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Yu Gu and K. N. Liou

Abstract

A two-dimensional cirrus cloud model has been developed to investigate the interaction and feedback of radiation, ice microphysics, and turbulence-scale turbulence, and their influence on the evolution of cirrus clouds. The model is designed for the study of cloud-scale processes with a 100-m grid spacing. The authors have incorporated a numerical scheme for the prediction of ice crystal size distributions based on calculations of nucleation, diffusional growth, advection, gravitational sedimentation, and turbulent mixing. The radiative effect on the diffusional growth of an individual ice crystal is also taken into account in the model. The model includes an advanced interactive radiative transfer scheme that employs the δ-four-stream approximation for radiative transfer, the correlated k-distribution method for nongray gaseous absorption, and the scattering and absorption properties of hexagonal ice crystals. This radiation scheme is driven by ice water content and mean effective ice crystal size that represents the ice crystal size distribution. To study the effects of entrainment and mixing on the cloud, a second-order turbulence closure has been developed and incorporated into the model. Simulation results show that initial cloud formation occurs through ice nucleation associated with dynamic and thermodynamic forcings. Radiation becomes important for cloud evolution once a sufficient amount of ice water is generated. Radiative processes enhance both the growth of ice crystals at the cloud top by radiative cooling and the sublimation of ice crystals in the lower region by radiative heating. The simulated ice crystal size distributions depend strongly on the radiation fields. In addition, the radiation effect on individual ice crystals through diffusional growth is shown to be significant. Turbulence begins to play a substantial role in cloud evolution during the maintenance and dissipation period of the cirrus cloud life cycle. The inclusion of turbulence tends to generate more intermediate-to-large ice crystals, especially in the middle and lower parts of the cloud. Incorporation of the second-order closure scheme enhances instability below the initial cloud layer and brings more moisture to the region above the cloud, relative to the use of the traditional eddy mixing theory.

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Yu Gu and K. N. Liou

Abstract

A three-dimensional (3D) radiative transfer model has been developed to simulate the transfer of solar and thermal infrared radiation in inhomogeneous cirrus clouds. The model utilizes a diffusion approximation approach (four-term expansion in the intensity) for application to inhomogeneous media, employing Cartesian coordinates. The extinction coefficient, single-scattering albedo, and asymmetry factor are functions of spatial position and wavelength and are parameterized in terms of the ice water content and mean effective ice crystal size. The correlated k-distribution method is employed for incorporation of gaseous absorption in multiple-scattering atmospheres. Delta-function adjustment is used to account for the strong forward-diffraction nature in the phase function of ice particles to enhance computational accuracy. Comparisons of the model results with those from plane-parallel (PP) and other 3D models show reasonable agreement for both broadband and monochromatic results. Three-dimensional flux and heating/cooling rate fields are presented for a number of cirrus cases in which the ice water content and ice crystal size are prescribed. The PP method is shown to be a good approximation under the homogeneous condition when the cloud horizontal dimension is much larger than the cloud thickness. As the horizontal dimension decreases, clouds produce less infrared warming at the bottom as well as less cooling at the top, while more solar heating is generated within the cloud. For inhomogeneous cases, upwelling and downwelling fluxes display patterns corresponding to the extinction coefficient field. Cloud inhomogeneity also plays an important role in determining both solar and IR heating rate distributions. The radiation parameterization is applied to potential cloud configurations generated from GCMs to investigate broken clouds and cloud-overlapping effects on the domain-averaged heating rates. Clouds with maximum overlap tend to produce less heating than those with random overlap. For the prescribed cloud configurations designed in this paper, broken clouds show more solar heating as well as more IR cooling as compared with a continuous cloud field.

