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Abstract
Latent and sensible heat fluxes over the oceans are believed to play an important role in the genesis and evolution of marine-based extratropical cyclones (ETCs) and affect rapid cyclogenesis. Observations of ocean surface heat fluxes are limited from existing in situ and remote sensing platforms, which may not offer sufficient spatial and temporal resolution. In addition, substantial precipitation frequently veils the ocean surface around ETCs, limiting the capacity of spaceborne instruments to observe the surface processes within maturing ETCs. Although designed as a tropics-focused mission, the Cyclone Global Navigation Satellite System (CYGNSS) can observe ocean surface wind speed and heat fluxes within a notable quantity of low-latitude extratropical fronts and cyclones. These observations can assist in understanding how surface processes may play a role in cyclogenesis and evolution. This paper illustrates CYGNSS’s capability to observe extratropical cyclones manifesting in various ocean basins throughout the globe and shows that the observations provide a robust sample of ETCs winds and surface fluxes, as compared with a reanalysis dataset.
Abstract
Latent and sensible heat fluxes over the oceans are believed to play an important role in the genesis and evolution of marine-based extratropical cyclones (ETCs) and affect rapid cyclogenesis. Observations of ocean surface heat fluxes are limited from existing in situ and remote sensing platforms, which may not offer sufficient spatial and temporal resolution. In addition, substantial precipitation frequently veils the ocean surface around ETCs, limiting the capacity of spaceborne instruments to observe the surface processes within maturing ETCs. Although designed as a tropics-focused mission, the Cyclone Global Navigation Satellite System (CYGNSS) can observe ocean surface wind speed and heat fluxes within a notable quantity of low-latitude extratropical fronts and cyclones. These observations can assist in understanding how surface processes may play a role in cyclogenesis and evolution. This paper illustrates CYGNSS’s capability to observe extratropical cyclones manifesting in various ocean basins throughout the globe and shows that the observations provide a robust sample of ETCs winds and surface fluxes, as compared with a reanalysis dataset.
Abstract
Surface latent and sensible heat fluxes are important for extratropical cyclone evolution and intensification. Because extratropical cyclone genesis often occurs at low latitudes, Cyclone Global Navigation Satellite System (CYGNSS) surface latent and sensible heat flux retrievals are composited to provide a mean picture of their spatial distribution in low-latitude oceanic extratropical cyclones. CYGNSS heat fluxes are not affected by heavy precipitation and offer observations of storms with frequent revisit times. Consistent with prior results obtained for cyclones in the Gulf Stream region, the fluxes are strongest in the wake of the cold fronts and are weakest to negative in the warm sector in advance of the cold fronts. As cyclone strength increases or mean precipitable water decreases, the maximum in surface heat fluxes increases while the minimum decreases. This affects the changes in fluxes during cyclone intensification: the post-cold-frontal surface heat flux maximum increases as a result of the increase in near-surface winds. During cyclone dissipation, the fluxes in this sector decrease because of the decrease in winds and in temperature and humidity contrast. The warm-sector minimum decreases throughout the entire cyclone lifetime and is mostly driven by sea–air temperature and humidity contrast changes. However, during cyclone dissipation, the surface heat fluxes increase along the cold front in a narrow band to the east, independent from changes in the cyclone characteristics. This result suggests that, during cyclone dissipation, energy transfers from the ocean to the atmosphere are linked to frontal processes in addition to synoptic-scale processes.
