Anomalous propagation conditions over eastern Pacific Ocean derived from MAGIC data

Abstract

This study characterizes the evaporation and elevated ducts, the most common types of ducts observed over the ocean, along a track of around 4000 km between the California coast and Hawaii. We analyzed 1 year (2012–2013) of ship-based measurements made during the Marine Atmospheric Radiation Measurement GPCI (GEWEX cloud system study Pacific Cross-Section Intercomparison) Investigation of Clouds (MAGIC) campaign. During this period, the ship made multiple transects between Southern California and Hawaii. While the ship-based in situ measurements and the 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 is 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 ≈ 25 and 35 M units near Hawaii and about 15 M 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 70–80% 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.