Abstract

Updraft speeds of thermals have always been difficult to measure, despite the significant role they play in transporting pollutants and in cloud formation and precipitation. In this study, updraft speeds in buoyancy-driven planetary boundary layers (PBLs) measured by Doppler lidar are found to be correlated with properties of the PBL and surface over the Southern Great Plains (SGP) site operated by the U.S. Department of Energy’s Atmospheric Radiation Measurement Program (ARM). Based on the relationships found here, two approaches are proposed to estimate both maximum (W _max) and cloud-base (W _cb) updraft speeds using satellite data together with some ancillary meteorological data of PBL depth, wind speed at 10-m height, and air temperature at 2-m height. The required satellite input data are cloud-base and surface skin temperatures. PBL depth can be determined by using cloud-base temperature in combination with European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis data. Validation against lidar-measured updraft speeds demonstrated the feasibility of retrieving W _max and W _cb using high-resolution Suomi–National Polar-Orbiting Partnership Visible Infrared Imaging Radiometer Suite (Suomi-NPP VIIRS) measurements over land for PBLs with thermally driven convective clouds during the satellite overpass time. The root-mean-square errors (RMSE) of W _max and W _cb are 0.32 and 0.42 m s⁻¹, respectively. This method does not work for a stable or a mechanically driven PBL.

Abstract

Cloud overlapping has been a major issue in climate studies owing to a lack of reliable information available over both oceans and land. This study presents the first near-global retrieval and analysis of single-layer and overlapped cloud vertical structures and their optical properties retrieved by applying a new method to the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Taking full advantage of the MODIS multiple channels, the method can differentiate cirrus overlapping lower water clouds from single-layer clouds. Based on newly retrieved cloud products using daytime Terra/MODIS 5-km overcast measurements sampled in January, April, July, and October 2001, global statistics of the frequency of occurrence, cloud-top pressure/temperature (Pc/Tc), visible optical depth (τ _VIS), and infrared emissivity (ε) are presented and discussed. Of all overcast scenes identified over land (ocean), the MODIS data show 61% (52%) high clouds (Pc < 500 hPa), 39% (48%) lower clouds (Pc > 500 hPa), and an extremely low occurrence (<4%) of Pc between 500 and 600 hPa. A distinct bimodal distribution of Pc is found and peaks at ∼275 and ∼725 hPa for high and low clouds, thus leaving a minimum in cloud in the middle troposphere. Out of the 61% (52%) of high clouds identified by MODIS, retrievals reveal that 41% (35%) are thin cirrus clouds (ε < 0.85 and Pc < 500 hPa) and the remaining 20% (17%) are thick high clouds (ε ≥ 0.85). Out of the 41% (35%) of thin cirrus, 29% (27%) are found to overlap with lower water clouds and 12% (8%) are single-layer cirrus. Total low-cloud amount (single-layer plus overlapped) out of all overcast scenes is thus 68% (39% + 29%) over land and 75% (48% + 27%) over ocean, which is greater than the cloud amounts reported by the MODIS and the International Satellite Cloud Climatology Project (ISCCP). Both retrieved overlapping and nonoverlapping cirrus clouds show similar mean τ _VIS of ∼1.5 and mean ε of ∼0.5. The optical properties of single-layer cirrus and single-layer water clouds agree well with the MODIS standard retrievals. Because the MODIS retrievals do not differentiate between cirrus and lower water clouds in overlap situations, large discrepancies are found for emissivity, cloud-top height, and optical depth for high cirrus overlapping lower water clouds.

Abstract

The disposition of mean July clear-sky solar radiation in the Canadian Climate Centre second-generation general circulation model (CCC-GCMII) was analyzed by comparing top of the atmosphere (TOA) net fluxes with earth radiation budget experiment (ERBE) data and atmospheric and surface net fluxes with values inferred from Li's algorithm using ERBE data and European Centre for Medium-Range Weather Forecasts precipitable water data. GCMII tended to reflect ˜5 W m⁻² too much to space. Corresponding atmospheric and surface absorption, however, tended to be too low and high, respectively, by ˜30 W m⁻² over much of the Northern Hemisphere. These results were echoed when GCMII atmospheric absorption was compared to estimated results from Li's algorithm driven by GCMII TOA albedo and precipitable water.

The latest version of the CCC-GCM (GCMIII) has numerous upgrades to its clear-sky solar radiative transfer algorithm, the most important of which involve water vapor transmittances and aerosols that tend to enhance atmospheric absorptance. GCMIII's water vapor transmittance functions derive from Geophysical Fluid Dynamics Laboratory line-by-line results, whereas GCMII's were based on Air Force Geophysical Laboratory data. GCMIII includes crude distributions of background tropospheric aerosols, whereas GCMII neglected aerosols.

