Relationships Between Inversion Strength, Lower-Tropospheric Moistening, and Low-Cloud Fraction in the Subtropical Southeast Pacific Derived From Stable Isotopologues of Water Vapor

2018 ◽  
Vol 45 (15) ◽  
pp. 7701-7710 ◽  
Author(s):  
Joseph Galewsky
Keyword(s):  
Water Vapor ◽  
Cloud Fraction ◽  
Southeast Pacific ◽  
Low Cloud ◽  
Inversion Strength

scholarly journals A comparison of ship and satellite measurements of cloud properties with global climate model simulations in the southeast Pacific stratus deck

2010 ◽  
Vol 10 (14) ◽  
pp. 6527-6536 ◽  
Author(s):  
M. A. Brunke ◽  
S. P. de Szoeke ◽  
P. Zuidema ◽  
X. Zeng
Keyword(s):  
Diurnal Cycle ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Cloud Fraction ◽  
Cloud Base ◽  
Liquid Water Path ◽  
Cloud Properties ◽  
Southeast Pacific ◽  
Cloud Thickness

Abstract. Here, liquid water path (LWP), cloud fraction, cloud top height, and cloud base height retrieved by a suite of A-train satellite instruments (the CPR aboard CloudSat, CALIOP aboard CALIPSO, and MODIS aboard Aqua) are compared to ship observations from research cruises made in 2001 and 2003–2007 into the stratus/stratocumulus deck over the southeast Pacific Ocean. It is found that CloudSat radar-only LWP is generally too high over this region and the CloudSat/CALIPSO cloud bases are too low. This results in a relationship (LWP~h9) between CloudSat LWP and CALIPSO cloud thickness (h) that is very different from the adiabatic relationship (LWP~h2) from in situ observations. Such biases can be reduced if LWPs suspected to be contaminated by precipitation are eliminated, as determined by the maximum radar reflectivity Zmax>−15 dBZ in the apparent lower half of the cloud, and if cloud bases are determined based upon the adiabatically-determined cloud thickness (h~LWP1/2). Furthermore, comparing results from a global model (CAM3.1) to ship observations reveals that, while the simulated LWP is quite reasonable, the model cloud is too thick and too low, allowing the model to have LWPs that are almost independent of h. This model can also obtain a reasonable diurnal cycle in LWP and cloud fraction at a location roughly in the centre of this region (20° S, 85° W) but has an opposite diurnal cycle to those observed aboard ship at a location closer to the coast (20° S, 75° W). The diurnal cycle at the latter location is slightly improved in the newest version of the model (CAM4). However, the simulated clouds remain too thick and too low, as cloud bases are usually at or near the surface.

scholarly journals Climatology of stratocumulus cloud morphologies: microphysical properties and radiative effects

2014 ◽  
Vol 14 (13) ◽  
pp. 6695-6716 ◽  
Author(s):  
A. Muhlbauer ◽  
I. L. McCoy ◽  
R. Wood
Keyword(s):  
Physical Properties ◽  
Global Scale ◽  
Cloud Fraction ◽  
Cloud Base ◽  
Trade Wind ◽  
Radiative Effects ◽  
Regional Variability ◽  
Microphysical Properties ◽  
Low Clouds ◽  
Low Cloud

Abstract. An artificial neural network cloud classification scheme is combined with A-train observations to characterize the physical properties and radiative effects of marine low clouds based on their morphology and type of mesoscale cellular convection (MCC) on a global scale. The cloud morphological categories are (i) organized closed MCC, (ii) organized open MCC and (iii) cellular but disorganized MCC. Global distributions of the frequency of occurrence of MCC types show clear regional signatures. Organized closed and open MCCs are most frequently found in subtropical regions and in midlatitude storm tracks of both hemispheres. Cellular but disorganized MCC are the predominant type of marine low clouds in regions with warmer sea surface temperature such as in the tropics and trade wind zones. All MCC types exhibit a pronounced seasonal cycle. The physical properties of MCCs such as cloud fraction, radar reflectivity, drizzle rates and cloud top heights as well as the radiative effects of MCCs are found highly variable and a function of the type of MCC. On a global scale, the cloud fraction is largest for closed MCC with mean cloud fractions of about 90%, whereas cloud fractions of open and cellular but disorganized MCC are only about 51% and 40%, respectively. Probability density functions (PDFs) of cloud fractions are heavily skewed and exhibit modest regional variability. PDFs of column maximum radar reflectivities and inferred cloud base drizzle rates indicate fundamental differences in the cloud and precipitation characteristics of different MCC types. Similarly, the radiative effects of MCCs differ substantially from each other in terms of shortwave reflectance and transmissivity. These differences highlight the importance of low-cloud morphologies and their associated cloudiness on the shortwave cloud forcing.

