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- Author or Editor: Guang Jun Zhang x
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Abstract
This study investigates to what extent monthly mean surface meteorological variables can be used to estimate surface turbulent fluxes in the equatorial Pacific. The two-year data from the TOGA TAO moored buoy array are used to compute the monthly mean surface fluxes using both daily mean and monthly mean data as input to the bulk aerodynamic formulas. A unique feature of the dataset is that it covers the western Pacific warm pool, a region of climatologically low surface winds and active convection. The results show that the ocean surface sensible and latent heat fluxes can he estimated using the monthly mean data to a very high accuracy and that the momentum flux is generally underestimated by about 10% when the monthly mean data instead of daily data is used. This study, together with previous studies by other researchers, suggests that the monthly mean data such as those derived from satellite measurements can be used to estimate the surface heat, moisture, and momentum fluxes over the global oceans.
Abstract
This study investigates to what extent monthly mean surface meteorological variables can be used to estimate surface turbulent fluxes in the equatorial Pacific. The two-year data from the TOGA TAO moored buoy array are used to compute the monthly mean surface fluxes using both daily mean and monthly mean data as input to the bulk aerodynamic formulas. A unique feature of the dataset is that it covers the western Pacific warm pool, a region of climatologically low surface winds and active convection. The results show that the ocean surface sensible and latent heat fluxes can he estimated using the monthly mean data to a very high accuracy and that the momentum flux is generally underestimated by about 10% when the monthly mean data instead of daily data is used. This study, together with previous studies by other researchers, suggests that the monthly mean data such as those derived from satellite measurements can be used to estimate the surface heat, moisture, and momentum fluxes over the global oceans.
Abstract
This study further examines the errors in estimating surface evaporation using monthly mean surface atmospheric variables from the moored buoys in the equatorial Pacific. Results from two significantly different bulk aerodynamic formulas are compared. It is shown that for both formulas the errors are within 2–3 W m−2 on average, representing a relative error of less than 3% in the equatorial Pacific. Although small, this error varies systematically across the equatorial Pacific and depends on the bulk formula used. It is also demonstrated that the individual nonlinear contributions associated with the strong dependence of exchange coefficient on surface wind to monthly mean surface evaporation are significant. However, they largely cancel each other, leaving small net effect.
Abstract
This study further examines the errors in estimating surface evaporation using monthly mean surface atmospheric variables from the moored buoys in the equatorial Pacific. Results from two significantly different bulk aerodynamic formulas are compared. It is shown that for both formulas the errors are within 2–3 W m−2 on average, representing a relative error of less than 3% in the equatorial Pacific. Although small, this error varies systematically across the equatorial Pacific and depends on the bulk formula used. It is also demonstrated that the individual nonlinear contributions associated with the strong dependence of exchange coefficient on surface wind to monthly mean surface evaporation are significant. However, they largely cancel each other, leaving small net effect.
Abstract
The effect of cumulus convection on the Asian summer monsoon circulation is investigated, using a general circulation model. Two simulations for the summer months (June, July, and August) are performed, one parameterizing convection using a mass flux scheme and the other without convective parameterization. Thee results show that convection has significant effects on the monsoon circulation and its associated precipitation. In the simulation with the mass flux convective parameterization, precipitation in the western Pacific is decreased, together with a decrease in surface evaporation and wind speed. In the Indian monsoon region it is almost the opposite. Comparison with a simulation using moist convective adjustment to parameterize convection shows that the monsoon circulation and precipitation distribution in the no-convection simulation are very similar to those in the simulation with moist convective adjustment.
The difference in the large-scale circulation with and without convective parameterization is interpreted in terms of convective stabilization of the atmosphere by convection, using dry and moist static energy budgets. It is shown that weakening of the low-level convergence in the western Pacific in the simulation with convection is closely associated with the stabilization of the atmosphere by convection, mostly through drying of the lower troposphere; changes in low-level convergence lead to changes in precipitation. The precipitation increase in the Indian monsoon region can be explained similarly.
Abstract
The effect of cumulus convection on the Asian summer monsoon circulation is investigated, using a general circulation model. Two simulations for the summer months (June, July, and August) are performed, one parameterizing convection using a mass flux scheme and the other without convective parameterization. Thee results show that convection has significant effects on the monsoon circulation and its associated precipitation. In the simulation with the mass flux convective parameterization, precipitation in the western Pacific is decreased, together with a decrease in surface evaporation and wind speed. In the Indian monsoon region it is almost the opposite. Comparison with a simulation using moist convective adjustment to parameterize convection shows that the monsoon circulation and precipitation distribution in the no-convection simulation are very similar to those in the simulation with moist convective adjustment.
