Dropsonde-Derived Moist Static Energy Variability in Atlantic Hurricanes

TITLE: Dropsonde-Derived Moist Static Energy Variability in Atlantic Hurricanes

AUTHORS: Michael Kopelman, Jacob Carstens, Allison A. Wing

Interactions between clouds, water vapor, radiation, and mesoscale circulations represent important feedbacks on tropical cyclone (TC) development, such as cloud longwave radiative effects. Understanding and quantifying these interactions requires knowing how moist static energy (MSE) and its column-integral (CMSE) vary spatially around the TC. Dropsondes from aircraft reconnaissance can directly sample MSE, providing a useful observational tool to address this question. Though dropsondes are limited in number and spatial coverage, recent modeling work suggests that they can faithfully resolve a TC’s radial MSE and CMSE variability. Therefore, this study uses upper-level reconnaissance dropsonde data from North Atlantic TCs spanning from the late 1990s until present to evaluate the spatial variability of MSE and CMSE around TCs. The Tropical Cyclone – Dropsonde Research and Operations Product Suite (TC-DROPS; Nguyen et al 2019) database contains thousands of dropsondes that sample TCs of different intensity, structure, size, and geographic location across the North Atlantic, which facilitates testing the sensitivity of MSE and CMSE variability to these properties. Ultimately, the objective of this work is to generate a TC-centric climatology of MSE, which is important for understanding TC development.

Despite differences in location, TC intensity, and flight pattern including inner and outer radial coverage, MSE has so far been found to have notably similar structures across different TCs. Therefore, we construct radius-azimuth-height composites of MSE over all TCs in the TC-DROPS dataset. MSE decreases with distance from the TC center due to the amplified warmth and moisture in the inner core. This decrease is sharper near the inner core, then more gradual at outer radii. MSE variability is also partitioned into separate contributions from temperature and moisture. Doing so reveals that moisture variability is dominant throughout the TC. However, the temperature contribution is non-negligible near the TC center, contributing about 25% of the overall MSE anomaly there due to the warm core, particularly at upper levels. With regards to intensity, we find an amplification of MSE radial variability as intensity increases from Tropical Storm-strength to Category 2-strength on the Saffir-Simpson Hurricane Wind Scale. Moist areas near the inner-core become moister and dry areas in the outer region become drier as TC intensity increases. The warm core temperature anomaly within 400 km of the TC center strengthens with TC intensity in the upper levels but weakens in the mid to low levels. We also consider MSE composited over TCs of different intensification rates which shows an overall increase of MSE in rapidly intensifying compared to steady state storms.

References: Nguyen, L. T., R. Rogers, J. Zawislak, and J. A. Zhang, 2019: Assessing the Influence of Convective Downdrafts and Surface Enthalpy Fluxes on Tropical Cyclone Intensity Change in Moderate Vertical Wind Shear. Mon. Wea. Rev., 147, 3519–3534.