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Judith Curry

Abstract

Cold-core anticyclones are a dominant feature of the circulation in the high latitudes during the cold half of the year. This paper focuses on how the radiative cooling associated with the formation of continental polar air masses contributes to the anticyclogenesis. The processes occurring in cold-core anticyclones are investigated with a nonlinear axisymmetric numerical model. The model experiments employ three different types of radiative cooling parameterization: 1) externally specified radiative cooling rates; 2) internally determined radiative cooling rates with all condensed water assumed to fall out immediately, not affecting the radiative transfer (after Wexler); and 3) fully interactive calculation, whereby the radiative effects of the condensate that forms in the cooling air are included (Curry). The following physical processes are considered in the fully interactive calculation: 1) condensation of water vapor and freezing of liquid water; 2) radiative transfer from water vapor, CO2, liquid water drops, and ice crystals; 3) gravitational fallout flux of water drops and ice crystals; and 4) surface enthalpy flux, including the heat received at the surface from the underlying snow/ice.

The model results show that after 5 days of integration, the central surface pressure increase is 7 mb for Wexler's cooling mechanism, and 10 mb for Curry's cooling mechanism. A positive feedback loop is shown to exist between the formation of condensate in the cooling air and anticyclogenesis; radiative cooling from condensate enhances anticyclogenesis; and the large-scale meridional circulation associated with anticyclone replenishes the moisture in the layer of condensate, thus enhancing the radiative cooling. Results from the experiments employing externally specified radiative cooling rates show that the central surface pressure increase is largest for (i) increased amount of cooling; (ii) increased horizontal extent of the cooling; (iii) decreased vertical extent of the cooling; (iv) decreased friction; and (v) increased latitude.

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Judith Curry

Abstract

This research investigates the transformation of maritime polar air into continental polar air in the Arctic during winter. The evolution of the vertical profiles of temperature and humidity is simulated using a one-dimensional model. The following physical processes are examined 1) surface enthalpy flux; 2) infrared radiative cooling due to emission by CO2, water vapor, water droplets, and ice crystals; 3) gravitational setting of the condensed water, 4) turbulent mixing, and 5) subsidence.

The modeled formation of continental polar air is dominated by the radiative cooling due to emission by ice crystals and water droplets. The model reproduces the formation of low-level clouds that are frequently observed in these cold air masses, and also the phenomenon of “cloudless” ice crystal precipitation. The model requires two weeks for the formation of fully-developed continental polar air, although after only four days of cooling the air has acquired most of the air mass properties.

The rate of cooling is shown to be very sensitive to the amount of condensed water in the atmosphere. Condensate is produced in the model primarily by radiative cooling, and is depleted from the atmosphere primarily by gravitational settling. Gravitational settling of the condensate significantly increase the amount of radiative cooling by decreasing the opacity of the condensate. Subsidence modifies the formation of continental polar air by reducing the supply of moisture for condensation, thus influencing the radiative cooling.

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Nicholas Lewis
and
Judith Curry

Abstract

Energy budget estimates of equilibrium climate sensitivity (ECS) and transient climate response (TCR) are derived based on the best estimates and uncertainty ranges for forcing provided in the IPCC Fifth Assessment Report (AR5). Recent revisions to greenhouse gas forcing and post-1990 ozone and aerosol forcing estimates are incorporated and the forcing data extended from 2011 to 2016. Reflecting recent evidence against strong aerosol forcing, its AR5 uncertainty lower bound is increased slightly. Using an 1869–82 base period and a 2007–16 final period, which are well matched for volcanic activity and influence from internal variability, medians are derived for ECS of 1.50 K (5%–95% range: 1.05–2.45 K) and for TCR of 1.20 K (5%–95% range: 0.9–1.7 K). These estimates both have much lower upper bounds than those from a predecessor study using AR5 data ending in 2011. Using infilled, globally complete temperature data give slightly higher estimates: a median of 1.66 K for ECS (5%–95% range: 1.15–2.7 K) and 1.33 K for TCR (5%–95% range: 1.0–1.9 K). These ECS estimates reflect climate feedbacks over the historical period, assumed to be time invariant. Allowing for possible time-varying climate feedbacks increases the median ECS estimate to 1.76 K (5%–95% range: 1.2–3.1 K), using infilled temperature data. Possible biases from non–unit forcing efficacy, temperature estimation issues, and variability in sea surface temperature change patterns are examined and found to be minor when using globally complete temperature data. These results imply that high ECS and TCR values derived from a majority of CMIP5 climate models are inconsistent with observed warming during the historical period.

