The following information was extracted from materials associated with the original S-PolKa Facility Request.
PI: Prof. Robert A. Houze, Jr., University of Washington
Co-Investigator: Dr. Socorro Medina, University of Washington
The initiation of the Madden-Julian Oscillation (MJO) is thought to be connected with the development of a convective cloud population over the equatorial Indian Ocean. Once developed the population propagates eastward to the equatorial Pacific Ocean and the disturbance is associated with intraseasonal variations of weather at remote latitudes, including North America. After the cloud population begins moving eastward, the atmosphere over central Indian Ocean becomes dry and the cloudiness is suppressed. The Dynamics of the MJO (DYNAMO) field campaign is aimed at understanding the stages of development of the cloud population in the Indian Ocean and its association with the recharging of the humidity field in the region after the population propagates away. The MJO is inadequately predicted by large-scale models and understanding of the behavior of the MJO and its representation in models is hampered by a lack of basic observations in the remote equatorial Indian Ocean.
DYNAMO will use radar observations to fill part of this observational gap. Atmospheric research radars have evolved to the point where they can not only observe the reflectivity and air motions of precipitating clouds but also detect and characterize nonprecipitating clouds, the microphysical characteristics of hydrometeors in clouds, and the humidity profile in the air surrounding the clouds. DYNAMO will capitalize on all these capabilities by employing a complement of radars that will document the presence and characteristics of small nonprecipitating convective clouds, deep convective elements, deep stratiform rain clouds of mesoscale systems, and nonprecipitating anvil clouds. The MJO is thought to be sensitive to the mix of these cloud types. The DYNAMO radars will therefore detect and statistically document all of the clouds making up the cloud population and their associated humidity field at each stage of MJO initiation. Hypotheses regarding how the cloud population feeds back to the largescale dynamics and how the clouds interact with the humidity field will be tested empirically through analysis of the statistics of the radar data collected at each stage of MJO development. Further, the cloud population observed by radar and processes diagnosed empirically from the observations will be compared with the output of several high-resolution cloud-resolving models.
No single radar can accomplish the task of comprehensively documenting the cloud population of the developing MJO. An experiment has therefore been designed to simultaneously employ a suite of radars of varying wavelengths and scanning capabilities on two ships, possibly one aircraft, and the island of Gan. The primary radar on Gan in this research will be the National Center for Atmospheric Research dual-wavelength Doppler polarimetric S-PolKa radar. Data from S-PolKa will be co-analyzed with data from four other radars on Gan and with the ship and aircraft radars. The island and ship radar sites will employ both centimeter and millimeter wavelength radars so as to detect both nonprecipitating and precipitating clouds. A dual-wavelength technique will be used to determine the humidity profiles associated with the developing cloud population. Polarimetric radar observations will be employed to determine microphysical properties of the clouds. The radar data will be further analyzed in close conjunction with DYNAMO sounding network data, which includes soundings at the ships and Gan.
In summary, the DYNAMO radar observations, analysis, and model comparison described in this proposal will add knowledge of the nonprecipitating clouds, microphysical properties, and relation to the humidity field at each stage of MJO development that are absent from previous studies of the tropical cloud population.
The results of this study will substantially improve basic understanding of the role of clouds and humidity in the MJO. The full use of modern radar technology in DYNAMO will expand the general knowledge of the oceanic tropical cloud from a past knowledge primarily of the gross properties of precipitating clouds to an understanding of the full spectrum of both nonprecipitating and precipitating clouds, their microphysics and dynamics, and their relation to ambient humidity.
Broader impacts resulting from the proposed activity: Improved observations and understanding of the MJO cloud population will lead to improved large-scale model prediction of the intraseasonal variations of weather and climate in both the tropics and midlatitudes.
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