Abstract
Surface latent and sensible heat fluxes are important for extratropical cyclone evolution and intensification. Because extratropical cyclone genesis often occurs at low latitudes, Cyclone Global Navigation Satellite System (CYGNSS) surface latent and sensible heat flux retrievals are composited to provide a mean picture of their spatial distribution in low-latitude oceanic extratropical cyclones. CYGNSS heat fluxes are not affected by heavy precipitation and offer observations of storms with frequent revisit times. Consistent with prior results obtained for cyclones in the Gulf Stream region, the fluxes are strongest in the wake of the cold fronts and are weakest to negative in the warm sector in advance of the cold fronts. As cyclone strength increases or mean precipitable water decreases, the maximum in surface heat fluxes increases while the minimum decreases. This affects the changes in fluxes during cyclone intensification: the post-cold-frontal surface heat flux maximum increases as a result of the increase in near-surface winds. During cyclone dissipation, the fluxes in this sector decrease because of the decrease in winds and in temperature and humidity contrast. The warm-sector minimum decreases throughout the entire cyclone lifetime and is mostly driven by sea–air temperature and humidity contrast changes. However, during cyclone dissipation, the surface heat fluxes increase along the cold front in a narrow band to the east, independent from changes in the cyclone characteristics. This result suggests that, during cyclone dissipation, energy transfers from the ocean to the atmosphere are linked to frontal processes in addition to synoptic-scale processes.
Abstract
The present study explores the ability of the Weather Research and Forecasting (WRF) Model to accurately reproduce the passage of extratropical cold fronts at the DOE ARM eastern North Atlantic (ENA) observation site on the Azores. An analysis of three case studies is performed in which the impact of the WRF domain size, position of the model boundary relative to the ENA site, grid spacing, and spectral nudging conditions are explored. The results from these case studies indicate that model biases in the timing and duration of cold front passages change with the distance between the model domain boundary and the ENA site. For these three cases, if the western model boundary is farther than 1500 km from the site, the front becomes too meridional and fails to reach the site, making 1000 or 1500 km the optimal distances. In contrast, integrations with small distances (e.g., 500 km) between the site and domain boundaries have inadequate spatial spinup (i.e., the domain is too small for the model to properly stabilize). For all three cases, regardless of domain size, the model has biases in its upper-level circulation that impact the position and timing of the front. However, this issue is most serious for 4000-km2 domains and larger. For these domains, prolonged spectral nudging can correct cold front biases. As such, this analysis provides a framework to optimize the WRF Model configuration necessary for a realistic hindcast of a cold front passage at a fixed location centered in a domain as large as computationally possible.
Abstract
The present study explores the ability of the Weather Research and Forecasting (WRF) Model to accurately reproduce the passage of extratropical cold fronts at the DOE ARM eastern North Atlantic (ENA) observation site on the Azores. An analysis of three case studies is performed in which the impact of the WRF domain size, position of the model boundary relative to the ENA site, grid spacing, and spectral nudging conditions are explored. The results from these case studies indicate that model biases in the timing and duration of cold front passages change with the distance between the model domain boundary and the ENA site. For these three cases, if the western model boundary is farther than 1500 km from the site, the front becomes too meridional and fails to reach the site, making 1000 or 1500 km the optimal distances. In contrast, integrations with small distances (e.g., 500 km) between the site and domain boundaries have inadequate spatial spinup (i.e., the domain is too small for the model to properly stabilize). For all three cases, regardless of domain size, the model has biases in its upper-level circulation that impact the position and timing of the front. However, this issue is most serious for 4000-km2 domains and larger. For these domains, prolonged spectral nudging can correct cold front biases. As such, this analysis provides a framework to optimize the WRF Model configuration necessary for a realistic hindcast of a cold front passage at a fixed location centered in a domain as large as computationally possible.
Abstract
Extratropical cyclones (ETCs) produce most of the winter precipitation at midlatitudes and are often associated with the most extreme winter weather events. For climate models to accurately predict the occurrence and severity of these extreme events in a changing climate, they need to accurately represent moist processes in general and ice processes in particular. To provide an observational constraint for model evaluation, because cloud cover and precipitation are prevalent in warm-frontal regions, a compositing method is applied to ice retrievals from satellite observations to explore the ice distribution across warm fronts in both hemispheres. Ice water path (IWP) and its variability are compared between Northern Hemisphere (NH) and Southern Hemisphere (SH) warm fronts for different ETC-wide characteristics, as well as for different ETC origination regions. Results reveal that warm-frontal IWP and its variability tend to be higher in the NH than the SH, even when controlling for the ETC strength and environmental precipitable water (PW). IWP differences between NH and SH are found to be primarily related to where the cyclones originate. As the intertropical convergence zone is shifted north, ETCs that originate close to the northern tropics have more PW than those that originate close to the southern tropics. This, in turn, seems to lead to larger IWP in NH frontal clouds than in the SH frontal clouds at a later time. This highlights the importance, for ice amounts generated in warm-frontal regions, of the environmental conditions that an ETC encounters during its genesis phase.