scholarly journals Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud fraction over the southeast Atlantic

2021 ◽  
Author(s):  
Jianhao Zhang ◽  
Paquita Zuidema
Keyword(s):  
Boundary Layer ◽  
Seasonal Cycle ◽  
Large Scale ◽  
Cloud Fraction ◽  
Small Scale ◽  
Aerosol Cloud ◽  
Ascension Island ◽  
Tropospheric Aerosol ◽  
Low Cloud ◽  
Aerosol Cloud Interactions

Abstract. Many studies examining shortwave-absorbing aerosol-cloud interactions over the southeast Atlantic apply a seasonal averaging. This disregards a meteorology that raises the mean altitude of the smoke layer from July to October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass-burning aerosol loading at Ascension Island from July to October, based on measurements from Ascension Island (8º S, 14.5º W), satellite retrievals and reanalysis. In July and August, variability in the smoke loading predominantly occurs in the boundary layer. During both months, the low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. The meteorology varies little, suggesting aerosol-cloud interactions consistent with a boundary-layer semi-direct effect can explain the cloudiness changes. September marks a transition month during which mid-latitude disturbances can intrude into the Atlantic subtropics, constraining the land-based anticyclonic circulation transporting free-tropospheric aerosol to closer to the coast. Stronger boundary layer winds help deepen, dry, and cool the boundary layer near the main stratocumulus deck compared to that on days with high smoke loadings, with stratocumulus reducing everywhere but at the northern deck edge. Longwave cooling rates generated by a sharp water vapor gradient at the aerosol layer top facilitates small-scale vertical mixing, and could help to maintain a better-mixed September free troposphere. The October meteorology is more singularly dependent on the strength of the free-tropospheric winds advecting aerosol offshore. Free-tropospheric aerosol is less, and moisture variability more, compared to September. Low-level clouds increase and are more stratiform, when the smoke loadings are higher. The increased free-tropospheric moisture can help sustain the clouds through reducing evaporative drying during cloud-top entrainment. Enhanced subsidence above the coastal upwelling region increasing cloud droplet number concentrations may further prolong cloud lifetime through microphysical interactions. Reduced subsidence underneath stronger free-tropospheric winds at Ascension supports slightly higher cloud tops during smokier conditions. Overall the monthly changes in the large-scale aerosol and moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology, raising a climate importance as cloudiness changes dominate changes in the top-of-atmosphere radiation budget.

scholarly journals On the relationship between low cloud variability and lower tropospheric stability in the Southeast Pacific

2011 ◽  
Vol 11 (2) ◽  
pp. 3777-3811
Author(s):  
F. Sun ◽  
A. Hall ◽  
X. Qu
Keyword(s):  
Climate Change ◽  
Seasonal Cycle ◽  
Austral Summer ◽  
Cloud Amount ◽  
Anthropogenic Climate Change ◽  
Cloud Feedbacks ◽  
Southeast Pacific ◽  
Low Cloud ◽  
Lower Tropospheric Stability ◽  
The Relationship

Abstract. In this study, we examine observed marine low cloud variability in the southeast Pacific and its association with lower-tropospheric stability (LTS) across a spectrum of timescales. On both daily and interannual timescales, LTS and low cloud amount are very well correlated in austral summer (DJF). Meanwhile in winter (JJA), when ambient LTS increases, the LTS-low cloud relationship disintegrates. The DJF LTS-low cloud relationship also weakens in years with unusually large ambient LTS values. These are generally strong El Niño years, in which DJF LTS values are comparable to those typically found in JJA. Thus the LTS-low cloud relationship is strongly modulated by the seasonal cycle and the ENSO phenomenon. We also investigate the origin of LTS anomalies closely associated with low cloud variability during austral summer. We find that the ocean and atmosphere are independently involved in generating anomalies in LTS and hence variability in the southeast Pacific low cloud deck. This highlights the coupled nature of the climate system in this region, and raises the possibility of cloud feedbacks related to LTS. We conclude by addressing the implications of the observed LTS-low cloud relationship in the southeast Pacific for low cloud feedbacks in anthropogenic climate change.

