scholarly journals Heuristic estimation of low-level cloud fraction over the globe based on a decoupling parameterization

2019 ◽  
Vol 19 (8) ◽  
pp. 5635-5660 ◽  
Author(s):  
Sungsu Park ◽  
Jihoon Shin
Keyword(s):  
General Circulation ◽  
Cloud Amount ◽  
Cloud Fraction ◽  
Spatiotemporal Variation ◽  
Interannual Variations ◽  
Marine Stratocumulus ◽  
Near Surface ◽  
Low Level ◽  
Inversion Strength ◽  
Lifting Condensation Level

Abstract. Based on the decoupling parameterization of the cloud-topped planetary boundary layer, a simple equation is derived to compute the inversion height. In combination with the lifting condensation level and the amount of water vapor in near-surface air, we propose a low-level cloud suppression parameter (LCS) and estimated low-level cloud fraction (ELF), as new proxies for the analysis of the spatiotemporal variation of the global low-level cloud amount (LCA). Individual surface and upper-air observations are used to compute LCS and ELF as well as lower-tropospheric stability (LTS), estimated inversion strength (EIS), and estimated cloud-top entrainment index (ECTEI), three proxies for LCA that have been widely used in previous studies. The spatiotemporal correlations between these proxies and surface-observed LCA were analyzed. Over the subtropical marine stratocumulus deck, both LTS and EIS diagnose seasonal–interannual variations of LCA well. However, their use as a global proxy for LCA is limited due to their weaker and inconsistent relationship with LCA over land. EIS is anti-correlated with the decoupling strength more strongly than it is correlated with the inversion strength. Compared with LTS and EIS, ELF and LCS better diagnose temporal variations of LCA, not only over the marine stratocumulus deck but also in other regions. However, all proxies have a weakness in diagnosing interannual variations of LCA in several subtropical stratocumulus decks. In the analysis using all data, ELF achieves the best performance in diagnosing spatiotemporal variation of LCA, explaining about 60 % of the spatial–seasonal–interannual variance of the seasonal LCA over the globe, which is a much larger percentage than those explained by LTS (2 %) and EIS (4 %). Our study implies that accurate prediction of inversion base height and lifting condensation level is a key factor necessary for successful simulation of global low-level clouds in general circulation models (GCMs). Strong spatiotemporal correlation between ELF (or LCS) and LCA identified in our study can be used to evaluate the performance of GCMs, identify the source of inaccurate simulation of LCA, and better understand climate sensitivity.

scholarly journals Heuristic Estimation of Low-Level Cloud Fraction over the Globe Based on a Decoupling Parameterization

2019 ◽  
Author(s):  
Sungsu Park ◽  
Jihoon Shin
Keyword(s):  
General Circulation ◽  
Cloud Amount ◽  
Cloud Fraction ◽  
Spatiotemporal Variation ◽  
Interannual Variations ◽  
Marine Stratocumulus ◽  
Near Surface ◽  
Low Level ◽  
Inversion Strength ◽  
Lifting Condensation Level

Abstract. Based on the decoupling parameterization of the cloud-topped planetary boundary layer, a simple equation is derived to compute the inversion height. In combination with the lifting condensation level and the amount of water vapor in near-surface air, we propose a low-level cloud suppression parameter (LCS) and estimated low-level cloud fraction (ELF), as new proxies for the analysis of the spatiotemporal variation of the global low-level cloud amount (LCA). Individual surface and upper-air observations are used to compute LCS and ELF as well as lower-tropospheric stability (LTS), estimated inversion strength (EIS), and estimated cloud-top entrainment index (ECTEI), three proxies for LCA that have been widely used in previous studies. The spatiotemporal correlations between these proxies and surface-observed LCA were analyzed. Over the subtropical marine stratocumulus deck, both LTS and EIS well diagnose seasonal-interannual variations of LCA. However, their use as global proxy for LCA is limited due to their weaker and inconsistent relationship with LCA over land. EIS is anti-correlated with the decoupling strength more strongly than it is correlated with the inversion strength. Compared with LTS and EIS, ELF and LCS better diagnose temporal variations of LCA, not only over the marine stratocumulus deck but also in other regions. However, all proxies have a weakness in diagnosing interannual variations of LCA in several subtropical stratocumulus decks. In the analysis using all data, ELF achieves the best performance in diagnosing spatiotemporal variation of LCA, explaining about 60 % of the spatial-seasonal-interannnual variance of the seasonal LCA over the globe, which is a much larger percentage than those explained by LTS (2 %) and EIS (4 %). Our study implies that accurate prediction of inversion base height and lifting condensation level is a key factor necessary for successful simulation of global low-level clouds in general circulation models (GCMs). Strong spatiotemporal correlation between ELF (or LTS) and LCA identified in our study can be used to evaluate the performance of GCMs, identify the source of inaccurate simulation of LCA, and better understand climate sensitivity.