Significance Statement
Extratropical cyclones (ETCs) are responsible for most of the winter precipitation in the midlatitudes and are often associated with severe winter weather events. In order for climate models to accurately predict these extreme events in a changing climate, they need to correctly represent moist processes, especially those involving ice. To evaluate and improve these models, we apply a compositing method to satellite observations of ice profiles in warm-frontal regions, which are known for having high cloud cover and precipitation. This helps us understand the distribution of ice across warm fronts in both the Northern Hemisphere (NH) and the Southern Hemisphere (SH). We compare the ice water path (IWP) and its variability between NH and SH warm fronts, considering different characteristics of ETCs and their formation regions. Our findings show that NH warm fronts generally contain more ice, and the amount varies a lot more across warm fronts than for SH warm fronts. This is true even when accounting for the strength of the cyclones and the moisture available to them. These differences in IWP between NH and SH are found to be primarily related to the locations where the cyclones originate. As the intertropical convergence zone (ITCZ) is shifted northward, ETCs originating closer to the northern tropics tend to have more moisture available to them than those originating closer to the southern tropics. This leads to greater ice amounts in NH frontal clouds compared to SH frontal clouds at a later time. These results emphasize the importance of understanding the origin of ETCs in order to accurately characterize ice processes in warm-frontal regions.
Abstract
Extratropical cyclones (ETCs) produce most of the winter precipitation at midlatitudes and are often associated with the most extreme winter weather events. For climate models to accurately predict the occurrence and severity of these extreme events in a changing climate, they need to accurately represent moist processes in general and ice processes in particular. To provide an observational constraint for model evaluation, because cloud cover and precipitation are prevalent in warm-frontal regions, a compositing method is applied to ice retrievals from satellite observations to explore the ice distribution across warm fronts in both hemispheres. Ice water path (IWP) and its variability are compared between Northern Hemisphere (NH) and Southern Hemisphere (SH) warm fronts for different ETC-wide characteristics, as well as for different ETC origination regions. Results reveal that warm-frontal IWP and its variability tend to be higher in the NH than the SH, even when controlling for the ETC strength and environmental precipitable water (PW). IWP differences between NH and SH are found to be primarily related to where the cyclones originate. As the intertropical convergence zone is shifted north, ETCs that originate close to the northern tropics have more PW than those that originate close to the southern tropics. This, in turn, seems to lead to larger IWP in NH frontal clouds than in the SH frontal clouds at a later time. This highlights the importance, for ice amounts generated in warm-frontal regions, of the environmental conditions that an ETC encounters during its genesis phase.
Significance Statement
Extratropical cyclones (ETCs) are responsible for most of the winter precipitation in the midlatitudes and are often associated with severe winter weather events. In order for climate models to accurately predict these extreme events in a changing climate, they need to correctly represent moist processes, especially those involving ice. To evaluate and improve these models, we apply a compositing method to satellite observations of ice profiles in warm-frontal regions, which are known for having high cloud cover and precipitation. This helps us understand the distribution of ice across warm fronts in both the Northern Hemisphere (NH) and the Southern Hemisphere (SH). We compare the ice water path (IWP) and its variability between NH and SH warm fronts, considering different characteristics of ETCs and their formation regions. Our findings show that NH warm fronts generally contain more ice, and the amount varies a lot more across warm fronts than for SH warm fronts. This is true even when accounting for the strength of the cyclones and the moisture available to them. These differences in IWP between NH and SH are found to be primarily related to the locations where the cyclones originate. As the intertropical convergence zone (ITCZ) is shifted northward, ETCs originating closer to the northern tropics tend to have more moisture available to them than those originating closer to the southern tropics. This leads to greater ice amounts in NH frontal clouds compared to SH frontal clouds at a later time. These results emphasize the importance of understanding the origin of ETCs in order to accurately characterize ice processes in warm-frontal regions.
