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Abstract
Synoptic variability of low-cloud properties, temperature advection, and thermodynamic soundings of the trade wind boundary layer are analyzed, using the long data record from ocean weather station November (30°N, 140°W). The variations in low-cloud amount at this subtropical site are most strongly correlated with variations in temperature advection, the stability of the lower troposphere, and the relative humidity of the cloud layer. No single predictor is capable of explaining more than 13% of the variance in low-cloud amount. However, the amount of variance explained increases considerably when the data are averaged over several days. Four parameterizations for the amount of stratiform cloud under a subsidence inversion are tested against the observed amount of low clouds. The four parameterizations are based upon relative humidity, the inversion strength, a mixing line slope, and the amount of condensed water. All parameterizations are positively correlated with the observed cloud amounts, although the variance explained is less than 16%.
Abstract
Synoptic variability of low-cloud properties, temperature advection, and thermodynamic soundings of the trade wind boundary layer are analyzed, using the long data record from ocean weather station November (30°N, 140°W). The variations in low-cloud amount at this subtropical site are most strongly correlated with variations in temperature advection, the stability of the lower troposphere, and the relative humidity of the cloud layer. No single predictor is capable of explaining more than 13% of the variance in low-cloud amount. However, the amount of variance explained increases considerably when the data are averaged over several days. Four parameterizations for the amount of stratiform cloud under a subsidence inversion are tested against the observed amount of low clouds. The four parameterizations are based upon relative humidity, the inversion strength, a mixing line slope, and the amount of condensed water. All parameterizations are positively correlated with the observed cloud amounts, although the variance explained is less than 16%.
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Abstract
Summertime observations for 11 yr from the Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains (SGP) site are used to investigate mechanisms controlling the transition from shallow to deep convection over land. It is found that a more humid environment immediately above the boundary layer is present before the start of late afternoon heavy precipitation events. The higher moisture content is brought by wind from the south. Greater boundary layer inhomogeneity in moist static energy, temperature, moisture, and horizontal wind before precipitation begins is correlated to larger rain rates at the initial stage of precipitation. In an examination of afternoon rain statistics, higher relative humidity above the boundary layer is correlated to an earlier onset and longer duration of afternoon precipitation events, whereas greater boundary layer inhomogeneity and atmospheric instability in the 2–4-km layer above the surface are positively correlated to the total rain amount and the maximum rain rate. Although other interpretations may be possible, these observations are consistent with theories for the transition from shallow to deep convection that emphasize the role of a moist lower free troposphere and boundary layer inhomogeneity.
Abstract
Summertime observations for 11 yr from the Atmospheric Radiation Measurement (ARM) Climate Research Facility Southern Great Plains (SGP) site are used to investigate mechanisms controlling the transition from shallow to deep convection over land. It is found that a more humid environment immediately above the boundary layer is present before the start of late afternoon heavy precipitation events. The higher moisture content is brought by wind from the south. Greater boundary layer inhomogeneity in moist static energy, temperature, moisture, and horizontal wind before precipitation begins is correlated to larger rain rates at the initial stage of precipitation. In an examination of afternoon rain statistics, higher relative humidity above the boundary layer is correlated to an earlier onset and longer duration of afternoon precipitation events, whereas greater boundary layer inhomogeneity and atmospheric instability in the 2–4-km layer above the surface are positively correlated to the total rain amount and the maximum rain rate. Although other interpretations may be possible, these observations are consistent with theories for the transition from shallow to deep convection that emphasize the role of a moist lower free troposphere and boundary layer inhomogeneity.
Abstract
Summertime observations for 13 yr at the Atmospheric Radiation Measurement Southern Great Plains site are used to study fair-weather shallow cumuli (ShCu). To roughly separate forced from active ShCu, days are categorized into “thin-” or “thick-” ShCu days according to whether the daytime-average cloud depth exceeds 300 m. By comparing diurnal-cycle composites of these two regimes, the authors document differences in cloud properties and their radiative impacts. The differences in environmental conditions provide clues as to what controls ShCu vertical extent.
Higher boundary layer (BL) relative humidity (RH) is found on thick-cloud days, associated with large-scale moisture advection before sunrise. This higher BL RH not only contributes to a lower cloud base but also to the penetrating ability of an air parcel to reach higher levels, and thus leads to larger cloud vertical extent.