Li's algorithm was then driven by GCMIII data, and atmospheric absorption of solar radiation by GCMIII was assessed. Differences between GCMIII's and Li's atmospheric absorption over land were almost always within 5 W m⁻². Over oceans, differences were mostly between −5 W m⁻² and −15 W m⁻². This apparent underestimation over GCMIII's oceans probably stems from the algorithm's use of a thin, highly absorbing aerosol.

Solar energy disposition (SED) concerns the amount of solar radiation reflected to space, absorbed in the atmosphere, and absorbed at the surface. The state of knowledge on SED is examined by comparing eight datasets from surface and satellite observation and modeling by general circulation models. The discrepancies among these contemporary estimates of SED are so large that wisdom on conventional SED is wanting. Thanks to satellite observations, the earth's radiation budget (ERB) at the top of the atmosphere is reasonably well known. Current GCMs manage to reproduce a reasonable global and annual mean ERB, but often fail to simulate the variations in ERB associated with certain cloud regimes such as tropical convection and storm tracks. In comparison to ERB, knowledge of the surface radiation budget (SRB) and the atmospheric radiation budget (ARB) is still rather poor, owing to the inherent problems in both in situ observations and remote sensing. The major shortcoming of in situ observations lies in insufficient sampling, while the remote sensing techniques suffer from lack of information on some variables affecting the radiative transfer process, and dependence, directly or indirectly, on radiative transfer models. Nevertheless, satellite-based SRB products agree fairly well overall with ground-based observations. GCM-simulated SRBs and ARBs are not only subject to large regional uncertainties associated with clouds, but also to systematic errors of the order of 25 W m⁻2, due possibly to the neglect of aerosol and/or inaccurate computation of water vapor absorption. Analyses of various datasets suggest that the SED based on ERBE satellite data appears to be more reliable, indicating 30% reflection to space, 24% absorption in the atmosphere, and 46% absorption at the surface.

Abstract

Surface latent heat flux (LHF) has been considered as the determinant driver of the stratocumulus-to-cumulus transition (SCT). The distinct signature of the LHF in driving the SCT, however, has not been found in observations. This motivates us to ask, How determinant is the LHF to SCT? To answer this question, we conduct large-eddy simulations in a Lagrangian setup in which the sea surface temperature increases over time to mimic a low-level cold-air advection. To isolate the role of LHF, we conduct a mechanism-denial experiment in which the LHF adjustment is turned off. The simulations confirm the indispensable roles of LHF in sustaining (although not initiating) the boundary layer decoupling (first stage of SCT) and driving the cloud regime transition (second stage of SCT). However, using theoretical arguments and LES results, we show that decoupling can happen without the need for LHF to increase as long as the capping inversion is weak enough to ensure high entrainment efficiency. The high entrainment efficiency alone cannot sustain the decoupled state without the help of LHF adjustment, leading to the recoupling of the boundary layer that eventually becomes cloud-free. Interestingly, the stratocumulus sheet is sustained longer without LHF adjustment. The mechanisms underlying the findings are explained from the perspectives of cloud-layer budgets of energy (first stage) and liquid water path (second stage).

Abstract

An earlier parameterization that relates the outgoing solar flux at the top of the atmosphere to the flux absorbed at the surface is modified and extended to allow for variations in atmospheric properties that were not considered in the original parameterization. Changes to the parameterization have also been introduced as a result of better treatment of water vapor absorption in the detailed radiative transfer calculations. Corrections are introduced that account for the height of the surface (surface pressure), ozone amount, aerosol type and amount, and cloud height and cloud type, which is characterized by an effective cloud droplet radius. Finally, the results of applying the parameterization to Earth Radiation Budget Satellite measurements are compared with the measurements of the net solar flux measured from two instrumented towers.

Abstracts

This is the Part II of a two-part study that seeks a theoretical understanding of an empirical relationship for shallow cumulus clouds: subcloud updraft velocity covaries linearly with the cloud-base height. This work focuses on continental cumulus clouds that are more strongly forced by surface fluxes and more deviated from equilibrium than those over oceans (Part I). We use a simple analytical model for shallow cumulus that is well tested against a high-resolution (25 m in the horizontal) large-eddy simulation model. Consistent with a conventional idea, we find that surface Bowen ratio is the key variable that regulates the covariability of both parameters: under the same solar insolation, a drier surface allows for stronger buoyancy flux, triggering stronger convection that deepens the subcloud layer. We find that the slope of the Bowen-ratio-regulated relationship between the two parameters (defined as λ) is dependent on both the local time and the stability of the lower free atmosphere. The value of λ decreases with time exponentially from sunrise to early afternoon and linearly from early afternoon to sunset. The value of λ is larger in a more stable atmosphere. In addition, continental λ in the early afternoon more than doubles the oceanic λ. Validation of the theoretical results against ground observations over the Southern Great Plains shows a reasonable agreement. Physical mechanisms underlying the findings are explained from the perspective of different time scales at which updrafts and cloud-base height respond to a surface flux forcing.