Abstract
Precipitation (from TMPA) and cloud structures (from MODIS) in extratropical cyclones (ETCs) are modulated by phases of large-scale moisture flux convergence (from MERRA-2) in the sectors of ETCs, which are studied in a new coordinate system with directions of both surface warm fronts (WFs) and surface cold fronts (CFs) fixed. The phase of moisture flux convergence is described by moisture dynamical convergence Q cnvg and moisture advection Q advt. Precipitation and occurrence frequencies of deep convective clouds are sensitive to changes in Q cnvg, while moisture tendency is sensitive to changes in Q advt. Increasing Q cnvg and Q advt during the advance of the WF is associated with increasing occurrences of both deep convective and high-level stratiform clouds. A rapid decrease in Q advt with a relatively steady Q cnvg during the advance of the CF is associated with high-level cloud distribution weighting toward deep convective clouds. Behind the CF (cold sector or area with polar air intrusion), the moisture flux is divergent with abundant low- and midlevel clouds. From deepening to decaying stages, the pre-WF and WF sectors experience high-level clouds shifting to more convective and less stratiform because of decreasing Q advt with relatively steady Q cnvg, and the CF experiences shifting from high-level to midlevel clouds. Sectors of moisture flux divergence are less influenced by cyclone evolution. Surface evaporation is the largest in the cold sector and the CF during the deepening stage. Deepening cyclones are more efficient in poleward transport of water vapor.
Abstract
Precipitation (from TMPA) and cloud structures (from MODIS) in extratropical cyclones (ETCs) are modulated by phases of large-scale moisture flux convergence (from MERRA-2) in the sectors of ETCs, which are studied in a new coordinate system with directions of both surface warm fronts (WFs) and surface cold fronts (CFs) fixed. The phase of moisture flux convergence is described by moisture dynamical convergence Q cnvg and moisture advection Q advt. Precipitation and occurrence frequencies of deep convective clouds are sensitive to changes in Q cnvg, while moisture tendency is sensitive to changes in Q advt. Increasing Q cnvg and Q advt during the advance of the WF is associated with increasing occurrences of both deep convective and high-level stratiform clouds. A rapid decrease in Q advt with a relatively steady Q cnvg during the advance of the CF is associated with high-level cloud distribution weighting toward deep convective clouds. Behind the CF (cold sector or area with polar air intrusion), the moisture flux is divergent with abundant low- and midlevel clouds. From deepening to decaying stages, the pre-WF and WF sectors experience high-level clouds shifting to more convective and less stratiform because of decreasing Q advt with relatively steady Q cnvg, and the CF experiences shifting from high-level to midlevel clouds. Sectors of moisture flux divergence are less influenced by cyclone evolution. Surface evaporation is the largest in the cold sector and the CF during the deepening stage. Deepening cyclones are more efficient in poleward transport of water vapor.
