Effect of Background Atmospheric Conditions on Convectively Coupled Kelvin Wave Phase Speed

Suny At Albany, Albany NY

Investigators

Abstract

Kelvin waves are a well-documented synoptic weather phenomenon in the Tropics, accounting for 70% of the rainfall associated with the Madden-Julian oscillation (MJO) near the equator, which is a prominent mode of deep convection in the Tropics in its own right with important impacts on the global circulation and global weather patterns. Kelvin waves can also directly impact circulations beyond the Tropics through teleconnections associated with deep convection coupled to these disturbances. The propagation characteristics of synoptic disturbances in the Tropics including Kelvin waves and the MJO are poorly represented in climate models, which has consequences for the global circulation, water and energy budgets, and the ability of operational global models to optimize the subseasonal to seasonal predictability inherent in the climate system. This project aims to improve fundamental understanding of the phase speed of Kelvin waves, including the influence of background advection by the mean wind, moist processes, and static stability, using gridded data products and satellite observations of deep clouds. Project outcomes include documentation of Kelvin wave phase speed under varying background conditions such as the El Niño Southern Oscillation (ENSO) and the MJO, as well as secular trends in the background state over time present in the observational data sets. The Principal Investigator has a strong background in Kelvin wave dynamics and in training the next generation of scientists for a range of STEM positions including academia, private sector, government laboratories, and forecasting agencies. The project will identify Kelvin waves in satellite outgoing longwave radiation (OLR) using a method well-established by the Principal Investigator and colleagues over many years. Average phase speeds will be estimated from Hovmöller diagrams (longitude vs time) of Kelvin wave filtered OLR for select background conditions including different seasons and ENSO and MJO phases. Results will be assessed at different pressure levels to understand how phase speed changes with height under different background wind, moisture and static stability conditions based on gridded data. Quality control measures will be taken to assure the select wave cases used for compositing are consistent with the expected horizontal structure and equivalent depth for a Kelvin wave. Background advective winds will be defined as the 20-day lowpass filtered wind at each level over the dates of the wave events. Contributions to the phase speed by moist processes will be determined by regressing the advection corrected phase speed onto profiles of atmospheric moisture and satellite rainfall for wave events. Understanding how Kelvin wave phase speed associates with tropospheric background conditions will give us benchmarks to assess how well global models simulate eddy mean flow interactions in the Tropics. Techniques developed to improve models based on these benchmarks could be applicable to mid-latitude eddy mean flow interactions. The results will also provide benchmarks for how convection quantitatively impacts wave phase speeds, providing insight into model representation of coupled convection in Kelvin waves. The project will train two PhD students, who will carry out the analysis as part of their thesis work. 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.