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Judith A. Curry

Abstract

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Judith A. Curry

Abstract

The Arctic Stratus Experiment, conducted during June 1980 over the Beaufort Sea, produced an extensive set of simultaneous measurements of boundary layer structure, radiation fluxes, and cloud microphysical properties. In this paper these data are used to determine the interactions between mixing, radiative transfer, and cloud microphysics for four cloud decks. The thermodynamic structure and fluxes of the thermodynamic quantities in the cloudy boundary layer are examined, including liquid water fluxes. Net radiative heating profiles are also determined. A detailed analysis of the fine-scale structure of the cloud microphysics is presented, including correlations between the cloud microphysical parameters (droplet concentration, liquid water content, mean radius, spectral dispersion, and the 95% volume liquid water drop radius), which are used to infer the nature of the mixing processes and the local effects of radiative heating/cooling. A comparison is then made with other observations and existing model conceptions of the cloudy boundary layer and cloud microphysical processes.

Due to the large static stability and frequent occurrence of a humidity inversion, these clouds are not maintained by surface fluxes of moisture. The net radiative cooling at the cloud top is balanced differently for each of the cases examined, although in all four cases at least a portion of the radiative cooling was found to promote mixed-layer convection. The effects of turbulent entrainment do not penetrate beyond 50 m below mean cloud top, therefore not directly affecting the evolution of the drop spectra except for right near cloud top. Significant liquid water production due to radiative cooling is indicated by the profiles of buoyancy flux, entropy flux, water fluxes, and vertical velocity variance, and also by the large drop spectral dispersions and the correlations between the cloud microphysical parameters. Liquid water fluxes are determined to be nearly as large as the vapor fluxes. The liquid water flux divergences introduce significant structure into the profiles of liquid water content and drop spectra, and also enhance coalescence processes in the lower portion of the clouds.

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Nicholas Lewis
and
Judith Curry

Abstract

Cowtan and Jacobs assert that the method used by Lewis and Curry in 2018 (LC18) to estimate the climate system’s transient climate response (TCR) from changes between two time windows is less robust—in particular against sea surface temperature bias correction uncertainty—than a method that uses the entire historical record. We demonstrate that TCR estimated using all data from the temperature record is closely in line with that estimated using the LC18 windows, as is the median TCR estimate using all pairs of individual years. We also show that the median TCR estimate from all pairs of decade-plus-length windows is closely in line with that estimated using the LC18 windows and that incorporating window selection uncertainty would make little difference to total uncertainty in TCR estimation. We find that, when differences in the evolution of forcing are accounted for, the relationship over time between warming in CMIP5 models and observations is consistent with the relationship between CMIP5 TCR and LC18’s TCR estimate but fluctuates as a result of multidecadal internal variability and volcanism. We also show that various other matters raised by Cowtan and Jacobs have negligible implications for TCR estimation in LC18.

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Yeun-Chung Tan
and
Judith A. Curry

Abstract

The primary goal of this study is to examine the formation mechanisms for the intense North American anticyclone that developed in late January and early February 1989 and to assess the relative importance of adiabatic versus diabatic processes. The complete height tendency equation is used as the diagnostic tool in this study. Results of this analysis show that diabatic processes are relatively unimportant and that the principal forcing mechanisms in the anticyclogenesis are vorticity advection and differential thermal advection. The cold low-level air over Alaska enhances the anticyclogenesis by promoting a positive contribution from the differential thermal advection. The cold low-level temperatures preclude strong cold-air advection at low levels in the anticyclone; strong upper-level cold-air advection thus results in a large positive contribution from the differential thermal advection. The vertical advection of static stability opposes the other forcings, acting to slow the development. The ageostrophic vorticity tendency term makes a substantial contribution; during some portions of the anticyclogenesis, the ageostrophic vorticity tendency enhances the anticyclogenesis and at other times opposes the development. A comparison is made with the model results of a purely cold-core anticyclone forced solely by radiative cooling. It is determined that the forcing mechanisms are substantially different for the intense North American anticyclone when compared with the cold-core anticyclone driven by radiative cooling, even though the thermodynamic structures of the two anticyclones are similar.