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Jinwon Kim, Yu Gu, and K. N. Liou

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To understand the regional impact of the atmospheric aerosols on the surface energy and water cycle in the southern Sierra Nevada characterized by extreme variations in terrain elevation, the authors examine the aerosol radiative forcing on surface insolation and snowmelt for the spring of 1998 in a regional climate model experiment. With a prescribed aerosol optical thickness of 0.2, it is found that direct aerosol radiative forcing influences spring snowmelt primarily by reducing surface insolation and that these forcings on surface insolation and snowmelt vary strongly following terrain elevation. The direct aerosol radiative forcing on surface insolation is negative in all elevations. It is nearly uniform in the regions below 2000 m and decreases with increasing elevation in the region above 2000 m. This elevation dependency in the direct aerosol radiative forcing on surface insolation is related to the fact that the amount of cloud water and the frequency of cloud formation are nearly uniform in the lower elevation region, but increase with increasing elevation in the higher elevation region. This also suggests that clouds can effectively mask the direct aerosol radiative forcing on surface insolation. The direct aerosol radiative forcing on snowmelt is notable only in the regions above 2000 m and is primarily via the reduction in the surface insolation by aerosols. The effect of this forcing on low-level air temperature is as large as −0.3°C, but its impact on snowmelt is small because the sensible heat flux change is much smaller than the insolation change. The direct aerosol radiative forcing on snowmelt is significant only when low-level temperature is near the freezing point, between −3° and 5°C. When low-level temperature is outside this range, the direct aerosol radiative forcing on surface insolation has only a weak influence on snowmelt. The elevation dependency of the direct aerosol radiative forcing on snowmelt is related with this low-level temperature effect as the occurrence of the favored temperature range is most frequent in high elevation regions.

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Di Liu, Guiling Wang, Rui Mei, Zhongbo Yu, and Huanghe Gu

Abstract

This paper focuses on diagnosing the strength of soil moisture–atmosphere coupling at subseasonal to seasonal time scales over Asia using two different approaches: the conditional correlation approach [applied to the Global Land Data Assimilation System (GLDAS) data, the Climate Forecast System Reanalysis (CFSR) data, and output from the regional climate model, version 4 (RegCM4)] and the Global Land–Atmosphere Coupling Experiment (GLACE) approach applied to the RegCM4. The conditional correlation indicators derived from the model output and the two observational/reanalysis datasets agree fairly well with each other in the spatial pattern of the land–atmosphere coupling signal, although the signal in CFSR data is stronger and spatially more extensive than the GLDAS data and the RegCM4 output. Based on the impact of soil moisture on 2-m air temperature, the land–atmosphere coupling hotspots common to all three data sources include the Indochina region in spring and summer, the India region in summer and fall, and north-northeastern China and southwestern Siberia in summer. For precipitation, all data sources produce a weak and spatially scattered signal, indicating the lack of any strong coupling between soil moisture and precipitation, for both precipitation amount and frequency. Both the GLACE approach and the conditional correlation approach (applied to all three data sources) identify evaporation and evaporative fraction as important links for the coupling between soil moisture and precipitation/temperature. Results on soil moisture–temperature coupling strength from the GLACE-type experiment using RegCM4 are in good agreement with those from the conditional correlation analysis applied to output from the same model, despite substantial differences between the two approaches in the terrestrial segment of the land–atmosphere coupling.

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Zhaolin Gu, Yongzhi Zhao, Yun Li, Yongzhang Yu, and Xiao Feng

Abstract

Based on an advanced dust devil–scale large-eddy simulation (LES) model, the atmosphere flow of a modeled dust devil in a quasi–steady state was first simulated to illustrate the characteristics of the gas phase field in the mature stage, including the prediction of the lower pressure and higher temperature in the vortex core. The dust-lifting physics is examined in two aspects. Through the experimental data analysis, it is verified again that the horizontal swirling wind can only make solid particles saltate along the ground surface. Based on a Lagrangian reference frame, the tracks of dust grains with different density (material) and diameter are calculated to show the effect of dust particles entrained by the vertical swirling wind field. The movement of solid particles depends on the interactions between the aloft dust particles and the airflow field of dust devils, in which the drag and the centrifugal force component on the horizontal plane are the key force components. There is the trend of the fine dust grains rising along the inner helical tracks while the large dust grains are lifting along the outer helical tracks and then descending beyond the corner region, resulting in the impact between different-sized dust grains in the swirling atmospheric flow. This trend will make the dust stratification, developing a top small-sized grain domain and a bottom large-sized grain domain in dust devils.