Abstract
Extratropical cyclones are the primary cause of midlatitude winds, precipitation, and clouds. Surface latent and sensible heat fluxes from the ocean impact cyclone intensity, but their role in moisture transport leading to cloud and precipitation is still under investigation. While numerical simulations can help establish such links, evaluating and constraining these simulations require observations. To this end, satellite-based observations of cloud and precipitation in low-latitude extratropical cyclones are examined for four distinct classifications based on the Cyclone Global Navigation Satellite System (CYGNSS) latent and sensible heat fluxes, averaged in the post-cold-frontal or warm-sector areas of the cyclones. Using a cyclone compositing approach, contrasts in cloud and precipitation in strong- versus weak-surface-flux conditions are examined. In the post-cold-frontal region, stronger latent or sensible heat fluxes are associated with lower precipitation rates and higher cloud opacity, indicating more vigorous shallow convection. However, larger sensible heat flux cases display larger cloud fraction, while larger latent heat flux cases exhibit lower cloud fraction, which could indicate differing cloud morphologies. In the comma region of the cyclones, clouds and precipitation depend on both cyclone strength and moisture availability. Consistent with this, larger cloud amount and precipitation are found for strong fluxes in the post-cold-frontal region, and weak or negative sensible heat fluxes, indicative of poleward warm advection, in the warm sector. The strong regional differences in the surface heat flux–cloud and precipitation relationships highlight the need for further investigation into moisture supply and transport in cyclones, while providing guidance for future work.
Abstract
Extratropical cyclones are the primary cause of midlatitude winds, precipitation, and clouds. Surface latent and sensible heat fluxes from the ocean impact cyclone intensity, but their role in moisture transport leading to cloud and precipitation is still under investigation. While numerical simulations can help establish such links, evaluating and constraining these simulations require observations. To this end, satellite-based observations of cloud and precipitation in low-latitude extratropical cyclones are examined for four distinct classifications based on the Cyclone Global Navigation Satellite System (CYGNSS) latent and sensible heat fluxes, averaged in the post-cold-frontal or warm-sector areas of the cyclones. Using a cyclone compositing approach, contrasts in cloud and precipitation in strong- versus weak-surface-flux conditions are examined. In the post-cold-frontal region, stronger latent or sensible heat fluxes are associated with lower precipitation rates and higher cloud opacity, indicating more vigorous shallow convection. However, larger sensible heat flux cases display larger cloud fraction, while larger latent heat flux cases exhibit lower cloud fraction, which could indicate differing cloud morphologies. In the comma region of the cyclones, clouds and precipitation depend on both cyclone strength and moisture availability. Consistent with this, larger cloud amount and precipitation are found for strong fluxes in the post-cold-frontal region, and weak or negative sensible heat fluxes, indicative of poleward warm advection, in the warm sector. The strong regional differences in the surface heat flux–cloud and precipitation relationships highlight the need for further investigation into moisture supply and transport in cyclones, while providing guidance for future work.
ABSTRACT
Using a high-spatial- and high-temporal-resolution precipitation dataset, Integrated Multi-satellite Retrievals for GPM (IMERG), extratropical cyclone precipitation is evaluated in two reanalyses and two climate models. Based on cyclone-centered composites, all four models overestimate precipitation in the western subsiding and dry side of the cyclones, and underestimate the precipitation in the eastern ascending and moist side. By decomposing the composites into frequency of occurrence and intensity (mean precipitation rate when precipitating), the analysis reveals a tendency for all four models to overestimate frequency and underestimate intensity, with the former issue dominating in the western half and the latter in the eastern half of the cyclones. Differences in frequency are strongly dependent on cyclone environmental moisture, while the differences in intensity are strongly impacted by the strength of ascent within the cyclone. There are some uncertainties associated with the observations: IMERG might underreport frozen precipitation and possibly exaggerate rates in vigorously ascending regions. Nevertheless, the analysis suggests that all models produce extratropical cyclone precipitation too often and too lightly. These biases have consequences when evaluating the changes in precipitation characteristics with changes in cyclone properties: the models disagree on the magnitude of the change in precipitation intensity with a change in environmental moisture and in precipitation frequency with a change in cyclone strength. This complicates accurate predictions of precipitation changes in a changing climate.
