scholarly journals Cluster analysis of midlatitude oceanic cloud regimes: mean properties and temperature sensitivity

2010 ◽  
Vol 10 (13) ◽  
pp. 6435-6459 ◽  
Author(s):  
N. D. Gordon ◽  
J. R. Norris
Keyword(s):  
Large Scale ◽  
Longwave Radiation ◽  
Cloud Fraction ◽  
Climate System ◽  
Radiative Properties ◽  
Upper Troposphere ◽  
Low Level ◽  
Increasing Temperature ◽  
Cloud Regimes ◽  
Cloud Response

Abstract. Clouds play an important role in the climate system by reducing the amount of shortwave radiation reaching the surface and the amount of longwave radiation escaping to space. Accurate simulation of clouds in computer models remains elusive, however, pointing to a lack of understanding of the connection between large-scale dynamics and cloud properties. This study uses a k-means clustering algorithm to group 21 years of satellite cloud data over midlatitude oceans into seven clusters, and demonstrates that the cloud clusters are associated with distinct large-scale dynamical conditions. Three clusters correspond to low-level cloud regimes with different cloud fraction and cumuliform or stratiform characteristics, but all occur under large-scale descent and a relatively dry free troposphere. Three clusters correspond to vertically extensive cloud regimes with tops in the middle or upper troposphere, and they differ according to the strength of large-scale ascent and enhancement of tropospheric temperature and humidity. The final cluster is associated with a lower troposphere that is dry and an upper troposphere that is moist and experiencing weak ascent and horizontal moist advection. Since the present balance of reflection of shortwave and absorption of longwave radiation by clouds could change as the atmosphere warms from increasing anthropogenic greenhouse gases, we must also better understand how increasing temperature modifies cloud and radiative properties. We therefore undertake an observational analysis of how midlatitude oceanic clouds change with temperature when dynamical processes are held constant (i.e., partial derivative with respect to temperature). For each of the seven cloud regimes, we examine the difference in cloud and radiative properties between warm and cold subsets. To avoid misinterpreting a cloud response to large-scale dynamical forcing as a cloud response to temperature, we require horizontal and vertical temperature advection in the warm and cold subsets to have near-median values in three layers of the troposphere. Across all of the seven clusters, we find that cloud fraction is smaller and cloud optical thickness is mostly larger for the warm subset. Cloud-top pressure is higher for the three low-level cloud regimes and lower for the cirrus regime. The net upwelling radiation flux at the top of the atmosphere is larger for the warm subset in every cluster except cirrus, and larger when averaged over all clusters. This implies that the direct response of midlatitude oceanic clouds to increasing temperature acts as a negative feedback on the climate system. Note that the cloud response to atmospheric dynamical changes produced by global warming, which we do not consider in this study, may differ, and the total cloud feedback may be positive.

scholarly journals Cluster analysis of midlatitude oceanic cloud regimes – Part 2: Temperature sensitivity of cloud properties

2010 ◽  
Vol 10 (1) ◽  
pp. 1595-1629
Author(s):  
N. D. Gordon ◽  
J. R. Norris
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Radiation Flux ◽  
Global Climate ◽  
Global Climate Models ◽  
Radiative Properties ◽  
Radiation Budget ◽  
Climate Feedback ◽  
Cloud Regimes ◽  
Cloud Response

Abstract. Clouds have a large impact on Earth's radiation budget by reflecting incoming solar radiation and trapping longwave radiation emitted from the surface. The present balance could change as the atmosphere warms from increasing anthropogenic greenhouse gases, thus altering the net radiation flux and mitigating or exacerbating the initial temperature increase. To ascertain the sign and magnitude of cloud-climate feedback, we must better understand the way in which clouds interact with their environment and how temperature modifies cloud and radiative properties. Since global climate models do not consistently and correctly simulate clouds, we undertake an observational analysis of how midlatitude oceanic clouds change with temperature when dynamical processes are held constant (i.e., partial derivative with respect to temperature). For each of the seven cloud regimes identified through k-means clustering of daily satellite data in the companion study, we examine the difference in cloud and radiative properties between warm and cold subsets. To avoid misinterpreting a cloud response to large-scale dynamical forcing as a cloud response to temperature, we require horizontal and vertical temperature advection in the warm and cold subsets to have near-median values in three layers of the troposphere. Across all of the seven clusters, we find that cloud fraction is smaller and cloud optical thickness is mostly larger for the warm subset. Cloud top pressure is higher for the three low-level cloud regimes and lower for the cirrus regime. The net upwelling radiation flux at the top of the atmosphere is larger for the warm subset in every cluster except cirrus, and larger when averaged over all clusters. This implies that the direct response of midlatitude oceanic clouds to increasing temperature acts as a negative feedback on the climate system. Note that the cloud response to atmospheric dynamical changes produced by global warming, which we do not consider in this study, may differ, and the total cloud feedback may be positive.