Although not as significant as BL RH, ShCu vertical extent also varies with thermal stability and surface fluxes. Enhanced stability above cloud on thin-cloud days may limit cloud vertical extent. A larger sensible heat flux on thin-cloud days encourages greater entrainment of dry air into the BL, whereas a larger latent heat flux on thick-cloud days helps sustain higher afternoon BL RH. These heat flux differences help maintain the BL RH differences that appear to control cloud vertical extent.
This study provides observational evidence that forced clouds are related to BL large-eddy overshoots limited by a stronger inversion whereas higher moisture and a weaker stability above favor active cumuli with greater vertical extent.
Abstract
Summertime observations for 13 yr at the Atmospheric Radiation Measurement Southern Great Plains site are used to study fair-weather shallow cumuli (ShCu). To roughly separate forced from active ShCu, days are categorized into “thin-” or “thick-” ShCu days according to whether the daytime-average cloud depth exceeds 300 m. By comparing diurnal-cycle composites of these two regimes, the authors document differences in cloud properties and their radiative impacts. The differences in environmental conditions provide clues as to what controls ShCu vertical extent.
Higher boundary layer (BL) relative humidity (RH) is found on thick-cloud days, associated with large-scale moisture advection before sunrise. This higher BL RH not only contributes to a lower cloud base but also to the penetrating ability of an air parcel to reach higher levels, and thus leads to larger cloud vertical extent.
Although not as significant as BL RH, ShCu vertical extent also varies with thermal stability and surface fluxes. Enhanced stability above cloud on thin-cloud days may limit cloud vertical extent. A larger sensible heat flux on thin-cloud days encourages greater entrainment of dry air into the BL, whereas a larger latent heat flux on thick-cloud days helps sustain higher afternoon BL RH. These heat flux differences help maintain the BL RH differences that appear to control cloud vertical extent.
This study provides observational evidence that forced clouds are related to BL large-eddy overshoots limited by a stronger inversion whereas higher moisture and a weaker stability above favor active cumuli with greater vertical extent.
Abstract
Clouds simulated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model are composited to derive the typical organization of clouds surrounding a midlatitude baroclinic system. Comparison of this composite of about 200 cyclones with that based on satellite data reveals that the ECMWF model quite accurately simulates the general positioning of clouds relative to a low pressure center. However, the optical depths of the model’s high/low clouds are too small/large relative to the satellite observations, and the model lacks the midlevel topped clouds observed to the west of the surface cold front.
Sensitivity studies with the ECMWF model reveal that the error in high-cloud optical depths is more sensitive to the assumptions applied to the ice microphysics than to the inclusion of cloud advection or a change of horizontal resolution from 0.5625° to 1.69° lat. This reflects the fact that in the ECMWF model gravitational settling is the most rapid process controlling the abundance of ice in the high clouds of midlatitude cyclones. These results underscore the need for careful evaluation of the parameterizations of microphysics and radiative properties applied to ice in large-scale models.
Abstract
Clouds simulated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model are composited to derive the typical organization of clouds surrounding a midlatitude baroclinic system. Comparison of this composite of about 200 cyclones with that based on satellite data reveals that the ECMWF model quite accurately simulates the general positioning of clouds relative to a low pressure center. However, the optical depths of the model’s high/low clouds are too small/large relative to the satellite observations, and the model lacks the midlevel topped clouds observed to the west of the surface cold front.
Sensitivity studies with the ECMWF model reveal that the error in high-cloud optical depths is more sensitive to the assumptions applied to the ice microphysics than to the inclusion of cloud advection or a change of horizontal resolution from 0.5625° to 1.69° lat. This reflects the fact that in the ECMWF model gravitational settling is the most rapid process controlling the abundance of ice in the high clouds of midlatitude cyclones. These results underscore the need for careful evaluation of the parameterizations of microphysics and radiative properties applied to ice in large-scale models.
Abstract
A new method for representing subgrid-scale cloud structure in which each model column is decomposed into a set of subcolumns has been introduced into the Geophysical Fluid Dynamics Laboratory’s global atmospheric model AM2. Each subcolumn in the decomposition is homogeneous, but the ensemble reproduces the initial profiles of cloud properties including cloud fraction, internal variability (if any) in cloud condensate, and arbitrary overlap assumptions that describe vertical correlations. These subcolumns are used in radiation and diagnostic calculations and have allowed the introduction of more realistic overlap assumptions. This paper describes the impact of these new methods for representing cloud structure in instantaneous calculations and long-term integrations. Shortwave radiation computed using subcolumns and the random overlap assumption differs in the global annual average by more than 4 W m−2 from the operational radiation scheme in instantaneous calculations; much of this difference is counteracted by a change in the overlap assumption to one in which overlap varies continuously with the separation distance between layers. Internal variability in cloud condensate, diagnosed from the mean condensate amount and cloud fraction, has about the same effect on radiative fluxes as does the ad hoc tuning accounting for this effect in the operational radiation scheme. Long simulations with the new model configuration show little difference from the operational model configuration, while statistical tests indicate that the model does not respond systematically to the sampling noise introduced by the approximate radiative transfer techniques introduced to work with the subcolumns.