The difference in the large-scale circulation with and without convective parameterization is interpreted in terms of convective stabilization of the atmosphere by convection, using dry and moist static energy budgets. It is shown that weakening of the low-level convergence in the western Pacific in the simulation with convection is closely associated with the stabilization of the atmosphere by convection, mostly through drying of the lower troposphere; changes in low-level convergence lead to changes in precipitation. The precipitation increase in the Indian monsoon region can be explained similarly.
Abstract
The upper-air sounding data from PRE-STORM are used to investigate the convective stabilization effect on the large-scale atmosphere. To facilitate comparison between different stages of cumulus convection, the data are divided into four categories: environment, presystem, insystem, and postsystem. It is found that the convective available potential energy of the atmosphere is reduced substantially after cumulus convection, most of which is consumed during the transition from presystem to insystem. Examination of the temperature and moisture changes during cumulus convection suggests that cooling and drying in the subcloud layer are the most important factors in stabilizing the atmosphere. In general, virtual potential temperature profiles in all categories are close to reversible moist adiabats below the 600-mb level and nearly parallel to moist pseudoadiabats above it.
The effect of entrainment on parcel buoyancy is also studied. It is found that a small amount of entrainment of ambient air can lead to a pronounced decrease of parcel buoyancy. Furthermore, for diluted parcel ascent, the convective available potential energy is greater for the insystem category than for the postsystem one, whereas the opposite is true for undiluted parcel ascent.
Abstract
The upper-air sounding data from PRE-STORM are used to investigate the convective stabilization effect on the large-scale atmosphere. To facilitate comparison between different stages of cumulus convection, the data are divided into four categories: environment, presystem, insystem, and postsystem. It is found that the convective available potential energy of the atmosphere is reduced substantially after cumulus convection, most of which is consumed during the transition from presystem to insystem. Examination of the temperature and moisture changes during cumulus convection suggests that cooling and drying in the subcloud layer are the most important factors in stabilizing the atmosphere. In general, virtual potential temperature profiles in all categories are close to reversible moist adiabats below the 600-mb level and nearly parallel to moist pseudoadiabats above it.
The effect of entrainment on parcel buoyancy is also studied. It is found that a small amount of entrainment of ambient air can lead to a pronounced decrease of parcel buoyancy. Furthermore, for diluted parcel ascent, the convective available potential energy is greater for the insystem category than for the postsystem one, whereas the opposite is true for undiluted parcel ascent.
Abstract
The parameterization theory developed in Part I is applied to compute the vertical transport of momentum by cumulus clouds for the average of six convective periods in Phase III of GATE. Special attention is paid to the role of perturbation pressure field in vertical momentum transport. Good agreement between the parameterized and the observed apparent momentum sources is obtained. In general, cumulus convection tends to decrease the vertical wind shear of the environment. It is found that the cloud-mean momentum varies significantly with height in an environment with vertical wind shear, and the pressure gradient force is mostly responsible for this variation.
Sensitivity tests of the parameterization scheme show that the cloud-mean momentum obtained is fairly insensitive to some poorly represented parameters in the cloud model. Results using this scheme are compared with those using the Schneider and Lindzen scheme. Appreciable improvement is found with the use of the new scheme.
Abstract
The parameterization theory developed in Part I is applied to compute the vertical transport of momentum by cumulus clouds for the average of six convective periods in Phase III of GATE. Special attention is paid to the role of perturbation pressure field in vertical momentum transport. Good agreement between the parameterized and the observed apparent momentum sources is obtained. In general, cumulus convection tends to decrease the vertical wind shear of the environment. It is found that the cloud-mean momentum varies significantly with height in an environment with vertical wind shear, and the pressure gradient force is mostly responsible for this variation.
Sensitivity tests of the parameterization scheme show that the cloud-mean momentum obtained is fairly insensitive to some poorly represented parameters in the cloud model. Results using this scheme are compared with those using the Schneider and Lindzen scheme. Appreciable improvement is found with the use of the new scheme.
Abstract
A scheme is developed to parameterize the vertical transport of momentum by cumulus clouds, in which the effect of a cloud-induced pressure field is included. In addition, a new form for momentum exchange between clouds and their environment is used. The scheme incorporates a simple cloud model which includes both updraft and downdraft and specifies the cloud dynamic fields to enable us to determine the mean pressure gradient force across the clouds. It is shown that in a typical environment with vertical wind shear, the pressure gradient force is along the direction of the wind shear.