ABSTRACT
Using a high-spatial- and high-temporal-resolution precipitation dataset, Integrated Multi-satellite Retrievals for GPM (IMERG), extratropical cyclone precipitation is evaluated in two reanalyses and two climate models. Based on cyclone-centered composites, all four models overestimate precipitation in the western subsiding and dry side of the cyclones, and underestimate the precipitation in the eastern ascending and moist side. By decomposing the composites into frequency of occurrence and intensity (mean precipitation rate when precipitating), the analysis reveals a tendency for all four models to overestimate frequency and underestimate intensity, with the former issue dominating in the western half and the latter in the eastern half of the cyclones. Differences in frequency are strongly dependent on cyclone environmental moisture, while the differences in intensity are strongly impacted by the strength of ascent within the cyclone. There are some uncertainties associated with the observations: IMERG might underreport frozen precipitation and possibly exaggerate rates in vigorously ascending regions. Nevertheless, the analysis suggests that all models produce extratropical cyclone precipitation too often and too lightly. These biases have consequences when evaluating the changes in precipitation characteristics with changes in cyclone properties: the models disagree on the magnitude of the change in precipitation intensity with a change in environmental moisture and in precipitation frequency with a change in cyclone strength. This complicates accurate predictions of precipitation changes in a changing climate.
Abstract
The Cyclone Global Navigation Satellite System (CYGNSS) mission, launched in December 2016, is designed to estimate surface wind speeds over the global tropical oceans. Nevertheless, its orbit allows the constellation to view regions up to 40° latitude. As such, it is possible that CYGNSS will provide observations of a number of low-latitude extratropical cyclones and their associated fronts. In this study, one year of simulated CYGNSS specular point locations is combined with a database of objectively identified fronts and cyclones to assess the potential efficacy of CYGNSS for observing extratropical systems. It is found that, with the exception of regions poleward of warm fronts, the subset of locations in the simulated CYGNSS dataset nearly exactly matches the distribution of wind speeds and surface fluxes across frontal zones and near cyclone centers in the reanalysis database.
Abstract
The Cyclone Global Navigation Satellite System (CYGNSS) mission, launched in December 2016, is designed to estimate surface wind speeds over the global tropical oceans. Nevertheless, its orbit allows the constellation to view regions up to 40° latitude. As such, it is possible that CYGNSS will provide observations of a number of low-latitude extratropical cyclones and their associated fronts. In this study, one year of simulated CYGNSS specular point locations is combined with a database of objectively identified fronts and cyclones to assess the potential efficacy of CYGNSS for observing extratropical systems. It is found that, with the exception of regions poleward of warm fronts, the subset of locations in the simulated CYGNSS dataset nearly exactly matches the distribution of wind speeds and surface fluxes across frontal zones and near cyclone centers in the reanalysis database.
Abstract
An extratropical cyclone that crossed the United States on 9–11 April 2009 was successfully simulated at high resolution (3-km horizontal grid spacing) using the Colorado State University Regional Atmospheric Modeling System. The sensitivity of the associated warm front to increasing pollution levels was then explored by conducting the same experiment with three different background profiles of cloud-nucleating aerosol concentration. To the authors’ knowledge, no study has examined the indirect effects of aerosols on warm fronts. The budgets of ice, cloud water, and rain in the simulation with the lowest aerosol concentrations were examined. The ice mass was found to be produced in equal amounts through vapor deposition and riming, and the melting of ice produced approximately 75% of the total rain. Conversion of cloud water to rain accounted for the other 25%. When cloud-nucleating aerosol concentrations were increased, significant changes were seen in the budget terms, but total precipitation remained relatively constant. Vapor deposition onto ice increased, but riming of cloud water decreased such that there was only a small change in the total ice production and hence there was no significant change in melting. These responses can be understood in terms of a buffering effect in which smaller cloud droplets in the mixed-phase region lead to both an enhanced vapor deposition and decreased riming efficiency with increasing aerosol concentrations. Overall, while large changes were seen in the microphysical structure of the frontal cloud, cloud-nucleating aerosols had little impact on the precipitation production of the warm front.