Abstract
A new method for representing subgrid-scale cloud structure in which each model column is decomposed into a set of subcolumns has been introduced into the Geophysical Fluid Dynamics Laboratory’s global atmospheric model AM2. Each subcolumn in the decomposition is homogeneous, but the ensemble reproduces the initial profiles of cloud properties including cloud fraction, internal variability (if any) in cloud condensate, and arbitrary overlap assumptions that describe vertical correlations. These subcolumns are used in radiation and diagnostic calculations and have allowed the introduction of more realistic overlap assumptions. This paper describes the impact of these new methods for representing cloud structure in instantaneous calculations and long-term integrations. Shortwave radiation computed using subcolumns and the random overlap assumption differs in the global annual average by more than 4 W m−2 from the operational radiation scheme in instantaneous calculations; much of this difference is counteracted by a change in the overlap assumption to one in which overlap varies continuously with the separation distance between layers. Internal variability in cloud condensate, diagnosed from the mean condensate amount and cloud fraction, has about the same effect on radiative fluxes as does the ad hoc tuning accounting for this effect in the operational radiation scheme. Long simulations with the new model configuration show little difference from the operational model configuration, while statistical tests indicate that the model does not respond systematically to the sampling noise introduced by the approximate radiative transfer techniques introduced to work with the subcolumns.
Abstract
Composite large-scale dynamical fields contemporaneous with low cloud types observed at midlatitude Ocean Weather Station (OWS) C and eastern subtropical OWS N are used to establish representative relationships between low cloud type and the synoptic environment. The composites are constructed by averaging meteorological observations of surface wind and sea level pressure from volunteering observing ships (VOS) and analyses of sea level pressure, 1000-mb wind, and 700-mb pressure vertical velocity from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis project on those dates and times of day when a particular low cloud type was reported at the OWS.
VOS and NCEP results for OWS C during summer show that bad-weather stratus occurs with strong convergence and ascent slightly ahead of a surface low center and trough. Cumulus-under-stratocumulus and moderate and large cumulus occur with divergence and subsidence in the cold sector of an extratropical cyclone. Both sky-obscuring fog and no-low-cloud typically occur with southwesterly flow from regions of warmer sea surface temperature and differ primarily according to slight surface convergence and stronger warm advection in the case of sky-obscuring fog or surface divergence and weaker warm advection in the case of no-low-cloud. Fair-weather stratus and ordinary stratocumulus are associated with a mixture of meteorological conditions, but differ with respect to vertical motion in the environment. Fair-weather stratus occurs most commonly in the presence of slight convergence and ascent, while stratocumulus often occurs in the presence of divergence and subsidence.
Surface divergence and estimated subsidence at the top of the boundary layer are calculated from VOS observations. At both OWS C and OWS N during summer and winter these values are large for ordinary stratocumulus, less for cumulus-under-stratocumulus, and least (and sometimes slightly negative) for moderate and large cumulus. Subsidence interpolated from NCEP analyses to the top of the boundary layer does not exhibit such variation, but the discrepancy may be due to deficiencies in the analysis procedure or the boundary layer parameterization of the NCEP model. The VOS results suggest that decreasing divergence and subsidence in addition to increasing sea surface temperature may promote the transition from stratocumulus to trade cumulus observed over low-latitude oceans.
Abstract
Composite large-scale dynamical fields contemporaneous with low cloud types observed at midlatitude Ocean Weather Station (OWS) C and eastern subtropical OWS N are used to establish representative relationships between low cloud type and the synoptic environment. The composites are constructed by averaging meteorological observations of surface wind and sea level pressure from volunteering observing ships (VOS) and analyses of sea level pressure, 1000-mb wind, and 700-mb pressure vertical velocity from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis project on those dates and times of day when a particular low cloud type was reported at the OWS.