scholarly journals The relationship between low-level cloud amount and its proxies over the globe by cloud type

2020 ◽  
Vol 20 (5) ◽  
pp. 3041-3060
Author(s):  
Jihoon Shin ◽  
Sungsu Park
Keyword(s):  
Cloud Amount ◽  
Superior Performance ◽  
Cloud Type ◽  
Low Level ◽  
Incorrect Diagnosis ◽  
Stratiform Clouds ◽  
Extended Analysis ◽  
Inversion Strength ◽  
Lifting Condensation Level ◽  
The Relationship

Abstract. We extend upon previous work to examine the relationship between low-level cloud amount (LCA) and various proxies for LCA – estimated low-level cloud fraction (ELF), lower tropospheric stability (LTS), and estimated inversion strength (EIS) – by low-level cloud type (CL) over the globe using individual surface and upper-air observations. Individual CL has its own distinct environmental structure, and therefore our extended analysis by CL can provide insights into the strengths and weaknesses of various proxies and help to improve them. Overall, ELF performs better than LTS and EIS in diagnosing the variations in LCA among various CLs, indicating that a previously identified superior performance of ELF compared to LTS and EIS as a global proxy for LCA comes from its realistic correlations with various CLs rather than with a specific CL. However, ELF, LTS, and EIS have a problem in diagnosing the changes in LCA when noCL (no low-level cloud) is reported and also when Cu (cumulus) is reported over deserts where background stratus does not exist. This incorrect diagnosis of noCL as a cloudy condition is more clearly seen in the analysis of individual CL frequencies binned by proxy values. If noCL is excluded, ELF, LTS, and EIS have good inter-CL correlations with the amount when present (AWP) of individual CLs. In the future, an advanced ELF needs to be formulated to deal with the decrease in LCA when the inversion base height is lower than the lifting condensation level to diagnose cumulus updraft fraction, as well as the amount of stratiform clouds and detrained cumulus, and to parameterize the scale height as a function of appropriate environmental variables.

scholarly journals The Relationship between Low-Level Cloud Amount and Its Proxies over the Globe by Cloud Types

2019 ◽  
Author(s):  
Jihoon Shin ◽  
Sungsu Park
Keyword(s):  
Cloud Amount ◽  
Superior Performance ◽  
Low Level ◽  
Incorrect Diagnosis ◽  
Stratiform Clouds ◽  
Extended Analysis ◽  
Inversion Strength ◽  
Lifting Condensation Level ◽  
The Relationship ◽  
Better Than

Abstract. We extend upon previous work to examine the relationship between low-level cloud amount (LCA) and various proxies for LCA – estimated low-level cloud fraction (ELF), lower-tropospheric stability (LTS), estimated inversion strength (EIS), and estimated cloud-top entrainment index (ECTEI) – by low-level cloud types (CL) over the globe using individual surface and upper-air observations. Individual CL has its own distinct environmental structure, and therefore our extended analysis by CL can provide insights into the strength and weakness of various proxies and help to improve them. Overall, ELF performs better than LTS/EIS in diagnosing the variations in LCA among various CLs, indicating that a previously identified superior performance of ELF to LTS/EIS as a global proxy for LCA comes from its realistic correlations with various CLs rather than with a specific CL. However, ELF as well as LTS/EIS has a problem in diagnosing the decrease in LCA when CL0 (no low-level cloud) is reported and the increase of LCA when CL12 (cumulus) is reported over the deserts where background stratus does not exist. This incorrect diagnosis of CL0 as a cloudy condition is more clearly seen in the analysis of individual CL frequencies binned by proxy values. If CL0 is excluded, all ELF/LTS/EIS have good inter-CL correlations with the amount-when-present (AWP) of individual CLs. In future, an advanced ELF needs to be formulated to deal with the dissipation of LCA when the inversion base height is lower than the lifting condensation level, to diagnose cumulus updraft fraction as well as the amount of stratiform clouds and detrained cumulus, and to parameterize the scale height as a function of appropriate environmental variables.