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Guosheng Liu
and
Judith A. Curry

Abstract

An ice water path retrieval algorithm, using airborne Millimeter-Wave Imaging Radiometer brightness temperatures at 89, 150, and 220 GHz, is developed for tropical clouds. This algorithm is based on the results of radiative transfer model simulations, using in situ ice particle properties measured from aircraft as model inputs. The scattering signatures at the 150- and 220-GHz channels are the primary inputs into the algorithm, while 89-GHz data are used for determining the nonice background radiation. The ice water path is first calculated from each of the 150- and 220-GHz scattering signatures, and then a combination of the two channels is used for the final retrieval, based on the consideration of the different channel sensitivities to the magnitude of the ice water path. The algorithm is evaluated by comparing the retrieved with in situ measured ice water paths for seven cases observed during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Theoretical analysis shows that the uncertainty due to particle size could be the largest error in the retrievals and this error could be as large as plus or minus 50%. As an application of this algorithm, the ice water path characteristics during TOGA COARE are studied, including assessment of the mean of ice water path, its frequency distribution, and its relationships with cloud-top temperature and liquid water amount. Although tropical clouds are the target of this study, this algorithm could be modified and extended to other climatological regions.

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Guosheng Liu
and
Judith A. Curry

Abstract

The method of simultaneously retrieving ice water path and mass median diameter using microwave data at two frequencies is examined and implemented for tropical nonprecipitating clouds. To develop the retrieval algorithm, the authors first derived a bulk mass–size relation for ice particles in tropical clouds based on microphysical data collected during the Central Equatorial Pacific Experiment. This relation effectively allows ice particle density to decrease with particle size. In implementing the retrieval algorithm, 150- and 220-GHz Millimeter-Wave Imaging Radiometer data collected during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment were used. Ice water path and mass median diameter are determined based on a lookup table generated by a radiative transfer model. The lookup table depends on cloud type, cloud liquid water path, and atmospheric temperature and humidity profiles. Only nonprecipitating clouds are studied in this paper. Error analyses were performed by a Monte Carlo procedure in which atmospheric profiles, ice cloud height, liquid water content, surface temperature, and instrument noise vary randomly within their uncertainty range through a Latin hypercube sampling scheme. The rms error in the retrievals is then assessed and presented in a two-dimensional diagram of ice water path and mass median diameter. It is shown that the simultaneous retrieval method using 150 and 220 GHz may be used for clouds with ice water path larger than 200 g m−2 and mass median diameter larger than 200 μm. To obtain meaningful retrievals for “thinner” clouds, higher microwave frequencies are needed. It is also shown that liquid water clouds that are at the same altitude as ice clouds interfere with the retrievals to a significant degree. To obtain reasonable ice water path and mass median size retrievals, it is necessary first to group clouds into several classes, then to apply separate algorithms to the different classes. The accuracy of the retrievals also depends on cloud type, with the best accuracy for cirrus and the worst for the midtop mixed-phase cloud among the clouds investigated in this study.

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Vitaly I. Khvorostyanov
and
Judith A. Curry

Abstract

In order to understand the mechanisms of formation of broad size spectra of cloud droplets and to develop a basis for the parameterization of cloud microphysical and optical properties, the authors derive a general kinetic equation of stochastic condensation that is applicable for various relationships between the supersaturation relaxation time τ f and the timescale of turbulence τ L . Supersaturation is considered as a nonconservative variable, and thus additional covariances and a turbulent diffusion coefficient tensor that is dependent on the supersaturation relaxation time, k ij (τ f ), are introduced into the kinetic equation. This equation can be used in cloud models with explicit microphysics or can serve as a basis for development of parameterizations for bulk cloud models and general circulation models.

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