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Francis P. Bretherton

An analysis shows the present distribution of employment of the estimated 1200 meteorologists holding doctoral degrees and also the year and field in which they received their training. The author speculates that at present there is an approximate balance between the total supply of new doctorates and the needs of the field at this level, subject to a continuing emphasis on quality and upon a stable level of federal support.

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A. J. Baran, P. N. Francis, and P. Yang

Abstract

The processes that contribute to the absorption of infrared radiation by atmospheric ice crystals are studied. The processes are separated into the geometric optics (i.e., refraction, internal, and external reflection) and above-edge (i.e., the capture of photons beyond the physical cross section of the particle via tunneling through an inertial barrier) contributions. The geometric optics and above-edge contributions to ice crystal absorption are compared and contrasted assuming ice spheres, randomly oriented hexagonal ice columns, and randomly oriented ice aggregates (i.e., systematically increasing particle complexity). The geometric optics and above-edge absorption coefficients have been calculated using the complex angular momentum approximation applied to the ice sphere, and a composite method has been used to calculate the two sets of absorption coefficients for the hexagonal ice column and ice aggregate based on the finite difference time domain and improved geometric optics methods. The impact of the geometric optics and above-edge contributions to retrieval of ice crystal effective size (r e) is studied for each particle geometry using aircraft-based downwelling radiometric measurements of cirrus at the wavelengths of 8.55 and 11.0 μm. The retrieved r e is compared with in situ measurements of crystal effective size. The profile averaged value of r e is estimated to be in the range 32–49 μm.

The retrieved r e assuming the ice sphere with the geometric optics contribution only is found to be 30.4 ± 14.2 μm, while with the above-edge contribution included, it is 15.5 ± 7.2 μm. The impact of the ice sphere above-edge contribution acts to reduce the retrieved r e by about half, and this results in the retrieved r e being about a factor of 2–3 less than the in situ measurements of r e. Interestingly, as the particle complexity increases from the ice sphere to the ice aggregate, the impact of the above-edge contribution on the retrieval of r e is found to systematically diminish. For the ice aggregate, the retrieved r e with the geometric optics contribution only is found to be 27.4 ± 4.3 μm. However, with the above-edge contribution included, it is found to be 28.6 ± 3.9 μm. Clearly, as particle complexity increases and particle symmetry decreases, the impact of the above-edge contribution on the retrieval of r e at the wavelengths of 8.55 and 11.0 μm is considerably diminished. However, in general the above-edge contribution should not be ignored and a full electromagnetic solution is still preferred in the resonance region. The findings also indicate that ice aggregates are a better representation of cirrus cloud midinfrared radiative properties than pristine solid hexagonal ice columns.

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Xuebin Zhang, Francis W. Zwiers, and P. A. Stott

Abstract

Using an optimal detection technique and climate change simulations produced with two versions of two GCMs, we have assessed the causes of twentieth-century temperature changes from global to regional scales. Our analysis is conducted in nine spatial domains: 1) the globe; 2) the Northern Hemisphere; four large regions in the Northern Hemispheric midlatitudes covering 30°–70°N including 3) Eurasia, 4) North America, 5) Northern Hemispheric land only, 6) the entire 30°–70°N belt; and three smaller regions over 7) southern Canada, 8) southern Europe, and 9) China. We find that the effect of anthropogenic forcing on climate is clearly detectable at global through regional scales.

The effect of combined greenhouse gases and sulfate aerosol forcing is detectable in all nine domains in annual and seasonal mean temperatures observed during the second half of the twentieth century. The effect of greenhouse gases can also be separated from that of sulfate aerosols over this period at continental and regional scales. Uncertainty in these results is larger in the smaller spatial domains. Detection is improved when an ensemble of models is used to estimate the response to anthropogenic forcing and the underlying internal variability of the climate system. Our detection results hold after removal of North Atlantic Oscillation (NAO)-related variability in temperature observations—variability that may or may not be associated with anthropogenic forcing. They also continue to hold when our estimates of natural internal climate variability are doubled.

