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Abstract
In a model for unstable tropical cyclones, Charney and Eliassen have examined a type of convective instability which depends critically upon the equivalent static stability and a characteristic scale of the disturbance. However, the converging mass of moist air rising out of the boundary layer must first overcome the gravitational field imposed by the stratification of the environment before latent heating at greater heights will energize the circulation. When this effect is introduced into the model, the growth rate is sharply reduced, and may even be zero when ∂θ/∂p = constant ≠ 0. This sensitivity to low-level stratification suggests that the seasonal and geographical variations in the sea surface temperature and trade wind inversion in the tropical zone may exert an important controlling influence on the growth of incipient tropical disturbances.
Abstract
In a model for unstable tropical cyclones, Charney and Eliassen have examined a type of convective instability which depends critically upon the equivalent static stability and a characteristic scale of the disturbance. However, the converging mass of moist air rising out of the boundary layer must first overcome the gravitational field imposed by the stratification of the environment before latent heating at greater heights will energize the circulation. When this effect is introduced into the model, the growth rate is sharply reduced, and may even be zero when ∂θ/∂p = constant ≠ 0. This sensitivity to low-level stratification suggests that the seasonal and geographical variations in the sea surface temperature and trade wind inversion in the tropical zone may exert an important controlling influence on the growth of incipient tropical disturbances.
Abstract
The effect of the release of latent heat on the thermal vorticity of Hurricane Diane (1955) is examined when it passed over the east coast of the United States as a weakened tropical storm. A form of the thermodynamic energy equation is used as the diagnostic equation for this study. Because of uncertainty in the data, as well as incomplete physical understanding of the interaction, between synoptic and convective scales, several alternative models are used which employ different assumptions on the influence of convection and the vertical distribution of humidity and temperature.
In agreement with the results of previous investigations, Diane was found to weaken after landfall as a result of the lifting of negatively buoyant air at the storm center. However, as the storm readjusted to its new environment the stability at the center again decreased. The latent heating, as represented by its horizontal Laplacian, then becomes effective in the maintenance and redevelopment of the storm.
Abstract
The effect of the release of latent heat on the thermal vorticity of Hurricane Diane (1955) is examined when it passed over the east coast of the United States as a weakened tropical storm. A form of the thermodynamic energy equation is used as the diagnostic equation for this study. Because of uncertainty in the data, as well as incomplete physical understanding of the interaction, between synoptic and convective scales, several alternative models are used which employ different assumptions on the influence of convection and the vertical distribution of humidity and temperature.
In agreement with the results of previous investigations, Diane was found to weaken after landfall as a result of the lifting of negatively buoyant air at the storm center. However, as the storm readjusted to its new environment the stability at the center again decreased. The latent heating, as represented by its horizontal Laplacian, then becomes effective in the maintenance and redevelopment of the storm.
A few examples of scientific accomplishments in tropical meteorology and hurricane research are presented. Tropical field experiments such as GATE have greatly influenced observational studies of convection and tropical easterly waves. One application of the study of convection is the attempt to estimate precipitation from satellite platforms.
Research in tropical cyclones has further improved the definition of large-scale structure and the environment in which the hurricane grows. Radiation, convection, and air-sea interaction studies are directed at the forcing and possible feedback of the hurricane with its environment. With this improved physical understanding, numerical modeling of hurricanes can now produce position forecasts of reasonable accuracy that are becoming competitive with current statistical-dynamical methods. There is a continuing effort to attempt hurricane modification experiments in conjunction with an adequate measurement program.
A few examples of scientific accomplishments in tropical meteorology and hurricane research are presented. Tropical field experiments such as GATE have greatly influenced observational studies of convection and tropical easterly waves. One application of the study of convection is the attempt to estimate precipitation from satellite platforms.
Research in tropical cyclones has further improved the definition of large-scale structure and the environment in which the hurricane grows. Radiation, convection, and air-sea interaction studies are directed at the forcing and possible feedback of the hurricane with its environment. With this improved physical understanding, numerical modeling of hurricanes can now produce position forecasts of reasonable accuracy that are becoming competitive with current statistical-dynamical methods. There is a continuing effort to attempt hurricane modification experiments in conjunction with an adequate measurement program.
The 10th Prospectus Development Team (PDT-10) of the U.S. Weather Research Program was charged with identifying research needs and opportunities related to the short-term prediction of weather and air quality in urban forecast zones. Weather has special and significant impacts on large numbers of the U.S. population who live in major urban areas. It is recognized that urban users have different weather information needs than do their rural counterparts. Further, large urban areas can impact local weather and hydrologic processes in various ways. The recommendations of the team emphasize that human life and well-being in urban areas can be protected and enjoyed to a significantly greater degree. In particular, PDT-10 supports the need for 1) improved access to real-time weather information, 2) improved tailoring of weather data to the specific needs of individual user groups, and 3) more user-specific forecasts of weather and air quality. Specific recommendations fall within nine thematic areas: 1) development of a user-oriented weather database; 2) focused research on the impacts of visibility and icing on transportation; 3) improved understanding and forecasting of winter storms; 4) improved understanding and forecasting of convective storms; 5) improved forecasting of intense/severe lightning; 6) further research into the impacts of large urban areas on the location and intensity of urban convection; 7) focused research on the application of mesoscale forecasting in support of emergency response and air quality; 8) quantification and reduction of uncertainty in hydrological, meteorological, and air quality modeling; and 9) the need for improved observing systems. An overarching recommendation of PDT-10 is that research into understanding and predicting weather impacts in urban areas should receive increased emphasis by the atmospheric science community at large, and that urban weather should be a focal point of the U.S. Weather Research Program.