scholarly journals Thermodynamic control of anvil cloud amount

2016 ◽  
Vol 113 (32) ◽  
pp. 8927-8932 ◽  
Author(s):  
Sandrine Bony ◽  
Bjorn Stevens ◽  
David Coppin ◽  
Tobias Becker ◽  
Kevin A. Reed ◽  
...  
Keyword(s):  
Thermodynamic Properties ◽  
General Circulation ◽  
Deep Convection ◽  
Cloud Amount ◽  
General Circulation Models ◽  
Cloud Fraction ◽  
Thermodynamic Control ◽  
Upper Troposphere ◽  
High Clouds ◽  
Enhanced Stability

General circulation models show that as the surface temperature increases, the convective anvil clouds shrink. By analyzing radiative–convective equilibrium simulations, we show that this behavior is rooted in basic energetic and thermodynamic properties of the atmosphere: As the climate warms, the clouds rise and remain at nearly the same temperature, but find themselves in a more stable atmosphere; this enhanced stability reduces the convective outflow in the upper troposphere and decreases the anvil cloud fraction. By warming the troposphere and increasing the upper-tropospheric stability, the clustering of deep convection also reduces the convective outflow and the anvil cloud fraction. When clouds are radiatively active, this robust coupling between temperature, high clouds, and circulation exerts a positive feedback on convective aggregation and favors the maintenance of strongly aggregated atmospheric states at high temperatures. This stability iris mechanism likely contributes to the narrowing of rainy areas as the climate warms. Whether or not it influences climate sensitivity requires further investigation.

scholarly journals Coastal Clouds at the Eastern Margin of the Southeast Pacific: Climatology and Trends

2016 ◽  
Vol 29 (12) ◽  
pp. 4525-4542 ◽  
Author(s):  
Ricardo C. Muñoz ◽  
Juan Quintana ◽  
Mark J. Falvey ◽  
José A. Rutllant ◽  
René Garreaud
Keyword(s):  
Cloud Cover ◽  
Marine Boundary Layer ◽  
Onset Time ◽  
Cloud Fraction ◽  
Cloud Base ◽  
Physical Factors ◽  
Near Surface ◽  
Low Level ◽  
Drying Trend ◽  
Long Term Trends

Abstract The climatology and recent trends of low-level coastal clouds at three sites along the northern Chilean coast (18.3°–23.4°S) are documented based upon up to 45 years of hourly observations of cloud type, coverage, and heights. Consistent with the subtropical location, cloud types are dominated by stratocumuli having greatest coverage (>7 oktas) and smaller heights (600–750 m) during the nighttime of austral winter and spring. Meridionally, nighttime cloud fraction and cloud-base heights increase from south to north. Long-term trends in mean cloud cover are observed at all sites albeit with a seasonal modulation, with increasing (decreasing) coverage in the spring (fall). Consistent trend patterns are also observed in independent sunshine hour measurements at the same sites. Cloud heights show negative trends of about 100 m decade−1 (1995–2010), although the onset time of this tendency differs between sites. The positive cloud fraction trends during the cloudy season reported here disagree with previous studies, with discrepancies attributed to differences in datasets used or to methodological differences in data analysis. The cloud-base height tendency, together with a less rapid lowering of the subsidence inversion base height, suggests a deepening of the coastal cloud layer. While consistent with the tendency toward greater low-level cloud cover and the known cooling of the marine boundary layer in this region, these tendencies are at odds with a drying trend of the near-surface air documented here as well. Assessing whether this intriguing result is caused by physical factors or by limitations of the data demands more detailed observations, some of which are currently under way.

Low-Level Cloud Variability over the Equatorial Cold Tongue in Observations and Models

2007 ◽  
Vol 20 (8) ◽  
pp. 1555-1570 ◽  
Author(s):  
David K. Mansbach ◽  
Joel R. Norris
Keyword(s):  
Boundary Layer ◽  
Surface Temperature ◽  
Climate Models ◽  
Cloud Amount ◽  
Global Climate Models ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Cold Tongue ◽  
Multilinear Regression ◽  
Near Surface ◽  
Low Level

