Anomalous Propagation Conditions over Eastern Pacific Ocean Derived from MAGIC Data
Observations from the ARM MAGIC campaign provide insight into atmospheric conditions that may impact shipborne radio communications and navigation radars.
Sharp vertical gradients in temperature and moisture can lead to the electromagnetic wave energy from transmitting radio or microwave sources becoming trapped within a shallow and nearly horizontal layer known as a ducting layer. These ducting layers, or ducts, lead to enhanced propagation range and weak propagation loss of electromagnetic energy within the ducting layer and a corresponding reduction in the electromagnetic signal strength above or below the duct. Knowledge on ducting is imperative for radio communication links, ocean navigation radars used by ships, and correcting weather radar returns for accurate estimation of precipitation rates. However, few in situ observations have been made of ducting conditions in the eastern and central North Pacific Ocean.
Scientists used observations from the year-long Marine Atmospheric Radiation Measurement GPCI (GEWEX cloud system study Pacific Cross-Section Inter-comparison) Investigation of Clouds (MAGIC) campaign to study the spatial and temporal variability of evaporation and elevated ducts observed over the North Pacific Ocean. They found that the spatial variability of evaporation duct thickness shows a persistent increase at a gradient of 1 m for every 5 degrees longitude from California to Hawaii, due to changes in the marine atmospheric boundary layer along this transect. An existing climatological evaporation duct height product based on model and reanalysis data shows similar trends of variability as the MAGIC observations generally underestimate duct height.
This study characterizes the evaporation and elevated ducts, the most common types of ducts observed over the ocean, along a track of around 4,000 km between the California coast and Hawaii. The team analyzed 1 year (2012-2013) of ship-based measurements made during the MAGIC campaign. During this period, the ship made multiple transects between southern California and Hawaii. While ship-based in situ measurements and radiosonde data served as the primary data source, a marine atmospheric surface layer model adapted from the Coupled Ocean-Atmosphere Response Experiment 3.0 surface flux scheme was used to diagnose evaporative duct properties. Calculated mean evaporation duct heights based on shipboard measurements were found to increase steadily from 7 m offshore California to about 15 m near Hawaii. Overall, 78% of duct heights are below 20 m. On average, the evaporation duct strength is between ≈ 25M and 35M units near Hawaii and about 15M units near the California coast. A gradual transition from stratocumulus (Sc) dominated offshore California to trade wind regime with cumulus (Cu) clouds takes place along MAGIC track. The measured marine atmospheric boundary layer (MABL) height and the capping inversion characteristics are significantly different in the two regimes with MABL decoupling occurring in the longitude range between 125°W and 140°W along the track. The characteristics of the elevated ducts, obtained from the rawinsonde sounding profiles, are also different in the two regions west and east of the MABL decoupling region. Approximately 70% to 80% of the elevated ducts occur below 1.5 km east of the decoupling region, whereas almost equal percentage of ducts forms at heights above 1.5 km on the west side. Further research is warranted to understand the factors affecting the spatial and temporal variability of duct parameters such as the semi-permanent North Pacific high and the trade wind inversion.
Naval Postgraduate School
MAGIC data used for the analyses were downloaded from the ARM archive (www.arm.gov). D. P. Alappattu is sponsored by the National Research Council Research Associateship Program. This work is part of the Coupled Air-Sea Processes and EM ducting Research (CASPER) project sponsored by the Office of Naval Research (ONR) under grant N0001416WX00469 through Multidisciplinary University Research Initiative (MURI).
Alappattu, D. P., Q. Wang, and J. Kalogiros. 2016. “Anomalous Propagation Conditions over Eastern Pacific Ocean Derived from MAGIC Data,” Radio Science 51(7), 1142-56. DOI: 10.1002/2016RS005994.