Abstract

Aerosols contribute to Earth’s radiative budget both directly and indirectly, and large uncertainties remain in quantifying aerosol effects on climate. Variability in aerosol distribution and properties, as might result from changing emissions and transport processes, must be characterized. In this study, variations in aerosol loading across the eastern seaboard of the United States and the North Atlantic Ocean during 2002 to 2012 are analyzed to examine the impacts of anthropogenic emission control measures using monthly mean data from MODIS, AERONET, and IMPROVE observations and Goddard Chemistry Aerosol Radiation and Transport (GOCART) model simulation. MODIS observes a statistically significant negative trend in aerosol optical depth (AOD) over the midlatitudes (−0.030 decade⁻¹). Correlation analyses with surface AOD from AERONET sites in the upwind region combined with trend analysis from GOCART component AOD confirm that the observed decrease in the midlatitudes is chiefly associated with anthropogenic aerosols that exhibit significant negative trends from the eastern U.S. coast extending over the western North Atlantic. Additional analysis of IMPROVE surface PM_2.5 observations demonstrates statistically significant negative trends in the anthropogenic components with decreasing mass concentrations over the eastern United States. Finally, a seasonal analysis of observational datasets is performed. The negative trend seen by MODIS is strongest during spring (MAM) and summer (JJA) months. This is supported by AERONET seasonal trends and is identified from IMPROVE seasonal trends as resulting from ammonium sulfate decreases during these seasons.

Abstract

An angular dependence model (ADM) describes the anisotropy in the reflectance field. ADMs are a key element in determining the top-of-the-atmosphere (TOA) albedos and radiative fluxes. This study utilizes 1-yr satellite data from the Scanner for Radiation Budget (ScaRaB) for overcast scenes to examine the variation of ADMs with cloud properties. Using ScaRaB shortwave (SW) overcast radiance measurements, an SW mean overcast ADM, similar to the Earth Radiation Budget Experiment (ERBE) ADM, was generated. Differences between the ScaRaB and ERBE overcast ADMs lead to biases of ∼0.01–0.04 in mean albedos inferred from specific angular bins. The largest biases are in the backward scattering direction. Overcast ADMs for the visible (VIS) wavelength were also generated using ScaRaB VIS measurements. They are very similar to, but a little smaller at large viewing angles and a little larger at nadir, than the SW overcast ADMs. To evaluate the effect of cloud properties on ADMs, ScaRaB overcast observations were further classified into thin, thick, warm, and cold cloud categories to generate four subsets of ADMs. The resulting ADMs for thin and thick clouds show opposite trends and deviate significantly from the overall mean ADM by more than 10%. Deviations from the mean ADM were also noted for the ADMs developed for warm water clouds and cold ice clouds. These deviations were attributed to the different scattering phase functions of water and ice particles and were compared with results from model simulations. Use of a single mean overcast ADM results in albedo biases of 0.01–0.04, relative to the use of specific ADMs for particular cloud types. The biases reduced to ∼0.005 when averaged over all cloud types and viewing geometry.

Abstract

This study examines the consistency and inconsistency in shortwave (SW) top-of-atmosphere (TOA) reflectances and albedos obtained from satellite measurements of the Earth Radiation Budget Experiment (ERBE) and radiation modeling based on cloud properties retrieved from the Advanced Very High Resolution Radiometer (AVHRR). The examination focuses on completely overcast scenes covered by low-level, single-layered, maritime stratus with uniform cloud-top heights as determined from AVHRR measurements at near nadir. A radiation model was then applied to the retrieved cloud optical depths, droplet effective radii, and top temperatures to compute the SW TOA reflectances and albedos that are compared with coincident ERBE observations. ERBE-observed and AVHRR-based modeled reflectances show excellent agreement in terms of both trend and magnitude, but the two albedos exhibit significant differences that have a strong dependence on cloud optical properties and solar zenith angle (SZA). To unravel the differences, two major factors, that is, scene identification and angular dependence model (ADM), involved in converting reflectance to albedo are examined. It is found that the dependence is mainly caused by the use of a single ERBE–ADM for all overcast scenes, regardless of cloud optical properties. The mean difference in SW TOA flux is about 4–12 W m⁻², depending on SZA, but individual differences may reach up to 40–50 W m⁻² for persistent large or small cloud optical depths. Nearly all of the uniform low-level overcast scenes as determined by AVHRR are identified as mostly cloudy by ERBE, but the misidentification does not have any adverse effect on the albedo differences. In fact, replacing the ERBE mostly cloudy ADM with the overcast ADM exacerbates the albedo comparisons. The mean fluxes obtained with the two ADMs differ by ∼8 W m⁻² at SZA ≈ 33° and by 30 W m⁻² at SZA ≈ 60°.