VOS and NCEP results for OWS C during summer show that bad-weather stratus occurs with strong convergence and ascent slightly ahead of a surface low center and trough. Cumulus-under-stratocumulus and moderate and large cumulus occur with divergence and subsidence in the cold sector of an extratropical cyclone. Both sky-obscuring fog and no-low-cloud typically occur with southwesterly flow from regions of warmer sea surface temperature and differ primarily according to slight surface convergence and stronger warm advection in the case of sky-obscuring fog or surface divergence and weaker warm advection in the case of no-low-cloud. Fair-weather stratus and ordinary stratocumulus are associated with a mixture of meteorological conditions, but differ with respect to vertical motion in the environment. Fair-weather stratus occurs most commonly in the presence of slight convergence and ascent, while stratocumulus often occurs in the presence of divergence and subsidence.
Surface divergence and estimated subsidence at the top of the boundary layer are calculated from VOS observations. At both OWS C and OWS N during summer and winter these values are large for ordinary stratocumulus, less for cumulus-under-stratocumulus, and least (and sometimes slightly negative) for moderate and large cumulus. Subsidence interpolated from NCEP analyses to the top of the boundary layer does not exhibit such variation, but the discrepancy may be due to deficiencies in the analysis procedure or the boundary layer parameterization of the NCEP model. The VOS results suggest that decreasing divergence and subsidence in addition to increasing sea surface temperature may promote the transition from stratocumulus to trade cumulus observed over low-latitude oceans.
Abstract
The seasonal cycle of low stratiform clouds is studied using data from surface-based cloud climatologies. The impact of low clouds on the radiation budget is illustrated by comparison of data from the Earth Radiation Budget Experiment with the cloud climatologies. Ten regions of active stratocumulus convection are identified. These regions fall into four categories: subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus. With the exception of the Chinese region, all the regions with high amounts of stratus clouds are over the oceans.
In all regions except the Arctic, the season of maximum stratus corresponds to the season of greatest lower-troposphere static stability. Interannual variations in stratus cloud amount also are related to changes in static stability. A linear analysis indicates that a 6% increase in stratus fractional area coverage is associated with each 1°C increase in static stability. Over midlatitude oceans, sky-obscuring fog is a large component of the summertime stratus amount. The amount of fog appears to be related to warm advection across sharp gradients of SST.
Abstract
The seasonal cycle of low stratiform clouds is studied using data from surface-based cloud climatologies. The impact of low clouds on the radiation budget is illustrated by comparison of data from the Earth Radiation Budget Experiment with the cloud climatologies. Ten regions of active stratocumulus convection are identified. These regions fall into four categories: subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus. With the exception of the Chinese region, all the regions with high amounts of stratus clouds are over the oceans.
In all regions except the Arctic, the season of maximum stratus corresponds to the season of greatest lower-troposphere static stability. Interannual variations in stratus cloud amount also are related to changes in static stability. A linear analysis indicates that a 6% increase in stratus fractional area coverage is associated with each 1°C increase in static stability. Over midlatitude oceans, sky-obscuring fog is a large component of the summertime stratus amount. The amount of fog appears to be related to warm advection across sharp gradients of SST.
Abstract
Marine stratiform clouds are important and highly variable contributors to earth's radiation budget over the eastern subtropical oceans and over middle- to high-latitude oceans in summer. Because these clouds influence the radiation budget primarily through their albedo, their diurnal cycle has an important influence on their radiative effectiveness.
The authors have analyzed the diurnal cycle in marine low-cloud fraction inferred from the International Satellite Cloud Climatology Project (ISCCP) dataset, after correcting for overlying clouds by using the assumption of random cloud overlap. The results have been compared with the diurnal cycle of low clouds at fixed ships and ships of opportunity. The diurnal cycles of ISCCP low clouds are in good agreement with surface observations of the diurnal cycle of low stratiform clouds almost everywhere. Analysis of the ISCCP data on a 2.5° × 2.5° grid shows that the largest diurnal range in low-cloud fraction occurs downwind in the mean flow, or westward and equatorward, of the subtropical maxima in low-cloud fraction. This is qualitatively consistent with control of the cloud amount by two competing processes in a partially decoupled cloud-topped planetary boundary layer: heating by solar radiation absorption and advection of moist boundary layer air. A radiative transfer code has been used to show that in eastern subtropical ocean regions, where the diurnal cycle of low clouds is large and the cloud has a small optical thickness, calculations with diurnally averaged cloud fraction overestimate total cloud radiative forcing by up to 3 W m−2 (16%) at the surface and 3 W m−2 (7%) at the top of the atmosphere compared with calculations that account for the diurnal cycle.
