Assessing the Impact of Diurnal Wind Variability

Principal Investigator: 
Collaborators: 
San Nguyen, Scripps Institution of Oceanography
Proposal Abstract: 

Winds vary diurnally throughout the tropics (equatorward of 30 degrees latitude) and along coastlines globally (where diurnally-varying winds are referred to as the land breeze/sea breeze circulation).  Diurnally varying winds play a potentially important role in the climate system, because they can interact with diurnally-varying radiative forcing and thus can have a rectified impact on the ocean, influencing mean mixed-layer depth and the properties of water within the mixed layer and controlling the exchange of mixed-layer water with the ocean interior. However, computational constraints have historically prevented many climate models from resolving the diurnal cycle of the mixed layer. One of the emerging challenges for our understanding of the climate system is to determine how best to resolve the diurnal cycle of winds and how to assess its true impact on upper ocean processes.  Most Earth-observing satellites fly in sun synchronous orbits, meaning that measurements are made at two local times per day (e.g. 06:00 and 18:00, in the case of QuikSCAT), exactly at the Nyquist frequency of the diurnal cycle.   The QuikSCAT/ADEOS-2 tandem mission in 2003 provided six months of high quality scatterometry, which have been used to explore diurnal variability, albeit only for Northern Hemisphere summer conditions.  More recently the launch of multiple wind missions, including ASCAT, WindSat, and OSCAT, has offered the possibility of carrying out multi-mission analyses of wind in order to assess diurnal variability.  The planned launch of RapidScat aboard the International Space Station, which does not follow a sun synchronous orbit, will offer further opportunities to assess diurnal variability of ocean winds.  The objective of this study will focus on first characterizing seasonal and interannual variability of diurnal winds both using multi-sensor measurements (from ASCAT, OSCAT, and WindSat) and also RapidScat measurements once they become available.   This will allow us to characterize seasonal changes in the diurnal amplitude and year-to-year variations in the structure and magnitude of diurnal winds.  Weather stations and mooring-based winds will be used to validate satellite-derived results.  While satellite-based assessments of diurnal winds have typically been limited to examining a single diurnally varying harmonic, the more extensive sampling should make it possible to consider semi-diurnal and higher frequencies as well.  The study will also use an one-dimensional upper-ocean model with high vertical resolution to quantify the role that diurnal winds play in upper-ocean processes.  Results will be used to develop improvements for a more idealized model used to represent upper ocean processes in prognostic climate models. Our goals are to assess the impact of including or neglecting diurnally varying winds on large-scale ocean circulation studies and to quantify the impact of the rectified effects of radiative forcing and diurnal winds on surface temperature and salinity, upper ocean heat content and air-sea heat fluxes.

Start and End Date: 
July 2014 to July 2018
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