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T. H. Cheng, X. F. Gu, L. F. Chen, T. Yu, and G. L. Tian

Abstract

Optically thin cirrus play a key role in the earth’s radiation budget and global climate change. Their radiative effects depend critically on the thin cirrus optical and microphysical properties. In this paper, inhomogeneous hexagonal monocrystals (IHMs), which consist of a pure hexagon with spherical air bubble or aerosol inclusions, are applied to calculate the single-scattering properties of individual ice crystals. The multiangular polarized characteristics of optically thin cirrus for the 0.865- and 1.38-μm spectral bands are simulated on the basis of an adding–doubling radiative transfer program. The sensitivity of total and polarized reflectance at the top of the atmosphere (TOA) to different aerosol, cirrus, and surface parameters is studied. A new sensitivity index is introduced to further quantify the sensitivity study. The TOA polarized reflectance measured by the Polarization and Directionality of the Earth’s Reflectance (POLDER) instruments is compared to simulated TOA total and polarized reflectance. The test results are reasonable, although small deviations caused by the change of aerosol properties and thin cirrus optical thickness do exist. Finally, on the basis of the sensitivity study, a conceptual approach is suggested to simultaneously retrieve thin cirrus clouds’ optical thickness, ice particle shape, and the underlying aerosol optical thickness using the TOA total and polarized reflectance of the 0.865- and 1.38-μm spectral bands measured at multiple viewing angles.

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James R. Campbell, Simone Lolli, Jasper R. Lewis, Yu Gu, and Ellsworth J. Welton

Abstract

One year of continuous ground-based lidar observations (2012) is analyzed for single-layer cirrus clouds at the NASA Micro Pulse Lidar Network site at the Goddard Space Flight Center to investigate top-of-the-atmosphere (TOA) annual net daytime radiative forcing properties. A slight positive net daytime forcing is estimated (i.e., warming): 0.07–0.67 W m−2 in sample-relative terms, which reduces to 0.03–0.27 W m−2 in absolute terms after normalizing to unity based on a 40% midlatitude occurrence frequency rate estimated from satellite data. Results are based on bookend solutions for lidar extinction-to-backscatter (20 and 30 sr) and corresponding retrievals of the 532-nm cloud extinction coefficient. Uncertainties due to cloud undersampling, attenuation effects, sample selection, and lidar multiple scattering are described. A net daytime cooling effect is found from the very thinnest clouds (cloud optical depth ≤ 0.01), which is attributed to relatively high solar zenith angles. A relationship involving positive/negative daytime cloud forcing is demonstrated as a function of solar zenith angle and cloud-top temperature. These properties, combined with the influence of varying surface albedos, are used to conceptualize how daytime cloud forcing likely varies with latitude and season, with cirrus clouds exerting less positive forcing and potentially net TOA cooling approaching the summer poles (not ice and snow covered) versus greater warming at the equator. The existence of such a gradient would lead cirrus to induce varying daytime TOA forcing annually and seasonally, making it a far greater challenge than presently believed to constrain the daytime and diurnal cirrus contributions to global radiation budgets.

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Qin Xu, Chong-Jian Qiu, Hong-Dao Gu, and Jin-Xiang Yu

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Although the simple adjoint (SA) method for retrieving the low-altitude winds from single-Doppler scans was previously examined with the Phoenix II data collected for rather uniform wind fields on nonstorm days with aluminum chaff dispensed from an aircraft (to enhance the reflectivity), the method has not been tested with storm data for complex flow fields. To examine this problem, the SA method is further developed and tested with the Denver airport microburst data through a series of numerical experiments. In addition to the earlier upgrading of the SA method, three new objectives are fulfilled to improve the retrievals. In particular, it is found that by imposing a weak vorticity constraint, by using the previous time-level retrieval as the initial guess of the retrieval at the current time level, and finally by incorporating the surface anemometer data into the method, the averaged rms difference between the retrieved (from FL-2 radar) and dual-Doppler observed winds reduces from 3.87 to 2.89 m s−1, then to 2.66 m s−1, and finally to 2.55 m s−1, while the averaged correlation coefficient increases from 88% to 94%, then to 95%, and finally to 96%. The surface anemometer data can make a significant improvement, especially when the retrieval purely based on radar data is relatively poor. Factors responsible for the retrieval error are, also examined and discussed with physical interpretations.