Abstract
Marine stratiform clouds are important and highly variable contributors to earth's radiation budget over the eastern subtropical oceans and over middle- to high-latitude oceans in summer. Because these clouds influence the radiation budget primarily through their albedo, their diurnal cycle has an important influence on their radiative effectiveness.
The authors have analyzed the diurnal cycle in marine low-cloud fraction inferred from the International Satellite Cloud Climatology Project (ISCCP) dataset, after correcting for overlying clouds by using the assumption of random cloud overlap. The results have been compared with the diurnal cycle of low clouds at fixed ships and ships of opportunity. The diurnal cycles of ISCCP low clouds are in good agreement with surface observations of the diurnal cycle of low stratiform clouds almost everywhere. Analysis of the ISCCP data on a 2.5° × 2.5° grid shows that the largest diurnal range in low-cloud fraction occurs downwind in the mean flow, or westward and equatorward, of the subtropical maxima in low-cloud fraction. This is qualitatively consistent with control of the cloud amount by two competing processes in a partially decoupled cloud-topped planetary boundary layer: heating by solar radiation absorption and advection of moist boundary layer air. A radiative transfer code has been used to show that in eastern subtropical ocean regions, where the diurnal cycle of low clouds is large and the cloud has a small optical thickness, calculations with diurnally averaged cloud fraction overestimate total cloud radiative forcing by up to 3 W m−2 (16%) at the surface and 3 W m−2 (7%) at the top of the atmosphere compared with calculations that account for the diurnal cycle.
Abstract
The physical mechanisms that affect the tropical sea surface temperature (SST) are investigated using a two-box equilibrium model of the Tropics. One box represents the convecting, warm SST, high humidity region of the Tropics, and the other box represents the subsidence region with low humidity, boundary layer clouds, and cooler SST. The two regions communicate by energy and moisture fluxes that are proportional to the strength of the overturning circulation that couples the two regions. The boundary layer properties in the subsiding region are predicted with a mixing line model. Humidity above the inversion in the subsiding region is predicted from moisture conservation.
The humidity above the inversion in the subsiding region increases rapidly with temperature, but this has less effect on the sensitivity than expected, because the inversion lowers as the humidity above the inversion is increased. Some of the increased greenhouse effect of the free troposphere can be offset by decreased greenhouse effect of the boundary layer. Increasing the area of the warm, convective region increases the SSTs, because of the greenhouse effect of the greater upper-tropospheric water vapor in the convective region. The circulation strength is constrained by radiative cooling in the cold pool. The strength of the circulation decreases with increasing convective area, because the increase in dry static stability overwhelms the increase in cooling rate. Although they have strong individual effects on longwave and shortwave radiation, high clouds in the convective region do not affect the tropical SSTs strongly, because their net radiative forcing at the top of the atmosphere is small. Low clouds in the subsidence region have a strong cooling affect on the tropical SST, because they strongly reduce net radiative heating at the top of the atmosphere. A negative feedback is produced if the low clouds are predicted from the observed relationship between stratus cloud amount and lower-tropospheric stability.
Abstract
The physical mechanisms that affect the tropical sea surface temperature (SST) are investigated using a two-box equilibrium model of the Tropics. One box represents the convecting, warm SST, high humidity region of the Tropics, and the other box represents the subsidence region with low humidity, boundary layer clouds, and cooler SST. The two regions communicate by energy and moisture fluxes that are proportional to the strength of the overturning circulation that couples the two regions. The boundary layer properties in the subsiding region are predicted with a mixing line model. Humidity above the inversion in the subsiding region is predicted from moisture conservation.
The humidity above the inversion in the subsiding region increases rapidly with temperature, but this has less effect on the sensitivity than expected, because the inversion lowers as the humidity above the inversion is increased. Some of the increased greenhouse effect of the free troposphere can be offset by decreased greenhouse effect of the boundary layer. Increasing the area of the warm, convective region increases the SSTs, because of the greenhouse effect of the greater upper-tropospheric water vapor in the convective region. The circulation strength is constrained by radiative cooling in the cold pool. The strength of the circulation decreases with increasing convective area, because the increase in dry static stability overwhelms the increase in cooling rate. Although they have strong individual effects on longwave and shortwave radiation, high clouds in the convective region do not affect the tropical SSTs strongly, because their net radiative forcing at the top of the atmosphere is small. Low clouds in the subsidence region have a strong cooling affect on the tropical SST, because they strongly reduce net radiative heating at the top of the atmosphere. A negative feedback is produced if the low clouds are predicted from the observed relationship between stratus cloud amount and lower-tropospheric stability.