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Megan C. Kirchmeier-Young, Francis W. Zwiers, and Nathan P. Gillett

Abstract

Arctic sea ice extent (SIE) has decreased over recent decades, with record-setting minimum events in 2007 and again in 2012. A question of interest across many disciplines concerns the extent to which such extreme events can be attributed to anthropogenic influences. First, a detection and attribution analysis is performed for trends in SIE anomalies over the observed period. The main objective of this study is an event attribution analysis for extreme minimum events in Arctic SIE. Although focus is placed on the 2012 event, the results are generalized to extreme events of other magnitudes, including both past and potential future extremes. Several ensembles of model responses are used, including two single-model large ensembles. Using several different metrics to define the events in question, it is shown that an extreme SIE minimum of the magnitude seen in 2012 is consistent with a scenario including anthropogenic influence and is extremely unlikely in a scenario excluding anthropogenic influence. Hence, the 2012 Arctic sea ice minimum provides a counterexample to the often-quoted idea that individual extreme events cannot be attributed to human influence.

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Francis P. Bretherton, Michael J. Mcphaden, and Eric B. Kraus

Abstract

Objective analysis of large-scale simulated anomalies is used to estimate statistically the effectiveness of different sampling arrays. The methodology, which includes a new use of Bayesian inference, is of fairly general applicability, but is illustrated by a specific requirement to measure five-year changes in the North Atlantic to an accuracy equivalent to ±10 W m−2. The numerical procedure require 1) specification of an overall measure of accuracy on an appropriate resolution, 2) a reference field approximating the long-term mean, 3) an assumed ensemble of large-scale anomalies, 4) the distribution of mesoscale eddy noise as derived from previous analyses of historical data and 5) a set of ship tracks and a sampling, in space and time along them. The numerical output is the expected accuracy and other diagnostic information.

The ship tracks and sampling are varied by trial and error until the expected accuracy is within requirements in an economical manner. The study indicates that the optimized sampling scheme is sensitive primarily to the specification of the overall accuracy requirements including both the resolution and the level of uncertainty. It is less sensitive to the distribution of mesoscale eddy noise and relatively insensitive to plausible changes in the reference field or in the assumptions about the ensemble of large-scale anomalies.

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Francis S. Binkowski, Saravanan Arunachalam, Zachariah Adelman, and Joseph P. Pinto

Abstract

A prototype online photolysis module has been developed for the Community Multiscale Air Quality (CMAQ) modeling system. The module calculates actinic fluxes and photolysis rates (j values) at every vertical level in each of seven wavelength intervals from 291 to 850 nm, as well as the total surface irradiance and aerosol optical depth within each interval. The module incorporates updated opacity at each time step, based on changes in local ozone, nitrogen dioxide, and particle concentrations. The module is computationally efficient and requires less than 5% more central processing unit time than using the existing CMAQ “lookup” table method for calculating j values. The main focus of the work presented here is to describe the new online module as well as to highlight the differences between the effective cross sections from the lookup-table method currently being used and the updated effective cross sections from the new online approach. Comparisons of the vertical profiles for the photolysis rates for nitrogen dioxide (NO2) and ozone (O3) from the new online module with those using the effective cross sections from a standard CMAQ simulation show increases in the rates of both NO2 and O3 photolysis.

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Aurélien Ribes, Nathan P. Gillett, and Francis W. Zwiers

Abstract

Climate change detection and attribution studies rely on historical simulations using specified combinations of forcings to quantify the contributions from greenhouse gases and other forcings to observed climate change. In the last CMIP5 exercise, in addition to the so-called all-forcings simulations, which are driven with a combination of anthropogenic and natural forcings, natural forcings–only and greenhouse gas–only simulations were prioritized among other possible experiments. This study addresses the question of optimally designing this set of experiments to estimate the recent greenhouse gas–induced warming, which is highly relevant to the problem of constraining estimates of the transient climate response. Based on Monte Carlo simulations and considering experimental designs with a fixed budget for the number of simulations that modeling centers can perform, the most accurate estimate of historical greenhouse gas–induced warming is obtained with a design using a combination of all-forcings, natural forcings–only, and aerosol forcing–only simulations. An investigation of optimal ensemble sizes, given the constraint on the total number of simulations, indicates that allocating larger ensemble sizes to weaker forcings, such as natural-only, is optimal.