The equation governing cloud mean momentum together with the diagnostic equation for cloud-induced pressure field is solved by iteration to evaluate quantitatively the horizontal pressure gradient force across the cloud and determine the cloud mean momentum. Application of the parameterization scheme to GATE convective events will be presented in Part II of this paper.
Abstract
A scheme is developed to parameterize the vertical transport of momentum by cumulus clouds, in which the effect of a cloud-induced pressure field is included. In addition, a new form for momentum exchange between clouds and their environment is used. The scheme incorporates a simple cloud model which includes both updraft and downdraft and specifies the cloud dynamic fields to enable us to determine the mean pressure gradient force across the clouds. It is shown that in a typical environment with vertical wind shear, the pressure gradient force is along the direction of the wind shear.
The equation governing cloud mean momentum together with the diagnostic equation for cloud-induced pressure field is solved by iteration to evaluate quantitatively the horizontal pressure gradient force across the cloud and determine the cloud mean momentum. Application of the parameterization scheme to GATE convective events will be presented in Part II of this paper.
Abstract
This study is directed to evaluating the feedback between evaporation (FL ) and sea surface temperature (Ts ) in the equatorial Pacific Ocean by looking at the components that control dFL /dTs , the variation of evaporation with Ts . First eddy correlation evaporation estimates obtained during long (∼1000–1500 km), low-level (30 m) traverses of the central equatorial Pacific by research aircraft during the Central Equatorial Pacific Experiment (CEPEX) are analyzed. From this limited dataset, extension to climate space- and timescales is made by comparing the aircraft measurements to bulk aerodynamic estimates of FL , using mean values from both the aircraft and Tropical Atmosphere–Ocean buoys.
Variation of surface evaporation with Ts is shown to be affected not only by surface saturation humidity deficit and its dependence on Ts , but also by variations of wind speed with Ts . Depending on the relative importance of the two contributions, surface evaporation can either increase or decrease with Ts . Intercomparison between the aircraft data and the buoy data indicates that the humidity deficit effect is dominant during, CEPEX, and in low Ts , where surface winds are only weakly related to Ts ; the effect of wind speed variation with Ts is much more important in the 2-yr buoy data for Ts ≥ 301 K. The discrepancy between the evaporation feedback in CEPEX and that from the 2-yr buoy data is shown to be largely due to oversampling of high winds and high evaporation during CEPEX for 302 ≤ Ts < 303 K. The long-tem buoy data show that for Ts < 301 K, dFL /dTs = +9 W m−2K−1, while for 304 K > Ts ≥ 301 K, dFL /dTs = −13 W m−2K−1. Furthermore, observations of FL are well below the values necessary for evaporation to be the primary limiting factor in the regulation of Ts in the equatorial Pacific.
Abstract
This study is directed to evaluating the feedback between evaporation (FL ) and sea surface temperature (Ts ) in the equatorial Pacific Ocean by looking at the components that control dFL /dTs , the variation of evaporation with Ts . First eddy correlation evaporation estimates obtained during long (∼1000–1500 km), low-level (30 m) traverses of the central equatorial Pacific by research aircraft during the Central Equatorial Pacific Experiment (CEPEX) are analyzed. From this limited dataset, extension to climate space- and timescales is made by comparing the aircraft measurements to bulk aerodynamic estimates of FL , using mean values from both the aircraft and Tropical Atmosphere–Ocean buoys.
Variation of surface evaporation with Ts is shown to be affected not only by surface saturation humidity deficit and its dependence on Ts , but also by variations of wind speed with Ts . Depending on the relative importance of the two contributions, surface evaporation can either increase or decrease with Ts . Intercomparison between the aircraft data and the buoy data indicates that the humidity deficit effect is dominant during, CEPEX, and in low Ts , where surface winds are only weakly related to Ts ; the effect of wind speed variation with Ts is much more important in the 2-yr buoy data for Ts ≥ 301 K. The discrepancy between the evaporation feedback in CEPEX and that from the 2-yr buoy data is shown to be largely due to oversampling of high winds and high evaporation during CEPEX for 302 ≤ Ts < 303 K. The long-tem buoy data show that for Ts < 301 K, dFL /dTs = +9 W m−2K−1, while for 304 K > Ts ≥ 301 K, dFL /dTs = −13 W m−2K−1. Furthermore, observations of FL are well below the values necessary for evaporation to be the primary limiting factor in the regulation of Ts in the equatorial Pacific.