scholarly journals Changes in clouds and thermodynamics under solar geoengineering and implications for required solar reduction

2018 ◽  
Author(s):  
Rick D. Russotto ◽  
Thomas P. Ackerman
Keyword(s):  
Longwave Radiation ◽  
Atmospheric Temperature ◽  
Cloud Fraction ◽  
Model Intercomparison ◽  
Solar Forcing ◽  
Solar Constant ◽  
Low Cloud ◽  
Solar Geoengineering ◽  
Stratospheric Cooling ◽  
Global Mean

Abstract. The amount of solar constant reduction required to offset the global warming from an increase in atmospheric CO2 concentration is an interesting question with implications for assessing the feasibility of solar geoengineering scenarios and for improving our theoretical understanding of Earth's climate response to greenhouse gas and solar forcings. This study investigates this question by analyzing the results of 11 coupled atmosphere-ocean global climate models running Experiment G1 of the Geoengineering Model Intercomparison Project, in which CO2 concentrations are abruptly quadrupled and the solar constant is simultaneously reduced by an amount tuned to maintain top of atmosphere energy balance and preindustrial global mean temperature. The required solar constant reduction in G1 is between 3.2 % and 5.0 %, depending on the model, and is uncorrelated with the models' equilibrium climate sensitivity, while a formula from the experiment specifications based on the models' effective CO2 forcing and planetary albedo is well-correlated with but consistently underpredicts the required solar reduction. We propose an alternative theory for the required solar reduction based on CO2 instantaneous forcing and the sum of radiative adjustments to the combined CO2 and solar forcings. We quantify these radiative adjustments in G1 using established methods and explore changes in atmospheric temperature, humidity and cloud fraction in order to understand the causes of these radiative adjustments. The zonal mean temperature response in G1 exhibits cooling in the tropics and warming in high latitudes at the surface; greater cooling in the upper troposphere at all latitudes; and stratospheric cooling which is mainly due to the CO2 increase. Tropospheric specific humidity decreases due to the temperature decrease, while stratospheric humidity may increase or decrease depending on the model's temperature change in the tropical tropopause layer. Low cloud fraction decreases in all models in G1, an effect that is robust and widespread across ocean and vegetated land areas. We attribute this to a reduction in boundary layer inversion strength over the ocean, and a reduction in the release of water from plants due to the increased CO2. High cloud fraction increases in the global mean in most models. The low cloud fraction reduction and atmospheric temperature decrease have strong warming effects on the planet, due to reduced reflection of shortwave radiation and reduced emission of longwave radiation, respectively. About 50 % to 75 % of the temperature effect is caused by the stratospheric cooling, while the reduction in atmospheric humidity results in increased outgoing longwave radiation that roughly offsets the tropospheric temperature effect. Taken together, the sum of the diagnosed radiative adjustments and the CO2 instantaneous forcing predicts the required solar forcing in G1 to within about 6 %. The cloud fraction response to the G1 experiment raises interesting questions about cloud rapid adjustments and feedbacks under solar versus greenhouse forcings, which would be best explored in a model intercomparison framework with a solar-forcing-only experiment.

Influence of the Subtropical High and Storm Track on Low-Cloud Fraction and Its Seasonality over the South Indian Ocean

2018 ◽  
Vol 31 (10) ◽  
pp. 4017-4039 ◽  
Author(s):  
Ayumu Miyamoto ◽  
Hisashi Nakamura ◽  
Takafumi Miyasaka
Keyword(s):  
Indian Ocean ◽  
Storm Track ◽  
Cloud Fraction ◽  
Subtropical High ◽  
The South ◽  
South Indian Ocean ◽  
Mascarene High ◽  
South Indian ◽  
Low Cloud ◽  
Storm Track Activity

Abstract The south Indian Ocean is characterized by enhanced midlatitude storm-track activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene high. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the storm-track activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced storm-track activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although its zonally extended distribution is correspondent with that of LCF, EIS alone cannot explain the wintertime LCF enhancement, which precedes the EIS maximum under continuous lowering of SST and enhanced SHF in winter. Basinwide cold advection associated with the wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 15°–25°S, while seasonally enhanced storm-track activity augments SHF around 30°S. The analysis highlights the significance of large-scale controls, particularly through SHF, on the seasonality of the climatological LCF distribution over the south Indian Ocean, which reflect the seasonality of the Mascarene high and storm-track activity.