Abstract
An extratropical cyclone that crossed the United States on 9–11 April 2009 was successfully simulated at high resolution (3-km horizontal grid spacing) using the Colorado State University Regional Atmospheric Modeling System. The sensitivity of the associated warm front to increasing pollution levels was then explored by conducting the same experiment with three different background profiles of cloud-nucleating aerosol concentration. To the authors’ knowledge, no study has examined the indirect effects of aerosols on warm fronts. The budgets of ice, cloud water, and rain in the simulation with the lowest aerosol concentrations were examined. The ice mass was found to be produced in equal amounts through vapor deposition and riming, and the melting of ice produced approximately 75% of the total rain. Conversion of cloud water to rain accounted for the other 25%. When cloud-nucleating aerosol concentrations were increased, significant changes were seen in the budget terms, but total precipitation remained relatively constant. Vapor deposition onto ice increased, but riming of cloud water decreased such that there was only a small change in the total ice production and hence there was no significant change in melting. These responses can be understood in terms of a buffering effect in which smaller cloud droplets in the mixed-phase region lead to both an enhanced vapor deposition and decreased riming efficiency with increasing aerosol concentrations. Overall, while large changes were seen in the microphysical structure of the frontal cloud, cloud-nucleating aerosols had little impact on the precipitation production of the warm front.
Abstract
The observation and representation in general circulation models (GCMs) of cloud vertical overlap are the objects of active research due to their impacts on the earth’s radiative budget. Previous studies have found that vertically contiguous cloudy layers show a maximum overlap between layers up to several kilometers apart but tend toward a random overlap as separations increase. The decorrelation length scale that characterizes the progressive transition from maximum to random overlap changes from one location and season to another and thus may be influenced by large-scale vertical motion, wind shear, or convection. Observations from the U.S. Department of Energy Atmospheric Radiation Measurement program ground-based radars and lidars in midlatitude and tropical locations in combination with reanalysis meteorological fields are used to evaluate how dynamics and atmospheric state influence cloud overlap. For midlatitude winter months, strong synoptic-scale upward motion maintains conditions closer to maximum overlap at large separations. In the tropics, overlap becomes closer to maximum as convective stability decreases. In midlatitude subsidence and tropical convectively stable situations, where a smooth transition from maximum to random overlap is found on average, large wind shears sometimes favor minimum overlap. Precipitation periods are discarded from the analysis but, when included, maximum overlap occurs more often at large separations. The results suggest that a straightforward modification of the existing GCM mixed maximum–random overlap parameterization approach that accounts for environmental conditions can capture much of the important variability and is more realistic than approaches that are only based on an exponential decay transition from maximum to random overlap.
Abstract
The observation and representation in general circulation models (GCMs) of cloud vertical overlap are the objects of active research due to their impacts on the earth’s radiative budget. Previous studies have found that vertically contiguous cloudy layers show a maximum overlap between layers up to several kilometers apart but tend toward a random overlap as separations increase. The decorrelation length scale that characterizes the progressive transition from maximum to random overlap changes from one location and season to another and thus may be influenced by large-scale vertical motion, wind shear, or convection. Observations from the U.S. Department of Energy Atmospheric Radiation Measurement program ground-based radars and lidars in midlatitude and tropical locations in combination with reanalysis meteorological fields are used to evaluate how dynamics and atmospheric state influence cloud overlap. For midlatitude winter months, strong synoptic-scale upward motion maintains conditions closer to maximum overlap at large separations. In the tropics, overlap becomes closer to maximum as convective stability decreases. In midlatitude subsidence and tropical convectively stable situations, where a smooth transition from maximum to random overlap is found on average, large wind shears sometimes favor minimum overlap. Precipitation periods are discarded from the analysis but, when included, maximum overlap occurs more often at large separations. The results suggest that a straightforward modification of the existing GCM mixed maximum–random overlap parameterization approach that accounts for environmental conditions can capture much of the important variability and is more realistic than approaches that are only based on an exponential decay transition from maximum to random overlap.