Abstract
Warm SST bias underlying the spurious southern ITCZ has long been recognized as one of the main causes for double-ITCZ bias in coupled GCMs in the central Pacific. This study demonstrates that the NCAR CESM1.2 can still simulate significant double-ITCZ bias even with cold SST bias in the southern ITCZ region, indicating that warm SST bias is not a necessary condition for double-ITCZ bias in the central Pacific. Further analyses suggest that the equatorial cold tongue (ECT) biases play important roles in the formation of double-ITCZ bias in the central Pacific. The severe cold SST biases in the ECT region in the central Pacific may enhance the SST gradient between the ECT and southern ITCZ region, strengthening the lower-troposphere dynamical convergence and hence convection in the southern ITCZ region. The formation mechanism of excessive ECT bias is further investigated. It is shown that the cold SST biases in the ECT region can be largely attributed to the anomalous cooling tendency produced by the upper-ocean zonal advection due to overly strong zonal currents. In the ECT region, the westward ocean surface zonal current is driven by the equatorial easterly surface winds. It is shown that convection bias simulated by the atmospheric model in the equatorial Amazon region may lead to easterly wind bias in the downwind side (west) of convection region. The mean Walker circulation transports these easterly wind momentum anomalies downward and westward to the surface, resulting in the overly strong surface easterly wind in the central equatorial Pacific.
Abstract
Warm SST bias underlying the spurious southern ITCZ has long been recognized as one of the main causes for double-ITCZ bias in coupled GCMs in the central Pacific. This study demonstrates that the NCAR CESM1.2 can still simulate significant double-ITCZ bias even with cold SST bias in the southern ITCZ region, indicating that warm SST bias is not a necessary condition for double-ITCZ bias in the central Pacific. Further analyses suggest that the equatorial cold tongue (ECT) biases play important roles in the formation of double-ITCZ bias in the central Pacific. The severe cold SST biases in the ECT region in the central Pacific may enhance the SST gradient between the ECT and southern ITCZ region, strengthening the lower-troposphere dynamical convergence and hence convection in the southern ITCZ region. The formation mechanism of excessive ECT bias is further investigated. It is shown that the cold SST biases in the ECT region can be largely attributed to the anomalous cooling tendency produced by the upper-ocean zonal advection due to overly strong zonal currents. In the ECT region, the westward ocean surface zonal current is driven by the equatorial easterly surface winds. It is shown that convection bias simulated by the atmospheric model in the equatorial Amazon region may lead to easterly wind bias in the downwind side (west) of convection region. The mean Walker circulation transports these easterly wind momentum anomalies downward and westward to the surface, resulting in the overly strong surface easterly wind in the central equatorial Pacific.
Abstract
Moored buoy data from the equatorial Pacific are analyzed to investigate the relationship between sea surface temperature (SST) and latent heat flux from the ocean. It is found that at low SST the latent heat flux increases with SST; at high SST the latent heat flux decreases with increasing SST, a relationship that cannot be explained by thermodynamic considerations alone. Analysis of the wind speeds and humidity differences between the surface air and the saturation humidity at the sea surface temperature indicates that while at low SST the humidity difference primarily determines the latent heat flux, and at high SST a sharp decrease in wind speed is mostly responsible for the low latent heat flux. A mechanism that leads to low latent heat flux at high SST is suggested; it involves the interaction between convection and the large-scale circulation.
The longitudinal distribution of SST, wind speed, humidity difference, and latent heat flux is found to be similar to that in previous studies. In the eastern Pacific, SST is the lowest, the wind speed is large, and the humidity difference is low; in the western Pacific, SST is the highest, whereas the wind speed is low and the humidity difference is large. Latent heat flux increases from the eastern Pacific westward, reaching a maximum in the central Pacific, and then decreases toward the western Pacific warm pool.
Through analyses of the data on different timescales, we found that the atmospheric processes leading to low latent heat flux over warm SST were mainly operative on seasonal timescales (periods longer than 90 days). On shorter timescales (periods of 30–90 days), the influence of intraseasonal Madden and Julian waves was evident. On this timescale, the relationship between SST and latent heat flux was characterized by a 10-day lag between atmospheric forcing (primarily related to winds) and the local oceanic response in the western and central Pacific. In the eastern Pacific cold tongue, SST and latent heat flux variations were nearly in phase on this timescale, indicating an atmospheric response to oceanic forcing. For periods less than 30 days, SST variations associated with tropical instability waves were likewise shown to be important in forcing latent heat flux variations in the eastern Pacific cold tongue.
