An Objective Climatology of Tropical Cyclone Diurnal Pulses in the Atlantic Basin

Abstract

Storm-centered IR brightness temperature imagery was used to create 6-h IR brightness temperature difference fields for all Atlantic basin tropical cyclones from 1982 to 2017. Pulses of colder cloud tops were defined objectively by determining critical thresholds for the magnitude of the IR differences, areal coverage of cold-cloud tops, and longevity. Long-lived cooling pulses (≥9 h) were present on 45% of days overall, occurring on 80% of major hurricane days, 64% of minor hurricane days, 46% of tropical storm days, and 24% of tropical depression days. These cooling pulses propagated outward between 8 and 14 m s−1. Short-lived cooling pulses (3–6 h) were found 26.4% of the time. Some days without cooling pulses had events of the opposite sign, which were labeled warming pulses. Long-lived warming pulses occurred 8.5% of the time and propagated outward at the same speed as their cooling pulse counterparts. Only 12.2% of days had no pulses that met the criteria, indicating that pulsing is nearly ubiquitous in tropical cyclones. The environment prior to outward propagation of cooling pulses differed from warming pulse and no pulse days by having more favorable conditions between 0000 and 0300 LT for enhanced inner-core convection: higher SST and ocean heat content, more moisture throughout the troposphere, and stronger low-level vorticity and upper-level divergence.

Quantification and Exploration of Diurnal Oscillations in Tropical Cyclones

“…In total, 41% and 51% of TS and mH cases have indications of in-phase diurnal oscillations, respectively. The percentages for TS (41%), mH (51%), and MH (61%) presented here are similar, although a bit lower, to those found by Ditchek et al (2019), who examined the same 6-hourly IR differences as D14 in the Atlantic basin , developed a different diurnal metric based on D14's clock for classification, and found TS (46.0%), mH (63.8%), and MH (79.5%) have long-lived cold pulses in IR images. The pervasiveness of diurnal variations in all TCs is a fortuitous result in that we can explore the significant differences found in this larger sample of ALL TC cases and look beyond the MH environments that are very similar among the different diurnal stratifications.…”

“…This allows for a closer examination of the latitudinal behavior of the environmental conditions that appears to be most related to the strength and regularity of the diurnal convection in TCs. Figure 7a shows that SD and MD cases are generally more intense, again agreeing with D14 and Ditchek et al (2019). Figure 7b shows the zonal means of RHHI-for most cases (e.g., in latitudes from 158 to 308), it appears that higher RHHI values are typically related to more pronounced diurnal oscillations.…”

“…specification of a radial region, differencing period, or brightness temperature threshold. Given our different approach, it is rather encouraging to see other recent work (see Ditchek et al 2019) also endorse categorizing the type of diurnal signal present in observed TCs.…”

Using persistent homology to quantify a diurnal cycle in hurricanes

“…Previous research has documented a clear diurnal cycle of cloudiness and rainfall in TCs: enhanced convection (i.e., thunderstorms) occurs overnight, precipitation peaks near sunrise, and upper-level cloudiness (i.e., the cirrus canopy) expands radially outward throughout the day, reaching its maximum areal coverage in the early evening hours ( [4][5][6][7]14,16,20,21,29,30] Despite the consistent signature and documentation of this diurnal cloud signature, open questions remain as to how the diurnal cycle is linked to inner-core convective processes and whether it is a column-deep phenomenon or mainly tied to upper-level TC cloud dynamics related to incoming solar radiation ( [4, 20, 21, 30]). Investigating these questions is relevant to TC forecasting as the diurnal cycle of clouds and rainfall has implications for forecasting storm structure and intensity, as evidenced by the diurnal cycle in objective measures of TC intensity and the extent of the 50-kt wind radius documented by Dunion et al Additionally, and especially relevant to the current work, most of the papers above have identified the pulse using subjective measures of cloud-top temperature change and timing ( [4,7]).…”

“…The area of cooling then takes on a ring-like appearance as marked cloud-top warming occurs on its inside edge and it moves away from the storm overnight, reaching several hundred kilometers from the TC center by the following afternoon. Observations and numerical model simulations indicate that TC diurnal pulses propagate through a deep layer of the TC environment, suggesting that they may have implications for TC structure and intensity ( [4][5][6][7]16]).…”