On the Diurnal Cycle in UTLS Mesoscale Potential Vorticity Variability Using 9-km Resolution ECMWF Operational Forecast Analyses: A Climatology of South Pacific Tropical Cyclones During 2016 – 2024
AbstractAbstract
This study investigates mesoscale potential vorticity (PV) variability in the upper troposphere and lower stratosphere (UTLS) in tropical cyclones (TCs) using 9-km-resolution, 6-hourly ECMWF operational forecast analyses, to characterize its behavior during TC life cycles. TCs exhibit a diurnal cycle in deep convection with a maximum overnight. Since mesoscale variability of UTLS PV is caused by deep convection, it should also exhibit a diurnal cycle. Spatial standard deviations
σPV
and means are calculated in a 6° × 6° domain following a storm track. The distinctive maximum in
σPV
at the tropopause is caused by the upward increase in mean static stability. A key time was chosen to be the first 1800 LT after reaching category 1 (C1). All storms reached category 2 (C2) the night after the key time. We required at least a 6-day track and compared 3 days before and after reaching C2. This method was first applied to TC Yasa. We then analyzed a total of 23 TCs which met these selection criteria in the South Pacific during 2016–24 [13 C1–C2 and 10 category 3 (C3)–category 5 (C5)]. A strong diurnal cycle in 100-hPa
σPV
was found, with a development phase (days 1–3) and a mature phase (days 4–6). From days 1–3 to days 4–6, there is a reduction in mean
σPV
by ∼25% and in diurnal range by ∼45% for C1–C2 and ∼60% for C3–C5 and elimination of the preference for a 0600 LT peak. These results are consistent with cloud-radiative forcing theories of TCs.
Significance StatementWe investigated variability of mesoscale potential vorticity (PV) in tropical cyclones (TCs) in high-resolution global forecast model output to characterize its behavior and contribute toward understanding TC life cycles. We analyzed 23 TCs in the South Pacific during 2016–24. The spatial standard deviation of PV (the product of relative vorticity and static stability) is a sensitive measure of updrafts which reach the tropopause. It exhibits a diurnal cycle at the tropopause, with a 1800 LT minimum. Prior to reaching strength C2 (development phase), peak amplitude tends to occur at 0600 LT. After reaching C2 (mature phase), this preference disappears, and its range decreases by half. These results are consistent with cloud-radiative forcing theories of the diurnal cycle in TCs.