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.

scholarly journals Global large-scale stratosphere-troposphere exchange in modern reanalyses

2016 ◽  
Author(s):  
Alexander C. Boothe ◽  
Cameron R. Homeyer
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Radiative Properties ◽  
Lapse Rate ◽  
Polar Regions ◽  
Transport Mechanisms ◽  
Upper Troposphere ◽  
Long Term Changes ◽  
The Tropics

Abstract. Stratosphere-troposphere exchange (STE) has important and significant impacts on the chemical and radiative properties of the upper troposphere and lower stratosphere. This study presents a 15-year climatology of global large-scale STE from four modern reanalyses: ERA-Interim, JRA-55, MERRA-2, and MERRA-1. STE is separated into four categories for analysis to identify the significance of known transport mechanisms: 1) vertical stratosphere-to-troposphere transport (STT), 2) vertical troposphere-to-stratosphere transport (TST), 3) lateral STT (that occurring between the tropics and the extratropics and across the tropopause "break"), and 4) lateral TST. In addition, this study employs a method to identify STE as that which crosses the lapse-rate tropopause (LRT), while most previous studies have used a potential vorticity (PV) isosurface as the troposphere-stratosphere boundary. PV-based and LRT based STE climatologies are compared using the same reanalysis output (ERA-Interim). The comparison reveals quantitative and qualitative differences, particularly in the geographic representation of TST in the polar regions. Based upon spatiotemporal integrations, we find STE to be STT-dominant in ERA-Interim and JRA-55 and TST-dominant in the MERRA reanalyses. Time series during the 15-year analysis period show long-term changes that are argued to correspond with changes in the Brewer-Dobson circulation.

Structures of Different Tibetan Plateau Vortex Types

2020 ◽  
Author(s):  
Xinyuan Feng ◽  
Changhai Liu ◽  
Guangzhou Fan ◽  
Jie Zhang
Keyword(s):  
Tibetan Plateau ◽  
Large Scale ◽  
Warm Season ◽  
Symmetric System ◽  
Type I ◽  
Upper Troposphere ◽  
Low Level ◽  
Category Type ◽  
Tibetan Plateau Vortex ◽  
Low Levels

<p>A Tibetan Plateau vortex (TPV) is defined as a shallow cyclonic meso-α-scale low-pressure system that originates over the main body of the Tibetan Plateau in the warm season and presents most notably at 500 hPa. It is the main precipitation-inducing weather system over the plateau in the warm season.</p><p>Knowledge of the TPV structure is of considerable importance for understanding the generation and development mechanisms of this mesoscale system. However, our understanding of vortex structures and our ability to classify them on a physical basis is limited due to insufficient observations. The high-resolution NCEP Climate Forecast System Reanalysis (CFSR) dataset is used in the present paper to investigate the general structural features of various types of mature TPV through classification and composite structure analysis. Results indicate that the dynamic and thermodynamic structures show regional and seasonal dependency, as well as being influenced by attributes of translation, associated precipitation, and the South Asian high (SAH).</p><p>The common precipitating TPV (type I), frequently occurring in the west–east-oriented zonal region between 33° and 36°N, is a notably low-level baroclinic and asymmetric system. It resides within a large-scale confluent zone and preferentially travels eastwards, potentially moving out of the plateau. The heavy rain vortex (type II) corresponds to a deep vortex circulation occurring in midsummer. The low-level baroclinic sub-category (type IIa) is associated with a low-level jet and mainly originates in the area (32°–35°N, 86°–94°E), preferentially moving east of 90°E and even away from the plateau; meanwhile, the nearly upright sub-category (type IIb), which has a cold center at low levels and a warm center at mid-upper levels, is a quasi-stationary and quasi-symmetric system favorably occurring west of 92°E. A western-pattern SAH exists in the upper troposphere for these two sub-categories. The springtime dry vortex in the western plateau (type III) is warm and shallow (~100 hPa deep), and zonal circulation dominates the large-scale environmental flows in the middle and upper troposphere. The precipitating vortex in the southern plateau occurring during July–August (type IV) is not affected by northerly flow at low levels. It is vertically aligned and controlled by a banded SAH.</p>

scholarly journals Global large-scale stratosphere–troposphere exchange in modern reanalyses

2017 ◽  
Vol 17 (9) ◽  
pp. 5537-5559 ◽  
Author(s):  
Alexander C. Boothe ◽  
Cameron R. Homeyer
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Radiative Properties ◽  
Lapse Rate ◽  
Polar Regions ◽  
Transport Mechanisms ◽  
Upper Troposphere ◽  
Subtropical Jet ◽  
The Tropics

