Groenen, D., & Misra, V. (2016). Characterization of the Rainy Season of Mesoamerica.. American Meteorological Society.
Groenen, D. E. (2019). Diagnosing the Atmospheric Phenomena Associated with the Onset and Demise of the Rainy Season in Mesoamerica.
Abstract: Mexico and Central America (Mesoamerica) are situated in a complex and unique geographical position with the Caribbean Sea to the East and the tropical Eastern Pacific Ocean to the West. The weather patterns of this region are driven by winds, temperatures, moisture, and orography of several mountain ranges. This study finds the dates of the onset and demise of rainfall regimes on a specific day using NASA’s Tropical Rainfall Measuring Mission (TRMM) rainfall for years 1998–2012, area-averaged over land. Using NASA’s MERRA-2 Reanalysis data, we also look at the phenomenology of the triggers of the rainy season onset and demise on the daily time-scale instead of the monthly scales used by previous studies.
We find that the Mesoamerican Rainy Season can be distinguished into two parts: the Early Spring Rainfall (ESR) associated with light rains and the Late Spring Rainfall (LSR) associated with heavy rains. Two algorithms are used to obtain these rainy season distinctions. A new algorithm was developed during this study, called the SLOPE algorithm, to calculate when the rain rates first start to increase. In the second method, the daily cumulative anomalies of rainfall are compared to the climatological rainfall to find the time of onset of the heavy rains, called the MINCA algorithm. To better understand the phenomenology associated with the timing of the rainfall, we look at the monsoon trough, moisture flux convergence, moist static energy anomalies, and the weakening/strengthening of the winds associated with the Caribbean Low-Level Jet and Panama Jet.
The light rain rates begin, on average, in mid-March, approximately one month after the peak of the winter Caribbean Low-Level Jet and the Panama Jet. The ramp-up between the light rains and heavy rains is associated with a significant weakening of both jets and the northward progression of a monsoon trough off the western coast of Central America. The heavy rain rates begin, on average, in mid-May, and are associated with the timing when the Panama Jet goes to near zero magnitude and a strong monsoon trough in the eastern Pacific. At the demise of the rainfall, approximately in mid-November, the Panama Jet strengthens again, the total moisture flux convergence decreases significantly, and the monsoon trough retreats southward and eastward. The results of this study have positive implications in agriculture and water resources for Mesoamerica, as this information may help resource managers better plan and adapt to climate variability.
Guerra, L. A. A., Paiva, A. M., & Chassignet, E. P. (2018). On the translation of Agulhas rings to the western South Atlantic Ocean.
Deep Sea Research Part I: Oceanographic Research Papers, 139, 104–113.
Abstract: The shedding of Agulhas rings is the primary process connecting the Indian and Atlantic oceans. The rings transport warm and salty waters that feed the surface limb of the Atlantic Meridional Overturning Circulation. Early studies suggest that Agulhas rings decay and diffuse their contents within the South Atlantic subtropical gyre. In this paper, we update the ring census using an automated algorithm to detect and track eddies over more than 23 years of satellite altimetry data (1993-2016) and calculate their main characteristics. While 140 rings spawned from the Agulhas Retroflection, their following splitting and merging resulted in 74 long-lived rings that crossed the Walvis Ridge and translated towards the west. Eventually, three rings reached the western boundary. For one of them, we use in situ measurements to document its interaction with the Brazil Current and two cyclonic eddies, which resulted in a current velocity increase by three times. Although already hypothesized, this interaction had not been demonstrated with in situ evidence until now.
Guimond, S. (2010).
Tropical Cyclone Inner-Core Dynamics: A Latent Heat Retrieval and Its Effects on Intensity and Structure Change; and the Impacts of Effective Diffusion on the Axisymmetrization Process. Ph.D. thesis, Florida State University, Tallahassee, FL.
