Petr Chylek

Refractive indices of water and ice in the 0.65- to 2.5-μm spectral range

New accurate values of the imaginary part, k, of the refractive index of water at T = 22 °C, supercooled water at T = -8 °C and polycrystalline ice at T = -25 °C are reported. The k spectrum for water in the spectral region 0.65-2.5 µm is found to be in excellent agreement with those of previous studies. The k values for polycrystalline ice in the 1.44-2.50-µm region eliminate the large uncertainties existing among previously published conflicting sets of data. The imaginary part of refractive index of supercooled water shows a systematic shift of absorption peaks toward the longer wavelengths compared with that of water at warmer temperatures.

Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development

A size-segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below-cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol-cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size-segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process-splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea-salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96-bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea-salt mass accompanies this paper

The Two-Stream Approximation in Radiative Transfer: Including the Angle of the Incident Radiation

Abstract The two-stream approximation has been applied to the equation of radiative transfer to obtain two–stream models for the transfer of radiation through an optically thin plane-parallel atmosphere. The models include the dependence of the reflection and the transmission of the atmosphere on the angle of the incident radiation and on the angular dependence of the scattering phase function of the medium. The two models arise from different methods for treating the incident radiation. It is shown that the models reduce to the thin-atmosphere approximation in the limit that the optical depth of the atmosphere approaches zero. In this limit the sign of the heating caused by the presence of a scattering and absorbing layer over a reflecting surface is derived. This reveals the importance of both the zenith angle and the angular dependence of the scattering phase function. The results obtained from the two-stream models are compared with those of numerical solutions to the equation of radiative transfer. I...

Arctic air temperature change amplification and the Atlantic Multidecadal Oscillation

] UnderstandingArctictemperaturevariabilityisessentialfor assessing possible future melting of the Greenland icesheet,ArcticseaiceandArcticpermafrost.Temperaturetrendreversals in 1940 and 1970 separate two Arctic warmingperiods(1910–1940and1970–2008)byasignificant1940–1970 cooling period. Analyzing temperature records of theArctic meteorological stations we find that (a) the Arcticamplification(ratiooftheArctictoglobaltemperaturetrends)is not a constant but varies in time on a multi-decadal timescale, (b) the Arctic warming from 1910–1940 proceededat a significantly faster rate than the current 1970–2008warming, and (c) the Arctic temperature changes are highlycorrelated with the Atlantic Multi-decadal Oscillation(AMO) suggesting the Atlantic Ocean thermohalinecirculation is linked to the Arctic temperature variability onamulti-decadaltimescale.

The Atlantic Multidecadal Oscillation as a dominant factor of oceanic influence on climate

A multiple linear regression analysis of global annual mean near-surface air temperature (1900–2012) using the known radiative forcing and the El Nino–Southern Oscillation index as explanatory variables account for 89% of the observed temperature variance. When the Atlantic Multidecadal Oscillation (AMO) index is added to the set of explanatory variables, the fraction of accounted for temperature variance increases to 94%. The anthropogenic effects account for about two thirds of the post-1975 global warming with one third being due to the positive phase of the AMO. In comparison, the Coupled Models Intercomparison Project Phase 5 (CMIP5) ensemble mean accounts for 87% of the observed global mean temperature variance. Some of the CMIP5 models mimic the AMO-like oscillation by a strong aerosol effect. These models simulate the twentieth century AMO-like cycle with correct timing in each individual simulation. An inverse structural analysis suggests that these models generally overestimate the greenhouse gases-induced warming, which is then compensated by an overestimate of anthropogenic aerosol cooling.

GLOBAL WARMING AND THE GREENLAND ICE SHEET

The Greenland coastal temperatures have followed the early 20th century global warming trend. Since 1940, however, the Greenland coastal stations data have undergone predominantly a cooling trend. At the summit of the Greenland ice sheet the summer average temperature has decreased at the rate of 2.2 °C per decade since the beginning of the measurements in 1987. This suggests that the Greenland ice sheet and coastal regions are not following the current global warming trend. A considerable and rapid warming over all of coastal Greenland occurred in the 1920s when the average annual surface air temperature rose between 2 and 4 °C in less than ten years (at some stations the increase in winter temperature was as high as 6 °C). This rapid warming, at a time when the change in anthropogenic production of greenhouse gases was well below the current level, suggests a high natural variability in the regional climate. High anticorrelations (r = −0.84 to−0.93) between the NAO (North Atlantic Oscillation) index and Greenland temperature time series suggest a physical connection between these processes. Therefore, the future changes in the NAO and Northern Annular Mode may be of critical consequence to the future temperature forcing of the Greenland ice sheet melt rates.

Simple Approximation for Infrared Emissivity of Water Clouds

Abstract We have derived a simple approximation for the emissivity and flux emissivity of water clouds inside the atmospheric window between 8 and 14 m. In our approximation the emissivity in the 8–11.5 m band is a function only of the clouds liquid water content and cloud thickness. When compared with the exact radiative transfer calculations the broad-band flux emissivities (in the 8–11.5 m region) differ by less than 10%. At wavelengths > 11.5 m the emissivity is a function of the droplet size distribution as well. By considering a typical droplet size distribution for stratus, altostratus and cumulus clouds, we have shown that the effect of the size distribution on the broad-band flux emissivity in the 8–14 m band is about 35%. Our approximation should be useful for treatment of cloud infrared properties in climate models.

Infrared emittance of water clouds

Abstract A simple approximation has been developed for the infrared emittance of clouds composed of water spheres based on the absorption approximation for the emittance and on the polynomial approximation to the Mie absorption efficiency. The expression for the IR emittance is obtained in a simple analytical form as a function of the liquid water content and two size distribution parameters, namely, the effective radius and effective variance. The approximation is suitable for numerical weather prediction, climate modeling, and radiative transfer calculations. The accuracy, when compared to the exact Mie calculation and integration over the size distribution, is within a few percent, while the required computer time is reduced by several orders of magnitude. In the limit of small droplet sizes, the derived IR emittance reduces to a term proportional to the liquid water content.

Refractive index of ice in the 1.4–7.8-μm spectral range

New accurate values of the imaginary part of the refractive index k of polycrystalline ice at T = -22 °C are reported. The k spectrum in the 1.43-2.89-µm region was found to be in excellent agreement with the most recent study, and the data in the 3.35-7.81-µm range eliminate the large existing uncertainty in the 3.5-4.3-µm region.

Enhancement of dust source area during past glacial periods due to changes of the Hadley circulation

Tropical deserts (e.g., Sahara, Arabian desert, Australian desert) are located within the Hadley circulation. Most of the dust uplifted from these deserts is carried by trade winds and deposited in tropical oceans with very little, if any, transported to polar regions. During glacial periods the dust concentrations in polar ice cores were a factor of 10 to 100 higher than during interglacial periods, including the current Holocene. The early general circulation model simulations of the past glacial climate were not able to reproduce these high mineral dust concentrations; the most recent attempts achieve an increased dust transport to polar regions by extending dust source areas to higher latitudes. We present a hypothesis that during glacial periods the Hadley cell is confined closer to the equator. This contraction of the Hadley circulation leads to the geographical change of the boundary between the tropical and the midlatitude circulation regimes. During the glacial periods a considerable fraction of the current tropical deserts was located outside the region of the Hadley circulation. This allowed the dust to be uplifted and transported by midlatitude storm systems to the polar regions. We present a model for the contraction of the Hadley circulation during the past glacial periods based on the Schneider-Lindzen and Held-Hou model of symmetric tropical circulation and on the assumption that the tropical sea surface temperatures were lower during glacial periods than they are today.