Abstract Examination of cloud and meteorological observations from satellite, surface, and reanalysis datasets indicates that monthly anomalies in low-level cloud amount and near-surface temperature advection are strongly negatively correlated on the southern side of the equatorial Pacific cold tongue. This inverse correlation occurs independently of relationships between cloud amount and sea surface temperature (SST) or lower tropospheric static stability (LTS), and the combination of advection plus SST or LTS explains significantly more interannual cloud variability in a multilinear regression than does SST or LTS alone. Warm anomalous advection occurs when the equatorial cold tongue is well defined and the southeastern Pacific trade winds bring relatively warm air over colder water. Ship meteorological reports and soundings show that the atmospheric surface layer becomes stratified under these conditions, thus inhibiting the upward mixing of moisture needed to sustain cloudiness against subsidence and entrainment drying. Cold anomalous advection primarily occurs when the equatorial cold tongue is weak or absent and the air–sea temperature difference is substantially negative. These conditions favor a more convective atmospheric boundary layer, greater cloud amount, and less frequent occurrence of clear sky. Examination of output from global climate models developed by the Geophysical Fluid Dynamics Laboratory (GFDL) and the National Center for Atmospheric Research (NCAR) indicates that both models generally fail to simulate the cloud–advection relationships observed on the northern and southern sides of the equatorial cold tongue. Although the GFDL atmosphere model does reproduce the expected signs of cloud-advection correlations when forced with prescribed historical SST variations, it does not consistently do so when coupled to an ocean model. The NCAR model has difficulty reproducing the observed correlations in both atmosphere-only and coupled versions. This suggests that boundary layer cloud parameterizations could be improved through better representation of the effects of advection over varying SST.

scholarly journals Interactions between Hydrological Sensitivity, Radiative Cooling, Stability, and Low-Level Cloud Amount Feedback

2018 ◽  
Vol 31 (5) ◽  
pp. 1833-1850 ◽  
Author(s):  
Mark J. Webb ◽  
Adrian P. Lock ◽  
F. Hugo Lambert
Keyword(s):  
Climate Model ◽  
Cloud Amount ◽  
Cloud Fraction ◽  
Radiative Cooling ◽  
Substantial Impact ◽  
Surface Evaporation ◽  
Low Level ◽  
Rate Of Increase ◽  
Hydrological Sensitivity ◽  
The Impact

Low-level cloud feedbacks vary in magnitude but are positive in most climate models, due to reductions in low-level cloud fraction. This study explores the impact of surface evaporation on low-level cloud fraction feedback by performing climate change experiments with the aquaplanet configuration of the HadGEM2-A climate model, forcing surface evaporation to increase at different rates in two ways. Forcing the evaporation diagnosed in the surface scheme to increase at 7% K−1 with warming (more than doubling the hydrological sensitivity) results in an increase in global mean low-level cloud fraction and a negative global cloud feedback, reversing the signs of these responses compared to the standard experiments. The estimated inversion strength (EIS) increases more rapidly in these surface evaporation forced experiments, which is attributed to additional latent heat release and enhanced warming of the free troposphere. Stimulating a 7% K−1 increase in surface evaporation via enhanced atmospheric radiative cooling, however, results in a weaker EIS increase compared to the standard experiments and a slightly stronger low-level cloud reduction. The low-level cloud fraction response is predicted better by EIS than surface evaporation across all experiments. This suggests that surface-forced increases in evaporation increase low-level cloud fraction mainly by increasing EIS. Additionally, the results herein show that increases in surface evaporation can have a very substantial impact on the rate of increase in radiative cooling with warming, by modifying the temperature and humidity structure of the atmosphere. This has implications for understanding the factors controlling hydrological sensitivity.

scholarly journals The Influence of Lifting Condensation Level on Low-Level Outflow and Rotation in Simulated Supercell Thunderstorms

2019 ◽  
Vol 76 (5) ◽  
pp. 1349-1372 ◽  
Author(s):  
Matthew Brown ◽  
Christopher J. Nowotarski
Keyword(s):  
Necessary Condition ◽  
Cold Pool ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Low Level ◽  
Cold Pools ◽  
Surface Circulation ◽  
Supercell Thunderstorms ◽  
Wind Profiles ◽  
Lifting Condensation Level

Abstract This paper reports on results of idealized numerical simulations testing the influence of low-level humidity, and thus lifting condensation level (LCL), on the morphology and evolution of low-level rotation in supercell thunderstorms. Previous studies have shown that the LCL can influence outflow buoyancy, which can in turn affect generation and stretching of near-surface vertical vorticity. A less explored hypothesis is tested: that the LCL affects the relative positioning of near-surface circulation and the overlying mesocyclone, thus influencing the dynamic lifting and intensification of near-surface vertical vorticity. To test this hypothesis, a set of three base-state thermodynamic profiles with varying LCLs are implemented and compared over a variety of low-level wind profiles. The thermodynamic properties of the simulations are sensitive to variations in the LCL, with higher LCLs contributing to more negatively buoyant cold pools. These outflow characteristics allow for a more forward propagation of near-surface circulation relative to the midlevel mesocyclone. When the mid- and low-level mesocyclones become aligned with appreciable near-surface circulation, favorable dynamic updraft forcing is able to stretch and intensify this rotation. The strength of the vertical vorticity generated ultimately depends on other interrelated factors, including the amount of near-surface circulation generated within the cold pool and the buoyancy of storm outflow. However, these simulations suggest that mesocyclone alignment with near-surface circulation is modulated by the ambient LCL, and is a necessary condition for the strengthening of near-surface vertical vorticity. This alignment is also sensitive to the low-level wind profile, meaning that the LCL most favorable for the formation of intense vorticity may change based on ambient low-level shear properties.