Abstract
Moored buoy data from the equatorial Pacific are analyzed to investigate the relationship between sea surface temperature (SST) and latent heat flux from the ocean. It is found that at low SST the latent heat flux increases with SST; at high SST the latent heat flux decreases with increasing SST, a relationship that cannot be explained by thermodynamic considerations alone. Analysis of the wind speeds and humidity differences between the surface air and the saturation humidity at the sea surface temperature indicates that while at low SST the humidity difference primarily determines the latent heat flux, and at high SST a sharp decrease in wind speed is mostly responsible for the low latent heat flux. A mechanism that leads to low latent heat flux at high SST is suggested; it involves the interaction between convection and the large-scale circulation.
The longitudinal distribution of SST, wind speed, humidity difference, and latent heat flux is found to be similar to that in previous studies. In the eastern Pacific, SST is the lowest, the wind speed is large, and the humidity difference is low; in the western Pacific, SST is the highest, whereas the wind speed is low and the humidity difference is large. Latent heat flux increases from the eastern Pacific westward, reaching a maximum in the central Pacific, and then decreases toward the western Pacific warm pool.
Through analyses of the data on different timescales, we found that the atmospheric processes leading to low latent heat flux over warm SST were mainly operative on seasonal timescales (periods longer than 90 days). On shorter timescales (periods of 30–90 days), the influence of intraseasonal Madden and Julian waves was evident. On this timescale, the relationship between SST and latent heat flux was characterized by a 10-day lag between atmospheric forcing (primarily related to winds) and the local oceanic response in the western and central Pacific. In the eastern Pacific cold tongue, SST and latent heat flux variations were nearly in phase on this timescale, indicating an atmospheric response to oceanic forcing. For periods less than 30 days, SST variations associated with tropical instability waves were likewise shown to be important in forcing latent heat flux variations in the eastern Pacific cold tongue.
Abstract
The role of convection parameterization in the formation of double ITCZ and associated upper-ocean biases in the NCAR Community Climate System Model, version 3 (CCSM3) is investigated by comparing the simulations using the original and revised Zhang–McFarlane (ZM) convection schemes. Ten-year model climatologies show that the simulation with the original ZM scheme produces a typical double ITCZ bias, whereas all biases related to the spurious double ITCZ and overly strong cold tongue in precipitation, sea surface temperature (SST), wind stress, ocean thermocline, upper-ocean currents, temperature, and salinity are dramatically reduced when the revised ZM scheme is used. These results demonstrate that convection parameterization plays a critical role in the formation of double ITCZ bias in the CCSM3. To understand the physical mechanisms through which the modifications of the convection scheme in the atmospheric model alleviate the double ITCZ bias in the CCSM3, the authors investigate the impacts of convection schemes on the atmospheric forcing and feedback in the uncoupled Community Atmospheric Model, version 3 (CAM3). It is shown that the CAM3 simulation with the original ZM scheme also produces a signature of double ITCZ bias in precipitation, whereas the simulation with the revised ZM scheme does not. Diagnostic analyses have identified three factors on the atmospheric side (i.e., the sensitivity of convection to SST, the convection–shortwave flux–SST feedback, and the convection–wind–evaporation–SST feedback) that may contribute to the differences in the coupled simulations.
Abstract
The role of convection parameterization in the formation of double ITCZ and associated upper-ocean biases in the NCAR Community Climate System Model, version 3 (CCSM3) is investigated by comparing the simulations using the original and revised Zhang–McFarlane (ZM) convection schemes. Ten-year model climatologies show that the simulation with the original ZM scheme produces a typical double ITCZ bias, whereas all biases related to the spurious double ITCZ and overly strong cold tongue in precipitation, sea surface temperature (SST), wind stress, ocean thermocline, upper-ocean currents, temperature, and salinity are dramatically reduced when the revised ZM scheme is used. These results demonstrate that convection parameterization plays a critical role in the formation of double ITCZ bias in the CCSM3. To understand the physical mechanisms through which the modifications of the convection scheme in the atmospheric model alleviate the double ITCZ bias in the CCSM3, the authors investigate the impacts of convection schemes on the atmospheric forcing and feedback in the uncoupled Community Atmospheric Model, version 3 (CAM3). It is shown that the CAM3 simulation with the original ZM scheme also produces a signature of double ITCZ bias in precipitation, whereas the simulation with the revised ZM scheme does not. Diagnostic analyses have identified three factors on the atmospheric side (i.e., the sensitivity of convection to SST, the convection–shortwave flux–SST feedback, and the convection–wind–evaporation–SST feedback) that may contribute to the differences in the coupled simulations.
