Robert E. Marshall

Mesoscale Modeling of Boundary Layer Refractivity and Atmospheric Ducting

Abstract In this study four mesoscale forecasting systems were used to investigate the four-dimensional structure of atmospheric refractivity and ducting layers that occur within evolving synoptic conditions over the eastern seaboard of the United States. The aim of this study was to identify the most important components of forecasting systems that contribute to refractive structures simulated in a littoral environment. Over a 7-day period in April–May of 2000 near Wallops Island, Virginia, meteorological parameters at the ocean surface and within the marine atmospheric boundary layer (MABL) were measured to characterize the spatiotemporal variability contributing to ducting. By using traditional statistical metrics to gauge performance, the models were found to generally overpredict MABL moisture, resulting in fewer and weaker ducts than were diagnosed from vertical profile observations. Mesoscale features in ducting were linked to highly resolved sea surface temperature forcing and associated changes i...

Effects of Initial and Boundary Conditions of Mesoscale Models on Simulated Atmospheric Refractivity

AbstractRadar ducting is caused by sharp vertical changes in temperature and, especially, water vapor at the top of the atmospheric boundary layer, both of which are sensitive to variations in the underlying surface conditions, local mesoscale weather, and synoptic weather patterns. High-resolution numerical weather prediction (NWP) models offer an alternative to observation to determine boundary layer (BL) structure and to assess the spatial variability of radar ducts. The benefit of using NWP models for simulating ducting conditions very much depends on the initial state of sea surface temperature (SST) and the model spinup time, both of which have a great impact on BL structure. This study investigates the effects of variation of NWP-model initial conditions and simulation length on the accuracy of simulating the atmosphere’s refractive index over the Wallops Island, Virginia, region, which has pronounced SST variability and complex BL structure. The Met Office Unified Model (MetUM) with horizontal res...

Four Dimensional System Engineering Demands on Radar Operating in a Coastal Sub-refractive Environment

Surface sub-refraction in the littorals reduces radio frequency (RF) horizon and introduces an expensive ameliorating system margin requirement. Along coastal Virginia, sub-refractive events are driven by synoptic scale meteorological features with three day time scales. Air/land/sea interactions introduce diurnal and mesoscale variations. This paper describes these inhomogeneities in terms of range dependent propagation factor loss relative to a standard atmosphere. Four dimensional thermodynamic profiles supporting the refractivity field calculation are provided by the Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS). The Advanced Refractivity Effects Prediction System (AREPS) is employed to predict the performance of notional S and X band radars operating 24 nm offshore.

Multi-wavelength radar target detection in an extreme advection duct event

Near sea surface radio frequency (RF) refraction is four dimensional (4D) and can significantly impact the performance of radar systems. The refractivity field is dictated by the vertical thermodynamic structure of the constantly evolving marine atmospheric boundary layer (MABL). Logistical and budgetary restraints on meteorological measurements over water to capture the spatio-temporal structure of refractivity fields influencing radar performance have limited the knowledge of how and why radar performance is azimuth, range, and time dependent. Rapidly increasing computer processing speeds and decreasing memory capacity costs have supported the horizontal and vertical resolution requirements for mesoscale numerical weather prediction (NWP) models to resolve the thermodynamic structure in the MABL. Once modeled, refractivity structure is easily calculated from the thermodynamic structure. Mesoscale NWP models coupled with modern parabolic equation radar performance models can support the prediction of 4D radar performance in challenging non-homogeneous, near surface refractivity fields at the time and location of the modelers choice. The NWP modeling presented in this paper demonstrates how large-scale offshore flow of warm and dry air over colder seas produces strong near surface RF trapping. Large land-sea temperature differences can produce near shore sea breezes and surface-based ducts. This paper describes modeled radar performance in such a complex ducting structure over the Persian Gulf during large-scale northwest atmospheric flow. The refractivity field was resolved by the Coupled Ocean/ Atmosphere Mesoscale Prediction System (COAMPS ® is a registered trademark of the Naval Research Laboratory) and the notional radar performance was modeled by the advanced refractive effects prediction system (AREPS). The results indicate strong spatial and wavelength-dependent enhancements and degradations in radar performance relative to a standard atmosphere.