scholarly journals How Has Subtropical Stratocumulus and Associated Meteorology Changed since the 1980s?*

2015 ◽  
Vol 28 (21) ◽  
pp. 8396-8410 ◽  
Author(s):  
C. Seethala ◽  
Joel R. Norris ◽  
Timothy A. Myers
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Meteorological Parameters ◽  
Cloud Amount ◽  
Sea Surface ◽  
Independent Effect ◽  
Multilinear Regression ◽  
Low Level ◽  
Observational Uncertainty ◽  
Inversion Strength

Abstract The importance of low-level cloud feedbacks to climate sensitivity motivates an investigation of how low-level cloud amount and related meteorological conditions have changed in recent decades in subtropical stratocumulus regions. Using satellite cloud datasets corrected for inhomogeneities, it is found that during 1984–2009 low-level cloud amount substantially increased over the northeastern Pacific, southeastern Pacific, and southeastern Atlantic; decreased over the northeastern Atlantic; and weakly increased over the southeastern Indian Ocean subtropical stratocumulus regions. Examination of meteorological parameters from four reanalyses indicates that positive trends in low-level cloud amount are associated with cooler sea surface temperature, greater inversion strength, and enhanced cold-air advection. The converse holds for negative trends in low-level cloud amount. A multilinear regression model based on these three meteorological variables reproduces the sign and magnitude of observed cloud amount trends in all stratocumulus regions within the range of observational uncertainty. Changes in inversion strength have the largest independent effect on cloud trends, followed by changes in advection strength. Changes in sea surface temperature have the smallest independent effect on cloud trends. Differing signs of cloud trends and differing contributions from meteorological parameters suggest that observed changes in subtropical stratocumulus since the 1980s may be due to natural variability rather than a systematic response to climate change.

scholarly journals Significance of a Midlatitude SST Frontal Zone in the Formation of a Storm Track and an Eddy-Driven Westerly Jet*

2010 ◽  
Vol 23 (7) ◽  
pp. 1793-1814 ◽  
Author(s):  
Takeaki Sampe ◽  
Hisashi Nakamura ◽  
Atsushi Goto ◽  
Wataru Ohfuchi
Keyword(s):  
General Circulation ◽  
Frontal Zone ◽  
Sensible Heat Flux ◽  
Lower Boundary ◽  
Storm Track ◽  
Lower Boundary Condition ◽  
Near Surface ◽  
Low Level ◽  
Sst Front ◽  
Sst Gradient

Abstract In a set of idealized “aquaplanet” experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature (SST) is prescribed globally as the lower boundary condition, an assessment is made of the potential influence of the frontal SST gradient upon the formation of a storm track and an eddy-driven midlatitude polar front jet (PFJ), and on its robustness against changes in the intensity of a subtropical jet (STJ). In experiments with the frontal midlatitude SST gradient as that observed in the southwestern Indian Ocean, transient eddy activity in each of the winter and summer hemispheres is organized into a deep storm track along the SST front with an enhanced low-level baroclinic growth of eddies. In the winter hemisphere, another storm track forms just below the intense STJ core, but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath. The near-surface westerlies are strongest near the midlatitude SST front as observed, consistent with westerly momentum transport associated with baroclinic eddy growth. The sharp poleward decline in the surface sensible heat flux across the SST frontal zone sustains strong near-surface baroclinicity against the relaxing effect by vigorous poleward eddy heat transport. Elimination of the midlatitude frontal SST gradient yields marked decreases in the activity of eddies and their transport of angular momentum into midlatitudes, in association with equatorward shifts of the PFJ-associated low-level westerlies and a subtropical high pressure belt, especially in the summer hemisphere. These impacts of the midlatitude frontal SST gradient are found to be robust against modest changes in the STJ intensity as observed in its interannual variability, suggesting the potential importance of midlatitude atmosphere–ocean interaction in shaping the tropospheric general circulation.

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