Studies of the Maritime Atmospheric Boundary Layer during DYNAMO

Colorado State University, Fort Collins CO

Investigators

Abstract

The atmospheric boundary layer (ABL), typically the lowest kilometer or two of the atmosphere, has an outsized influence on the development of deep convective clouds over the tropical oceans. The role of the ABL in the development of the Madden-Julian Oscillation (MJO), a large eastward moving region of convection that forms over the Indian Ocean, was a primary focus of the 2011 Dynamics of the MJO (DYNAMO) field campaign (see for example awards 1022899 and 1059899). The campaign collected observations from four island stations and two ship stations in the Central Indian Ocean. Work under this award uses the DYNAMO observations to characterize the behavior of the tropical marine ABL and its interactions with the underlying ocean. It also seeks to determine the extent to which observations taken from the island stations are influenced by island effects which do not represent the conditions over the surrounding ocean that matter for the development of widespread convection. ABL properties to be examined include temperature, moisture, stability, winds, and cloudiness, with a particular focus on the diurnal cycle of ABL depth. The diurnal cycle is assessed from weather balloons launches that took place every three hours during the intensive observing period of the campaign. The PI has developed a method to determine the height of the ABL from the signal-to-noise ratio of continuous observations from the wind profiler and Radio Acoustic Sounding System (RASS) deployed on the islands and ships. The technique could prove useful for filling in the gaps between balloon launches and verifying that the balloon observations capture the full diurnal cycle of ABL depth. Profiler and RASS data collected on the island of Diego Garcia and the research vessel Revelle, at the same latitude and separated by 10 degrees longitude, are compared to detect island effects. Two of the islands used for the campaign are atolls (the second is Gan Island in the Maldives), which have minimal land area but surround lagoons with a relatively strong diurnal temperature cycle which might influence ABL observations. Observations taken from the Revelle and moorings deployed from the ship are used to characterize air-sea interactions relevant to ABL development and subsequent convection. The work has societal value due to the many ways in which the MJO affects weather and climate worldwide, including the development of hurricanes in the Gulf of Mexico and extreme rainfall events in California. The MJO presents a challenge to current weather prediction models, and work performed here may lead to improved forecasts of the MJO and its impacts. More specifically, the project generates forcing datasets that can be used for model development. Products generated under this award are made openly available to the research and operational communities through a website at the National Center for Atmospheric Research. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.