Abstract: Despite the fact that latent heating in cloud systems drives many atmospheric circulations, including tropical cyclones, little is known of its magnitude and structure due in large part to inadequate observations. In this work, a reasonably high-resolution (2 km), four-dimensional airborne Doppler radar retrieval of the latent heat of condensation is presented for rapidly intensifying Hurricane Guillermo (1997). Several advancements in the retrieval algorithm are shown including: (1) analyzing the scheme within the dynamically consistent framework of a numerical model, (2) identifying algorithm sensitivities through the use of ancillary data sources and (3) developing a precipitation budget storage term parameterization. The determination of the saturation state is shown to be an important part of the algorithm for updrafts of ~ 5 m s-1 or less. The uncertainties in the magnitude of the retrieved heating are dominated by errors in the vertical velocity. Using a combination of error propagation and Monte Carlo uncertainty techniques, biases were found to be small, and randomly distributed errors in the heating magnitude were ~16 % for updrafts greater than 5 m s-1 and ~156 % for updrafts of 1 m s- 1. The impact of the retrievals is assessed by inserting the heating into realistic numerical simulations at 2 km resolution and comparing the generated wind structure to the Doppler radar observations of Guillermo. Results show that using the latent heat retrievals outperforms a simulation that relies on a state-of-the-art microphysics scheme (Reisner and Jeffery 2009), in terms of wind speed root-mean-square errors, explained variance and eye/eyewall structure. The incorrect transport of water vapor (a function of the sub-grid model and the numerical approximations to advection) and the restrictions on the magnitude of heat release that ensure the present model's stability are suggested as sources of error in the simulation without the retrievals. Motivated by the latent heat retrievals, the dynamics of vortex axisymmetrization from the perspective of thermal anomalies is investigated using an idealized, non-linear atmospheric model (HIGRAD). Attempts at reproducing the results of previous work (Nolan and Grasso 2003; NG03) revealed a discrepancy with the impacts of purely asymmetric forcing. While NG03 found that purely asymmetric heating led to a negligible, largely negative impact on the vortex intensification, in the present study the impacts of asymmetries are found to have an important, largely positive role. Absolute angular momentum budgets revealed that the essential difference between the present work and that of NG03 was the existence of a significant, axisymmetric secondary circulation in the basic-state vortex used in the HIGRAD simulations. This secondary circulation was larger than that present in NG03's simulations. The spin-up of the vortex caused by the asymmetric thermal anomalies was dominated by the axisymmetric fluxes of angular momentum at all times, indicating fundamentally different evolution of asymmetries in the presence of radial flow. Radial momentum budgets were performed to elucidate the mechanisms responsible for the formation of the physically significant secondary circulation. Results show that explicit (sub-grid) diffusion in the model was producing a gradient wind imbalance, which drives a radial inflow and associated secondary circulation in an attempt to re-gain balance. In addition, the production of vorticity anomalies from the asymmetric heating was found to be sensitive to the eddy diffusivity, with large differences between HIGRAD and the widely used WRF model for the exact same value of this uncertain parameter.
Guimond, S. R. (2007).
A diagnostic study of the effects of trough interactions on tropical cyclone QPF. Master's thesis, Florida State University, Tallahassee, FL.
Abstract: A composite study is presented analyzing the influence of upper-tropospheric troughs on the evolution of precipitation in twelve Atlantic tropical cyclones (TCs) between the years 2000 � 2005. The TRMM Multi-Satellite Precipitation Analysis (TMPA) is used to examine the enhancement of precipitation within a 24 h window centered on trough interaction (TI) time in a shear-vector relative coordinate system. Eddy angular momentum flux convergence (EFC) computed from European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses is employed to objectively determine the initiation of a TI while adding insight, along with vertical wind shear, into the intensification of TC vortices. The relative roles of the dynamics (EFC and vertical wind shear) and thermodynamics (moist static energy potential) in TIs are outlined in the context of precipitation enhancement that provides quantitative insight into the “good trough”/“bad trough” paradigm. The largest precipitation rates and enhancements are found in the down-shear left quadrant of the storm, consistent with previous studies of convective asymmetries. Maximum mean enhancement values of 1.4 mm/h are found at the 200 km radius in the down-shear left quadrant. Results indicate that the largest precipitation enhancements occur with “medium” TIs; comprised of EFC values between 17 � 22 (m/s)/day and vertical wind shear Sensitivity tests on the upper vertical wind shear boundary reveal the importance of using the tropopause for wind shear computations when a TC enters mid-latitude regions. Changes in radial mean precipitation ranging from 29 � 40 % across all storm quadrants are found when using the tropopause as the upper boundary on the shear vector. Tests on the lower boundary using QuikSCAT ocean surface wind vectors expose large sensitivities on the precipitation ranging from 42 � 60 % indicating that the standard level of 850 hPa, outside of the boundary layer in most storms, is more physically reliable for computing vertical wind shear. These results should help to improve TC quantitative precipitation forecasting (QPF) as operational forecasters routinely rely on crude statistical methods and rules of thumb for forecasting TC precipitation.
Guimond, S. R., Turk, J., Blankenship, C., & Hawkins, J. (2006). Detecting tropical cyclone structural change with the TRMM Precipitation Radar (PR) and Advanced Microwave Sounding Unit (AMSU). In
86th Annual American Meteorological Society Meeting, Office of Naval Research, Atlanta, Georgia, USA.
Guimond, S. R., Bourassa, M. A., & Reasor, P. D. (2011). A Latent Heat Retrieval and Its Effects on the Intensity and Structure Change of Hurricane Guillermo (1997). Part I: The Algorithm and Observations.
J. Atmos. Sci., 68(8), 1549–1567.
Guimond, S. R., Heymsfield, G. M., & Turk, F. J. (2010). Multiscale Observations of Hurricane Dennis (2005): The Effects of Hot Towers on Rapid Intensification.
J. Atmos. Sci., 67(3), 633–654.
Guimond, S. R., & Reisner, J. M. (2012). A Latent Heat Retrieval and Its Effects on the Intensity and Structure Change of Hurricane Guillermo (1997). Part II: Numerical Simulations.
J. Atmos. Sci., 69(11), 3128–3146.
Gulev, S. K., S. A. Josey, M. A. Bourassa, L.-A. Breivik, M. F. Cronin, C. Fairall, S. Gille, E. C. Kent, C. M. Lee, M. J. McPhaden, P. M. S. Monteiro, U. Schuster, S. R. Smith, K. E. Trenberth, D. Wallace, and S. D. Woodruff. (2010). Surface Energy, CO2 Fluxes and Sea Ice. In D. D.E. and Stammer Harrison J. Hall (Ed.),
Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society (Vol. 1).