Date of Award
5-1-2024
Language
English
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
College/School/Department
Department of Atmospheric and Environmental Sciences
Dissertation/Thesis Chair
Liming Zhou
Committee Members
Aiguo Dai, Brian E.J. Rose, Paul E. Roundy
Subject Categories
Atmospheric Sciences
Abstract
Tropical rainforests regulate global and regional climate systems and play a vital role in the world’s water and carbon cycles. The Congo Basin, situated in central equatorial Africa, is home to the second largest rainforest on the planet and is one of the most convective regions in the world. Recent studies indicate that a large-scale and long-term drying trend, characterized by an increase in dry season length, a decrease in forest greenness, and a decrease in precipitation, has been stressing the region since the 1980’s. Despite the alarming drying trend and the Congo’s important role in the planetary circulation and the global terrestrial water and biochemical cycles, the region remains largely understudied compared to other major rainforests. Motivated by the need to better understand changes in convection and its associations with the drying trend, this dissertation aims to investigate variations and trends in convection from a diurnal perspective and explores relevant physical processes over the region. First, trends in thunderstorm activity were investigated from 1983 to 2018 across all seasons, i.e., December-February (DJF), March-May (MAM), June-August (JJA), and September-November (SON). An analysis of Gridded Satellite (GridSat-B1) cold cloud fractions (i.e., areas with low brightness temperatures) and the Gálvez-Davison Index (GDI) reveals an increase in thunderstorm activity throughout all seasons, primarily linked to a decrease in hydrostatic stability. More specifically, the observed changes can be attributed to a decrease in temperature at 500 hPa, an increase in the temperature gradient between 700 and 950 hPa, and a decrease of the equivalent potential temperature (θe) gradient with height. These changes collectively act to decrease vertical stability and subsequently increase convective activity over the region. Second, to increase our understanding of trends in thunderstorms over the Congo, changes in the diurnal cycle of convection were investigated from 1979 to 2019, and possible connections to the observed drying trend were explored. An analysis of the fifth generation of the European Centre for Medium Range Weather Forecasts reanalysis data (ERA5) as well as GridSat-B1 and MODerate Resolution Imaging Spectroradiometer (MODIS) satellite data highlight a decrease in clouds in the morning, contrasted by an increase in clouds and convective activity in the afternoon, during all four seasons. Additionally, findings revealed a decrease in precipitation efficiency (i.e., the ratio between rainfall and total column water) and an increase in cloud base height. These detected changes can potentially be connected to the drying conditions over the region. Elevated surface temperatures and reduced surface relative humidity will likely raise the lifting condensation level, and ultimately decrease the precipitation efficiency. These factors would collectively exacerbate the existing drying trend. Third, the impacts of the Madden-Julian Oscillation (MJO) on the diurnal cycles of convection and precipitation were evaluated during October through March from 1985 to 2019. GridSat-B1 and Tropical Rainfall Measuring Mission (TRMM) satellite data were used in conjunction with ERA5 reanalysis data, aiming to investigate the differences in the diurnal cycles of convection and precipitation during the MJO enhanced convective phase (RMM phases 1 and 2) versus the MJO suppressed convective phase (RMM phases 5 and 6). Results indicate increased precipitation and convection during the MJO enhanced convective phase and decreased precipitation and convection during the MJO suppressed convective phase as would be expected, where the largest differences were discovered during the morning hours, the time of day when convective processes are weakest. Additionally, the MJO’s impact on stratiform precipitation was found to be more pronounced, while the convective precipitation fractions remain similar during both MJO phases. These identified changes were attributed to MJO induced shifts in circulation patterns: Enhanced upward air motion in the mid- and upper levels coupled with strong divergence in the upper levels act to increase stratiform rainfall during the enhanced phase. In contrast, the suppressed phase is characterized by strong mid-level divergence and upper- to mid-level subsidence, mainly during the nighttime and morning hours, resulting in decreased convection and precipitation. Finally, the impacts of drying soils associated with the Congo drying trend on atmospheric stability and convection were investigated by using the Weather Forecasting and Research (WRF) model. The WRF model was run at a 4 km convection permitting resolution, and hourly ERA5 reanalysis data were used for initial and boundary conditions. Two sets of WRF simulations were conducted, i.e., one dry season case (16-January to 21-January 2018) and one wet season case (21-October to 26-October 2019), resulting in two six-day case studies. For the experiment run (EXP), the soil moisture was decreased by 50% compared to the control run (CTL). No other changes were made to the EXP run. Results highlight a decrease in low-level clouds with drier soils, which was attributed to the simulated decrease in near surface water vapor and increase in temperature in the lower levels. Conversely, mid- and high-level clouds were increased. Factors contributing to the observed increase in clouds include increased lifting condensation level (LCL) and planetary boundary layer (PBL) heights and decreased atmospheric stability due to the warming of the lower levels. Additionally, simulated circulation changes such as increased vertical velocity as well as increased near surface convergence and mid-level divergence likely contribute to the increase in mid-level clouds. Comparing both case studies, the changes during the dry season were more pronounced, possibly due to the already limited moisture availability, while the changes during the wet season were comparatively more subtle.
Recommended Citation
Alber, Kathrin, "Understanding Variations And Trends In Convection Over The Congo Basin From A Diurnal Perspective" (2024). Legacy Theses & Dissertations (2009 - 2024). 3284.
https://scholarsarchive.library.albany.edu/legacy-etd/3284