Engineering demands placed on littoral radar due to non-standard propagation revealed by mesoscale numerical weather prediction technology

Rapid improvements in skill and spatio-temporal resolution of mesoscale numerical weather prediction models have recently created the ability to simulate notional radar design performance in four dimensional non-standard refractivity environments. This paper quantifies the engineering demands, relative to a standard atmosphere, placed on notional ship borne S band, C band and X band radars during a strong sub-refractive event in the Chesapeake Bay. The Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPStrade) provided the refractivity field and the Advanced Refractivity Effects Prediction System (AREPS) was employed to model the radars. Engineering demands are wavelength, spatially, and temporally dependent and often exceed 50 dB.

Extreme extended multi-wavelength propagation due to hot and dry air flowing over the Persian Gulf

Shallow surface radio frequency (RF) trapping layers or ducts form in stable internal boundary layers (SIBL) when warm dry air flows offshore over colder and more humid sea surfaces. Depending on wind speed and land sea temperature difference, these surface ducts can exist for hundreds of kilometers offshore and trap radar energy in layers below 100m above sea level. This paper describes the performance of notional shore based S, C and X band radars during a SIBL event. The three dimensional (3D) verified refractivity field is modeled by the Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS®). The radar performance is modeled by the Advanced Refractive Effects Prediction System (AREPS).

Multi-wavelength impacts on coastal radar performance during a sea breeze

Sea breeze circulations dramatically impact the detection performance of coastal radar and are forecast to increase in strength and frequency into the 21st century. Dry and warm air from the land flows offshore between 100 and 200 meters above sea level out to 100km offshore. This redistribution of water vapor and temperature creates strong vertical humidity and temperature gradients resulting in significant radar ducting. These thermodynamic gradients and resulting non-standard propagation structures vary in time and space as the sea breeze develops. Spatio-temporal radar skip zones develop within the ducts and reduced target detection ranges develop above the ducts. Notional S, C and X band radars are modeled with the Advanced Refractive Effects Prediction System and are located at the shoreline during a sea breeze. The azimuth and range dependent refractivity field is modeled by the Regional Atmospheric Mesoscale Prediction System.

Surface-based ducting due to a shift in air masses off the coast of Wallops Island, Virginia

On 14 November 2007, the Johns Hopkins University Applied Physics Laboratory (JHU/APL) provided environmental characterization support for a US Navy radar test event, off the coast of Wallops Island, VA. Two environmental sensor systems were deployed: (1) the Automated Environmental Assessment System installed aboard the research vessel Chessie and (2) the Helicopter Atmospheric Profiler System. Chessie was located on the test range to collect near-surface environmental data in support of evaporation ducting assessments, as well as atmospheric soundings via rocketsondes to characterize upper-air refractivity conditions. Two types of helicopter flight profiles characterized evaporation and surface-based ducts. This paper summarizes the collected data, the propagation analysis, and highlights how a subtle change in meteorological conditions can significantly affect radar propagation. In addition, an analysis of the impact on radar system performance, due to the change in radar propagation conditions, is presented.

Clutter forecast - a synthesis of mesoscale numerical weather prediction and empirical site specific radar clutter models

This paper introduces a new model approach that derives three-dimensional refractivity profiles from mesoscale numerical weather prediction (MSNWP), applies them to parabolic equation propagation models, and combines them with empirical surface clutter reflectivity to generate a more realistic model of surface clutter. Realistic site specific clutter models must be faithful to the energy propagation to the surfaces that are responsible for the return clutter power; it is not good enough to maintain validated databases of reflectivities, however important that empirical data might be. This synthesis of MSNWP and radar clutter models can provide clutter and propagation forecasts to military planners, retroactive prediction of propagation for clutter test data analysis, and realistic models for radar system design and performance analysis.