Low Cloud Cover Sensitivity to Biomass-Burning Aerosols and Meteorology over the Southeast Atlantic

2018 ◽  
Vol 31 (11) ◽  
pp. 4329-4346 ◽  
Author(s):  
Adeyemi A. Adebiyi ◽  
Paquita Zuidema
Keyword(s):  
Biomass Burning ◽  
Cloud Cover ◽  
Longwave Radiation ◽  
Cloud Fraction ◽  
Specific Humidity ◽  
Aerosol Layer ◽  
Stratocumulus Clouds ◽  
Low Cloud ◽  
Low Cloud Cover ◽  
Absorbing Aerosols

Abstract Shortwave-absorbing aerosols seasonally cover and interact with an expansive low-level cloud deck over the southeast Atlantic. Daily anomalies of the MODIS low cloud fraction, fine-mode aerosol optical depth (AODf), and six ERA-Interim meteorological parameters (lower-tropospheric stability, 800-hPa subsidence, 600-hPa specific humidity, 1000- and 800-hPa horizontal temperature advection, and 1000-hPa geopotential height) are constructed spanning July–October (2001–12). A standardized multiple linear regression, whereby the change in the low cloud fraction to each component’s variability is normalized by one standard deviation, facilitates comparison between the different variables. Most cloud–meteorology relationships follow expected behavior for stratocumulus clouds. Of interest is the low cloud–subsidence relationship, whereby increasing subsidence increases low cloud cover between 10° and 20°S but decreases it elsewhere. Increases in AODf increase cloudiness everywhere, independent of other meteorological predictors. The cloud–AODf effect is partially compensated by accompanying increases in the midtropospheric moisture, which is associated with decreases in low cloud cover. This suggests that the free-tropospheric moisture affects the low cloud deck primarily through longwave radiation rather than mixing. The low cloud cover is also more sensitive to aerosol when the vertical distance between the cloud and aerosol layer is relatively small, which is more likely to occur early in the biomass burning season and farther offshore. A parallel statistical analysis that does not include AODf finds altered relationships between the low cloud cover changes and meteorology that can be understood through the aerosol cross-correlations with the meteorological predictors. For example, the low cloud–stability relationship appears stronger if aerosols are not explicitly included.

scholarly journals Simulating the Role of Subtropical Stratocumulus Clouds in Driving Pacific Climate Variability

2014 ◽  
Vol 27 (13) ◽  
pp. 5119-5131 ◽  
Author(s):  
Katinka Bellomo ◽  
Amy Clement ◽  
Thorsten Mauritsen ◽  
Gaby Rädel ◽  
Bjorn Stevens
Keyword(s):  
Climate Variability ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Cloud Feedback ◽  
Decadal Climate Variability ◽  
Cloud Feedbacks ◽  
Southeast Pacific ◽  
Low Cloud ◽  
Pacific Climate Variability

This study examines the influence of the northeast and southeast Pacific subtropical stratocumulus cloud regions on the modes of Pacific climate variability simulated by an atmospheric general circulation model (ECHAM6) coupled to a slab ocean. The sensitivity of cloud liquid water to underlying SST is changed in the radiation module of the atmospheric model to increase the strength of positive low-cloud feedback in the two regions. Enhanced low-cloud feedback increases the persistence and variance of the leading modes of climate variability at decadal and longer time scales. Additional integrations show that the southeast Pacific influences climate variability in the equatorial ENSO region, whereas the effects of the northeast Pacific remain confined to the North Pacific. The results herein suggest that a positive feedback among SST, cloud cover, and large-scale atmospheric circulation can explain decadal climate variability in the Pacific Ocean. In particular, cloud feedbacks over the subtropical stratocumulus regions set the time scale of climate variability. A proper representation of low-level cloud feedbacks in the subtropical stratocumulus regions could therefore improve the simulation of Pacific climate variability.