Abstract. Stratosphere–troposphere exchange (STE) has important impacts on the chemical and radiative properties of the upper troposphere and lower stratosphere. This study presents a 15-year climatology of global large-scale STE from four modern reanalyses: ERA-Interim, JRA-55, MERRA-2, and MERRA. STE is separated into three regions (tropics, subtropics, and extratropics) and two transport directions (stratosphere-to-troposphere transport or STT and troposphere-to-stratosphere transport or TST) in an attempt to identify the significance of known transport mechanisms. The extratropics and tropics are separated by the tropopause break. Any STE occurring between the tropics and the extratropics through the tropopause break is considered subtropical exchange (i.e., in the vicinity of the subtropical jet). In addition, this study employs a method to identify STE as that which crosses the lapse-rate tropopause (LRT), while most previous studies have used a potential vorticity (PV) isosurface as the troposphere–stratosphere boundary. PV-based and LRT-based STE climatologies are compared using the ERA-Interim reanalysis output. The comparison reveals quantitative and qualitative differences, particularly for TST in the polar regions. Based upon spatiotemporal integrations, we find STE to be STT dominant in ERA-Interim and JRA-55 and TST dominant in MERRA and MERRA-2. The sources of the differences are mainly attributed to inconsistencies in the representation of STE in the subtropics and extratropics. Time series during the 15-year analysis period show long-term changes that are argued to correspond with changes in the Brewer–Dobson circulation.

scholarly journals The Properties and Formation of Cirrus Clouds over the Tibetan Plateau Based on Summertime Lidar Measurements

2013 ◽  
Vol 70 (3) ◽  
pp. 901-915 ◽  
Author(s):  
Q. S. He ◽  
C. C. Li ◽  
J. Z. Ma ◽  
H. Q. Wang ◽  
G. M. Shi ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Large Scale ◽  
Longwave Radiation ◽  
Mean Value ◽  
Cirrus Clouds ◽  
The Tibetan Plateau ◽  
Mean Values ◽  
Upper Troposphere ◽  
Lidar Ratio ◽  
Highly Correlated

Abstract As part of the Tibet Ozone, Aerosol and Radiation (TOAR) project, a micropulse lidar was operated in Naqu (31.5°N, 92.1°E; 4508 m MSL) on the Tibetan Plateau to observe cirrus clouds continuously from 19 July to 26 August 2011. During the experiment, the time coverage of ice clouds only was 15% in the upper troposphere (above 9.5 km MSL). The cirrus top/bottom altitudes (mean values of 15.6/14.7 km) are comparable to those measured previously at tropical sites but relatively higher than those measured at midlatitude sites. The majority of the cloud layers yielded a lidar ratio between 10 and 40 sr, with a mean value of 28 ± 15 sr, characterized by a bimodal frequency distribution. Subvisible, thin, and opaque cirrus formation was observed in 16%, 34%, and 50% of all cirrus cases, respectively. A mean cirrus optical depth of 0.33 was observed over the Tibetan Plateau, slightly higher than those in the subtropics and tropics. With decreasing temperature, the lidar ratio increased slightly, whereas the mean extinction coefficient decreased significantly. The occurrence of clouds is highly correlated with the outgoing longwave radiation and the strong cold perturbations in the upper troposphere. Deep convective activity and Rossby waves are important dynamical processes that control cirrus variations over the Tibetan Plateau, where both anvil cirrus outflowing from convective cumulonimbus clouds and large-scale strong cold perturbations in the upper troposphere should play an important role in cirrus formation.

Impacts of Cloud Droplet–Nucleating Aerosols on Shallow Tropical Convection

2015 ◽  
Vol 72 (4) ◽  
pp. 1369-1385 ◽  
Author(s):  
Stephen M. Saleeby ◽  
Stephen R. Herbener ◽  
Susan C. van den Heever ◽  
Tristan L’Ecuyer
Keyword(s):  
Large Scale ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Energy Budgets ◽  
Substantial Portion ◽  
Cumulus Clouds ◽  
Feedback Effects ◽  
Low Level ◽  
Stratocumulus Clouds ◽  
Cloud Regimes

Abstract Low-level warm-phase clouds cover a substantial portion of Earth’s oceans and play an important role in the global water and energy budgets. The characteristics of these clouds are controlled by the large-scale environment, boundary layer conditions, and cloud microphysics. Variability in the concentration of aerosols can alter cloud microphysical and precipitation processes that subsequently impact the system dynamics and thermodynamics and thereby create aerosol–cloud dynamic–thermodynamic feedback effects. In this study, three distinct cloud regimes were simulated, including stratocumulus, low-level cumulus (cumulus under stratocumulus), and deeper cumulus clouds. The simulations were conducted without environmental large-scale forcing, thereby allowing all three cloud types to freely interact with the environmental state in an undamped fashion. Increases in aerosol concentration in these unforced, warm-phase, tropical cloud simulations lead to the production of fewer low-level cumuli; thinning and erosion of the widespread stratocumulus layer; and the development of deeper, inversion-penetrating cumuli. The mechanisms for these changes are explored. Despite the development of deeper, more heavily precipitating cumuli, the reduction of the widespread moderately precipitating stratocumulus clouds leads to an overall reduction in domainwide accumulated precipitation when aerosol concentrations are enhanced.