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Wei Liu, Francis P. Bretherton, Zhengyu Liu, Leslie Smith, Hao Lu, and Christopher J. Rutland

Abstract

The breaking of a monochromatic two-dimensional internal gravity wave is studied using a newly developed spectral/pseudospectral model. The model features vertical nonperiodic boundary conditions that ensure a realistic simulation of wave breaking during the wave propagation. Isopycnal overturning is induced at a local wave steepness of sc = 0.75–0.79, which is below the conventional threshold of s = 1. Isopycnal overturning is a sufficient condition for subsequent wave breaking by convective instability. When s = sc, little primary wave energy is being transferred to high-mode harmonics. Beyond s = 1, high-mode harmonics grow rapidly. Primary wave energy is more efficiently transferred by waves of lower frequency. A local gradient Richardson number is defined as Ri = −(g/ρ 0)(/dz)/ζ 2 to isolate convective instability (Ri ≤ 0) and wave-induced shear instability (0 < Ri < 0.25), where /dz is the local vertical density gradient and ζ is the horizontal vorticity. Consistent with linear wave theory, the probability density function (PDF) for occurrence of convective instability has a maximum at wave phase ϕ = π/2, where the wave-induced density perturbations to the background stratification are the greatest, whereas the wave-induced shear instability has maxima around ϕ = 0 (wave trough) and ϕ = π (wave crest). Nonlinearities in the wave-induced flow broaden the phase span in PDFs of both instabilities. Diapycnal mixing in numerical simulations may be compared with that in realistic oceanic flows in terms of the Cox number. In the numerical simulations, the Cox numbers increase from 1.5 (s = 0.78) to 21.5 (s = 1.1), and the latter is in the lower range of reported values for the ocean.

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Jennifer A. Francis, Thomas P. Ackerman, Kristina B. Katsaros, Richard J. Lind, and Kenneth L. Davidson

Abstract

Measurements of surface radiation fluxes and meteorological conditions collected in the Fram Strait during the summer 1984 Marginal Ice Zone Experiment (MIZEX) are presented and analyzed. These data were combined with calculations from a radiative transfer model to estimate surface and atmospheric moan radiation budgets on a daily basis and for the early summer season over both sea ice and open water in the marginal ice zone (MIZ). Intensities of solar and infrared fluxes within the atmospheric column, radiative properties of Arctic stratus, and atmospheric cooling rates due to the net loss of radiation were computed by the model.

Results show significant differences between the radiation budgets of sea-ice and open-water regimes in the MIZ. Fluxes averaged over the experimental period (16 June to 10 July) indicate that the atmosphere-open water system gained approximately 60 W m−2, while the atmosphere-ice regime was nearly in equilibrium. The open water absorbed twice as much radiation as did the ice, and the mean cooling rate of the over-water atmosphere was approximately 15% larger than that over ice. Observations and model calculations agree that the effect of varying surface albedo on flux intensities is significantly reduced in overcast conditions as compared to under clear skies.

Fluxes and atmospheric cooling rates were compared to values computed by other investigators. Few studies of Arctic radiation exist due to the dearth of observations from polar regions, but available values compare well with those derived from MIZEX data. Cooling rates calculated for the Farm Strait MIZ are twice as large as estimates for the central Arctic in summer. Evidence suggests that this cooling may be offset by a relatively strong poleward atmospheric advection of sensible and latent heat from the Norwegian Sea area.

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Avishai Ben-David, Alan P. Force, Francis M. D'Amico, and Silvio L. Emery

Abstract

The temporal pulse shape varies at different wavelengths, within the same wavelength, and between identical lasers. In differential absorption lidar (DIAL) measurements the variance introduced into the deduced average pathlength concentration from variation of laser pulse shape through the approximation of the normalized energy of the lidar return by sampling peak power values is small but can be significant for detecting low-level concentrations. The magnitude of this uncertainty is on the order of that caused by measurement errors on the order of a few percent.

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