The 10th Prospectus Development Team (PDT-10) of the U.S. Weather Research Program was charged with identifying research needs and opportunities related to the short-term prediction of weather and air quality in urban forecast zones. Weather has special and significant impacts on large numbers of the U.S. population who live in major urban areas. It is recognized that urban users have different weather information needs than do their rural counterparts. Further, large urban areas can impact local weather and hydrologic processes in various ways. The recommendations of the team emphasize that human life and well-being in urban areas can be protected and enjoyed to a significantly greater degree. In particular, PDT-10 supports the need for 1) improved access to real-time weather information, 2) improved tailoring of weather data to the specific needs of individual user groups, and 3) more user-specific forecasts of weather and air quality. Specific recommendations fall within nine thematic areas: 1) development of a user-oriented weather database; 2) focused research on the impacts of visibility and icing on transportation; 3) improved understanding and forecasting of winter storms; 4) improved understanding and forecasting of convective storms; 5) improved forecasting of intense/severe lightning; 6) further research into the impacts of large urban areas on the location and intensity of urban convection; 7) focused research on the application of mesoscale forecasting in support of emergency response and air quality; 8) quantification and reduction of uncertainty in hydrological, meteorological, and air quality modeling; and 9) the need for improved observing systems. An overarching recommendation of PDT-10 is that research into understanding and predicting weather impacts in urban areas should receive increased emphasis by the atmospheric science community at large, and that urban weather should be a focal point of the U.S. Weather Research Program.
The National Weather Service intends to begin routinely issuing long-lead forecasts of 3-month mean U. S. temperature and precipitation by the beginning of 1995. The ability to produce useful forecasts for certain seasons and regions at projection times of up to 1 yr is attributed to advances in data observing and processing, computer capability, and physical understanding—particularly, for tropical ocean-atmosphere phenomena. Because much of the skill of the forecasts comes from anomalies of tropical SST related to ENSO, we highlight here long-lead forecasts of the tropical Pacific SST itself, which have higher skill than the U.S forecasts that are made largely on their basis.
The performance of five ENSO prediction systems is examined: Two are dynamical [the Cane-Zebiak simple coupled model of Lamont-Doherty Earth Observatory and the nonsimple coupled model of the National Centers for Environmental Prediction (NCEP)]; one is a hybrid coupled model (the Scripps Institution for Oceanography-Max Planck Institute for Meteorology system with a full ocean general circulation model and a statistical atmosphere); and two are statistical (canonical correlation analysis and constructed analogs, used at the Climate Prediction Center of NCEP). With increasing physical understanding, dynamically based forecasts have the potential to become more skillful than purely statistical ones. Currently, however, the two approaches deliver roughly equally skillful forecasts, and the simplest model performs about as well as the more comprehensive models. At a lead time of 6 months (defined here as the time between the end of the latest observed period and the beginning of the predict and period), the SST forecasts have an overall correlation skill in the 0.60s for 1982–93, which easily outperforms persistence and is regarded as useful. Skill for extra-tropical surface climate is this high only in limited regions for certain seasons. Both types of forecasts are not much better than local higher-order autoregressive controls. However, continual progress is being made in understanding relations among global oceanic and atmospheric climate-scale anomaly fields.
It is important that more real-time forecasts be made before we rush to judgement. Performance in the real-time setting is the ultimate test of the utility of a long-lead forecast. The National Weather Service's plan to implement new operational long-lead seasonal forecast products demonstrates its effectiveness in identifying and transferring “cutting edge” technologies from theory to applications. This could not have been accomplished without close ties with, and the active cooperation of, the academic and research communities.
The National Weather Service intends to begin routinely issuing long-lead forecasts of 3-month mean U. S. temperature and precipitation by the beginning of 1995. The ability to produce useful forecasts for certain seasons and regions at projection times of up to 1 yr is attributed to advances in data observing and processing, computer capability, and physical understanding—particularly, for tropical ocean-atmosphere phenomena. Because much of the skill of the forecasts comes from anomalies of tropical SST related to ENSO, we highlight here long-lead forecasts of the tropical Pacific SST itself, which have higher skill than the U.S forecasts that are made largely on their basis.
The performance of five ENSO prediction systems is examined: Two are dynamical [the Cane-Zebiak simple coupled model of Lamont-Doherty Earth Observatory and the nonsimple coupled model of the National Centers for Environmental Prediction (NCEP)]; one is a hybrid coupled model (the Scripps Institution for Oceanography-Max Planck Institute for Meteorology system with a full ocean general circulation model and a statistical atmosphere); and two are statistical (canonical correlation analysis and constructed analogs, used at the Climate Prediction Center of NCEP). With increasing physical understanding, dynamically based forecasts have the potential to become more skillful than purely statistical ones. Currently, however, the two approaches deliver roughly equally skillful forecasts, and the simplest model performs about as well as the more comprehensive models. At a lead time of 6 months (defined here as the time between the end of the latest observed period and the beginning of the predict and period), the SST forecasts have an overall correlation skill in the 0.60s for 1982–93, which easily outperforms persistence and is regarded as useful. Skill for extra-tropical surface climate is this high only in limited regions for certain seasons. Both types of forecasts are not much better than local higher-order autoregressive controls. However, continual progress is being made in understanding relations among global oceanic and atmospheric climate-scale anomaly fields.
It is important that more real-time forecasts be made before we rush to judgement. Performance in the real-time setting is the ultimate test of the utility of a long-lead forecast. The National Weather Service's plan to implement new operational long-lead seasonal forecast products demonstrates its effectiveness in identifying and transferring “cutting edge” technologies from theory to applications. This could not have been accomplished without close ties with, and the active cooperation of, the academic and research communities.
