable water vapor increased from ~5 mm at 69º S to ~18 mm at 43º S. The mean cloud liquid water paths over NSO were much larger than those over SSO. Most liquid clouds occurred over NSO with very few over SSO, whereas more mixed-phase clouds occurred over SSO than over NSO. There were no significant differences for ice cloud Freq between NSO and SSO. These results will be valuable for advancing our understanding of the meridional and vertical distributions of clouds and can be used to improve model simulations over the SO.


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Cloud properties over the Southern Ocean during the MARCUS field campaign

11 There has been a growing concern that most climate models predict too frequent precipitation, 12 likely due to lack of reliable sub-grid variability and vertical variations of microphysical processes 13 in low-level warm clouds. In this study, the warm cloud physics parameterizations in the singe- 14 column configurations of NCAR Community Atmospheric Model version 6 and 5 (SCAM6 and 15 SCAM5, respectively) are evaluated using ground-based and airborne observations from the DOE 16 ARM Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign 17 near the Azores islands during 2017-2018. Eight-month SCM simulations show that both SCAM6 18 and SCAM5 can generally reproduce marine boundary-layer cloud structure, major macrophysical 19 properties, and their transition. The improvement of warm cloud properties from CAM5 to CAM6 20 physics can be found compared to the observations. Meanwhile, both physical schemes 21 underestimate cloud liquid water content, cloud droplet size, and rain liquid water content, but 22 overestimate surface rainfall. Modeled cloud condensation nuclei (CCN) concentrations are 23 comparable with aircraft observed ones in the summer but overestimated by a factor of two in 24 winter, largely due to the biases in the long-range transport of anthropogenic aerosols like sulfate. 25 We also test the newly recalibrated autoconversion and accretion parameterizations that account 26 for vertical variations of droplet size. Compared to the observations, more significant improvement 27 is found in SCAM5 than in SCAM6. This result is likely explained by the introduction of sub-grid 28 variations of cloud properties in CAM6 cloud microphysics, which further suppresses the scheme 29 sensitivity to individual warm rain microphysical parameters. The predicted cloud susceptibilities 30 to CCN perturbations in CAM6 are within a reasonable range, indicating significant progress since 31 CAM5 which produces too strong aerosol indirect effect. The present study emphasizes the 32 importance of understanding biases in cloud physics parameterizations by combining SCM with 33 in situ observations.

Insights of warm cloud biases in CAM5 and CAM6 from the single-column modeling 1 framework and ACE-ENA observations 2

Atmospheric aerosols are widely recognized to give rise to a substantial radiative forcing to the climate by scattering and absorbing radiation and by modifying the microphysical, lifetime, and radiative properties of clouds. During the Marine ARM GPCI Investigation of Clouds (MAGIC) over the Eastern North Pacific (ENP), the ship‐based measured cloud condensation nuclei (CCN) concentration at 0.2% supersaturation (NCCN,0.2) and condensation nuclei concentration (NCN) had mean values of 116.7 and 219.4 cm−3, with the highest concentrations found closest to LA due to an increase in aerosol sources. Moving westward, both NCCN,0.2 and NCN gradually decreased until stabilizing near 100 cm−3 and 200 cm−3, respectively. Using the methods proposed by Ghan and Collins (2004), https://doi.org/10.1175/1520-0426(2004)021<0387:uoisdt>2.0.co;2 and Ghan et al. (2006), https://doi.org/10.1029/2004jd005752, NCCN,0.2 profiles are retrieved using the surface measured NCCN,0.2 as a constraint. For coupled conditions, correlations between the retrieved NCCN,0.2 profiles and cloud‐droplet number concentration (NC) increase from 0.26 at the surface to 0.38 near cloud base, particularly true for non‐drizzling clouds. Although the correlations are lower than expected, the percentage increase (46.2%) is encouraging. Finally, the relationships between cloud breakup (CB) and the stratocumulus to cumulus transition (SCT) with environmental conditions and associated aerosols are also studied. The decreased NCN trend east of CB is mainly caused by precipitation scavenging, while the increased NCN trend west of CB is strongly associated with the increased surface wind speed and fewer drizzle events. A further study is needed using high‐resolution models to simulate these events.

Baike Xi

Papers

Cloud phase and macrophysical properties over the Southern Ocean during the MARCUS field campaign

Estimation of Liquid Water Path in Stratiform Precipitation Systems using Radar Measurements during MC3E

Cloud properties over the Southern Ocean during the MARCUS field campaign

Insights of warm cloud biases in CAM5 and CAM6 from the single-column modeling 1 framework and ACE-ENA observations 2

Maritime Aerosol and CCN Profiles Derived From Ship‐Based Measurements Over Eastern North Pacific During MAGIC