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Simone Lolli, James R. Campbell, Jasper R. Lewis, Yu Gu, Jared W. Marquis, Boon Ning Chew, Soo-Chin Liew, Santo V. Salinas, and Ellsworth J. Welton

Abstract

Daytime top-of-the-atmosphere (TOA) cirrus cloud radiative forcing (CRF) is estimated for cirrus clouds observed in ground-based lidar observations at Singapore in 2010 and 2011. Estimates are derived both over land and water to simulate conditions over the broader Maritime Continent archipelago of Southeast Asia. Based on bookend constraints of the lidar extinction-to-backscatter ratio (20 and 30 sr), used to solve extinction and initialize corresponding radiative transfer model simulations, relative daytime TOA CRF is estimated at 2.858–3.370 W m−2 in 2010 (both 20 and 30 sr, respectively) and 3.078–3.329 W m−2 in 2011 and over water between −0.094 and 0.541 W m−2 in 2010 and −0.598 and 0.433 W m−2 in 2011 (both 30 and 20 sr, respectively). After normalizing these estimates for an approximately 80% local satellite-estimated cirrus cloud occurrence rate, they reduce in absolute daytime terms to 2.198–2.592 W m−2 in 2010 and 2.368–2.561 W m−2 in 2011 over land and −0.072–0.416 W m−2 in 2010 and −0.460–0.333 W m−2 in 2011 over water. These annual estimates are mostly consistent despite a tendency toward lower relative cloud-top heights in 2011. Uncertainties are described. Estimates support the open hypothesis of a meridional hemispheric gradient in cirrus cloud daytime TOA CRF globally, varying from positive near the equator to presumably negative approaching the non-ice-covered poles. They help expand upon the paradigm, however, by conceptualizing differences zonally between overland and overwater forcing that differ significantly. More global oceans are likely subject to negative daytime TOA CRF than previously implied.

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James R. Campbell, Erica K. Dolinar, Simone Lolli, Gilberto J. Fochesatto, Yu Gu, Jasper R. Lewis, Jared W. Marquis, Theodore M. McHardy, David R. Ryglicki, and Ellsworth J. Welton

Abstract

Cirrus cloud daytime top-of-the-atmosphere radiative forcing (TOA CRF) is estimated for a 2-yr NASA Micro-Pulse Lidar Network (532 nm; MPLNET) dataset collected at Fairbanks, Alaska. Two-year-averaged daytime TOA CRF is estimated to be between −1.08 and 0.78 W·m−2 (from −0.49 to 1.10 W·m−2 in 2017, and from −1.67 to 0.47 W·m−2 in 2018). This subarctic study completes a now trilogy of MPLNET ground-based cloud forcing investigations, following midlatitude and tropical studies by Campbell et al. at Greenbelt, Maryland, and Lolli et al. at Singapore. Campbell et al. hypothesize a global meridional daytime TOA CRF gradient that begins as positive at the equator (2.20–2.59 W·m−2 over land and from −0.46 to 0.42 W·m−2 over ocean at Singapore), becomes neutral in the midlatitudes (0.03–0.27 W·m−2 over land in Maryland), and turns negative moving poleward. This study does not completely confirm Campbell et al., as values are not found as exclusively negative. Evidence in historical reanalysis data suggests that daytime cirrus forcing in and around the subarctic likely once was exclusively negative. Increasing tropopause heights, inducing higher and colder cirrus, have likely increased regional forcing over the last 40 years. We hypothesize that subarctic interannual cloud variability is likely a considerable influence on global cirrus cloud forcing sensitivity, given the irregularity of polar versus midlatitude synoptic weather intrusions. This study and hypothesis lay the basis for an extrapolation of these MPLNET experiments to satellite-based lidar cirrus cloud datasets.

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