scholarly journals The Time Scales of Variability of Marine Low Clouds

2016 ◽  
Vol 29 (18) ◽  
pp. 6463-6481 ◽  
Author(s):  
Simon P. de Szoeke ◽  
Kathryn L. Verlinden ◽  
Sandra E. Yuter ◽  
David B. Mechem
Keyword(s):  
Time Scales ◽  
Cloud Amount ◽  
Sea Surface ◽  
Low Clouds ◽  
Spectral Peaks ◽  
Low Cloud ◽  
Inversion Strength ◽  
Radiative Feedbacks ◽  
The Tropics ◽  
And Inversion

Abstract Multidecade global regressions of inversion strength, vertical velocity, and sea surface temperature (SST) on low cloud amount, from subdaily to multiyear time scales, refute the dominance of seasonal inversion strength on marine low cloud variability. Multiday low cloud variance averaged over the eastern Pacific and Atlantic stratocumulus regions [5 × 10−2 (cloud amount)2] is twice the subdaily variance and 5 times larger than the multimonth variance. The broad multiday band contains most (60%) of the variance, despite strong seasonal (annual) and diurnal spectral peaks. Multiday low cloud amount over the eastern tropical and midlatitude oceans is positively correlated to inversion strength, with a slope of 2%–5% K−1. Anecdotes show multiday low cloud and inversion strength anomalies propagate equatorward from midlatitudes. Previously shown correlations of low clouds to strong inversions and cool SST on monthly and longer time scales in the stratocumulus regions imply positive cloud-radiative feedbacks, with e-folding time scales of 300 days for SST and 14 days for atmospheric boundary layer temperature. On multimonth time scales, removing the effect of SST on low clouds reduces the low cloud amount explained by inversion strength by a factor of 3, but SST has a small effect at other time scales. Contrary to their positive correlation in the stratocumulus cloud decks, low clouds are anticorrelated to inversion strength over most of the tropics on daily and subdaily time scales.

scholarly journals Subseasonal variability of low cloud radiative properties over the southeast Pacific Ocean

2010 ◽  
Vol 10 (8) ◽  
pp. 4047-4063 ◽  
Author(s):  
R. C. George ◽  
R. Wood
Keyword(s):  
Large Scale ◽  
Cloud Fraction ◽  
Radiative Properties ◽  
Pressure System ◽  
Cloud Albedo ◽  
Cloud Properties ◽  
Droplet Concentration ◽  
Subseasonal Variability ◽  
Low Cloud ◽  
Macrophysical Properties

Abstract. Subseasonal variability of cloud radiative properties in the persistent southeast Pacific stratocumulus deck is investigated using MODIS satellite observations and NCEP reanalysis data. A once-daily albedo proxy is derived based on the fractional coverage of low cloud (a macrophysical field) and the cloud albedo, with the latter broken down into contributions from microphysics (cloud droplet concentration) and macrophysics (liquid water path). Subseasonal albedo variability is dominated by the contribution of low cloud fraction variability, except within 10–15° of the South American coast, where cloud albedo variability contributes significantly. Covariance between cloud fraction and cloud albedo also contributes significantly and positively to the variance in albedo, which highlights how complex and inseparable the factors controlling albedo are. Droplet concentration variability contributes only weakly to the subseasonal variability of albedo, which emphasizes that attributing albedo variability to the indirect effects of aerosols against the backdrop of natural meteorological variability is extremely challenging. The dominant large scale meteorological variability is associated with the subtropical high pressure system. Two indices representing changes in the subtropical high strength and extent explain 80–90% of this variability, and significantly modulate the cloud microphysical, macrophysical, and radiative cloud properties. Variations in droplet concentration of up to 50% of the mean are associated with the meteorological driving. We hypothesize that these fluctuations in droplet concentration are a result of the large scale meteorology and their correlation with cloud macrophysical properties should not be used as evidence of aerosol effects. Mechanisms by which large scale meteorology affects cloud properties are explored. Our results support existing hypotheses linking cloud cover variability to changes in cold advection, subsidence, and lower tropospheric stability. Within 10° of the coast interactions between variability in the surface high pressure system and the orography appear to modulate both cloud macrophysical properties and aerosol transport through suppression of the marine boundary layer depth near the coast. This suggests one possible way in which cloud macrophysical properties and droplet concentration may be correlated independently of the second aerosol indirect effect. The results provide variability constraints for models that strive to represent both meteorological and aerosol impacts on stratocumulus clouds.

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