Total Heating Characteristics of the ISCCP Tropical and Subtropical Cloud Regimes

2013 ◽  
Vol 26 (18) ◽  
pp. 7097-7116 ◽  
Author(s):  
Justin P. Stachnik ◽  
Courtney Schumacher ◽  
Paul E. Ciesielski
Keyword(s):  
Large Scale ◽  
Marine Boundary Layer ◽  
Deep Convection ◽  
Future Research ◽  
Low Level ◽  
Convective Systems ◽  
Stratocumulus Clouds ◽  
Tropospheric Heating ◽  
Heating Profiles ◽  
Cloud Regimes

Abstract Composite profiles of the apparent heat source Q1 and moisture sink Q2 are calculated for the International Satellite Cloud Climatology Project (ISCCP) cloud regimes (or “weather states”) using sounding observations from 10 field campaigns comprising both tropical and subtropical domains. Distinct heating profiles were determined for each ISCCP cloud regime, ranging from strong, upper-tropospheric heating for mesoscale convective systems (WS1) to integrated cooling for populations typically associated with marine stratus and stratocumulus clouds (WS5, WS6, and WS7). Despite being primarily associated with thin cirrus, the corresponding regime (WS4) has heating maxima in the lower and midtroposphere due to the presence of underlying clouds. Regime-averaged Q2 profiles showed similar transitions with strong drying observed for deep convection and low-level moistening for marine boundary layer clouds. The derived profiles were generally similar over land and ocean with the notable exception of the fair-weather cumulus regime (WS8). Additional midlevel moistening was identified for several weather states over land, suggesting enhanced detrainment and more frequent congestus clouds compared to oceanic domains. A control simulation using the Community Atmosphere Model, version 4 (CAM4), was similar to the large-scale patterns of diabatic heating at low levels produced by the ISCCP composites. Differences were more pronounced at middle and upper levels and are largely attributed to the uncertainty in the heating profiles for the cumulus regime (WS8). Low-level heating anomalies were calculated for each phase of the Madden–Julian oscillation (MJO) and they precede upper-tropospheric heating from deep convection by 3–4 phases. Implications for future research using ISCCP heating reconstructions are also discussed.

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

2009 ◽  
Vol 9 (6) ◽  
pp. 25275-25321 ◽  
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.

scholarly journals Cluster analysis of midlatitude oceanic cloud regimes – Part 1: Mean cloud and meteorological properties

2010 ◽  
Vol 10 (1) ◽  
pp. 1559-1593 ◽  
Author(s):  
N. D. Gordon ◽  
J. R. Norris
Keyword(s):  
Large Scale ◽  
Clustering Algorithm ◽  
Storm Track ◽  
Lower Troposphere ◽  
Shortwave Radiation ◽  
Tropospheric Temperature ◽  
Free Troposphere ◽  
Upper Troposphere ◽  
Cloud Data ◽  
Cloud Regimes

Abstract. Clouds play an important role in the climate system by reducing the amount of shortwave radiation reaching the surface and the amount of longwave radiation escaping to space. Although dependent on type and location, clouds produce more cooling than warming in the global average. Accurate simulation of clouds in computer models remains elusive, however, pointing to a lack of understanding of the connection between large-scale dynamics and cloud properties. This study uses a k-means clustering algorithm to group 21-years of satellite cloud data over midlatitude oceans into seven clusters and demonstrates that the cloud clusters are associated with distinct large-scale dynamical conditions. Three clusters correspond to low-level cloud regimes with different cloud fraction and cumuliform or stratiform characteristics, but all occur under large-scale descent and a relatively dry free troposphere. The "small cumulus" regime is most prevalent equatorward of 40° in all seasons; the "large cumulus" regime is associated with a relatively cold troposphere and primarily occurs during winter; and the "stratocumulus/stratus" regime occurs under a temperature inversion and relatively warm free troposphere and predominates during summer. Three clusters correspond to vertically extensive cloud regimes with tops in the middle or upper troposphere. They differ according to the strength of large-scale ascent and enhancement of tropospheric temperature and humidity: "deep altostratus" has the smallest forcing, "weak frontal" is in the middle, and "strong frontal" has the largest forcing. The frontal cloud regimes occur most frequently in storm track regions. The final cluster, "cirrus" is associated with a lower troposphere that is dry and an upper troposphere that is moist and experiencing weak ascent and horizontal moist advection. This information builds a foundation for producing an observational estimate of the midlatitude ocean cloud response to warming that is independent of confounding meteorological influences.

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