Radar and Rainfall and Clouds, Oh My!
Posted: September 16th, 2014Authors: Amanda E.
Rain and thunderstorms have always amazed me. I used to sit on my grandparent’s balcony in Florida and watch the clouds build up as a storm rolled in, darkening the sky as giant rain drops would begin to fall and lightning would illuminate the sky. The magic of it all intrigued me to study atmospheric science in undergraduate and graduate school. While in graduate school at Texas A&M University, I got the opportunity to work on a research project involving tropical convection (i.e., intense, heavy rainfall) in the Indian Ocean. After I wrote and defended my thesis, and graduated, I worked with my two (2) advisors, Courtney Schumacher and Anita Rapp, to publish my work in a peer-reviewed scientific journal called the Journal of Geophysical Research: Atmospheres. If you have a nerdy side, or are just plain interested in understanding weather phenomena, read on and learn about my methods and results from analyzing in situ and remotely sensed data from the tropics!
A multinational field campaign took place in the tropical Indian Ocean from October 2, 2011 to February 9, 2012, to study the initiation of a large-scale convective event, called the Madden-Julian Oscillation (MJO). The MJO exists on a time scale of 30-90 days, and commonly initiates over the Indian Ocean and propagates eastward into the western Pacific Ocean. The MJO creates an “envelope” of convection that works to sustain the phenomenon as it moves slowly. The MJO has two (2) distinct phases, the active and the suppressed phase. An MJO is considered in an active phase when deep convection and precipitation occurs. A suppressed MJO occurs in regions around the active MJO, associated with decreased cloud cover and less precipitation.
The campaign was comprised of three (3) field experiments: the Dynamics of the Madden Julian Oscillation (DYNAMO), the Atmospheric Radiation Measurement (ARM) Madden-Julian Oscillation Investigation Experiment (AMIE), and the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY2011). Texas A&M deployed a mobile Doppler radar on a tiny island just below the equator, Addu Atoll (0.6°S, 73°E), part of the Maldives. The radar was accompanied by additional land-based radars and meteorological instruments deployed by the Department of Energy (DOE) and other universities. There were multiple aircraft with radar and dropsondes (a tool literally “dropped” from the aircraft to measure atmospheric properties as it falls to the surface), as well as ship-based radar, deployed by different U.S., European, and Asian agencies. The entire field campaign was planned to continue through March 2012, but due to political unrest, the program had to be cut short (right before I was supposed to join!).
The Texas A&M radar, known as SMART-R (Shared Mobile Atmospheric Research and Teaching Radar) was a C-band radar, which means that it has a 5-cm wavelength, and is best at detecting precipitation and raining clouds. Every three (3) hours, DOE launched radiosondes, or weather balloons, which retrieved temperature, pressure, wind speed and direction, and relative humidity as they ascended into the atmosphere. A vertical cloud radar, called KAZR (Ka Zenith Radar) was also located on the DOE site, with a smaller 8.6 mm wavelength, best for detecting smaller, cloud-sized particles.
The research I was involved in used the radar data from SMART-R and KAZR and the atmospheric profiles from the daily weather balloons to analyze the convective patterns during the MJO events. I was especially interested in observing how the MJO events interacted with other smaller equatorial atmospheric waves, known as convectively coupled Kelvin Waves (KWs). KWs also move eastward on a timescale of 2-20 days. They can be associated with convection, which was the basis of my study. My goal was to analyze the precipitation and environmental properties of KWs during the active and suppressed phases of the MJO.
Using the data collected during the field campaign, as well as some processed data provided by other atmospheric scientists, I wrote software to visualize and analyze the KWs during the MJO events that occurred. During the field campaign, three (3) strong MJO events and ten (10) strong KWs occurred, where four (4) KWs occurred during the active MJO, five (5) occurred during the suppressed MJO, and one (1) occurred as the MJO was developing. I examined these events both individually and by compositing them based on maximum convective rainfall. A short summary of these results is provided below.
When KWs occurred during the active MJO, they were accompanied by two (2) peaks of large amounts of convective (heavy) and stratiform (lighter) rain. There was no evident humidity buildup as the KW developed, since it was trapped in the moist envelope provided by the MJO. The extent and number of clouds was greater in KWs during the active MJO. During the suppressed MJO, KWs had fewer clouds and less rainfall. There was also a slower build up to maximum rainfall. These KWs displayed the expected moisture buildup as they developed. Low-level humidity preceded convective rain development, suggesting that low-level humidity preconditions the atmosphere for convective development.
You can read a much more detailed and in-depth account of my research methods and conclusions in my publication, found here. If you are unable to access the article, shoot me an email at firstname.lastname@example.org and I’d be happy to provide a PDF copy of it! I had the pleasure of presenting this research at the 2012 American Geophysical Union Conference in San Francisco, CA; the 2013 American Meteorological Society Conference in Austin, TX; and the MJO Field Data and Science Workshop in Kohala Coast, HI. This is on-going research within the atmospheric science community, so keep an eye out for more publications studying the tropical convection of the MJO and KWs!