Formation, Thermodynamic Structure, and Airflow of a Japan Sea Polar Airmass Convergence Zone

Abstract

The Sea of Japan (SOJ) coast and adjoining orography of central Honshu, Japan receive substantial snowfall each winter. A frequent contributor during cold-air outbreaks (CAOs) is the Japan Sea Polar-Airmass Convergence Zone (JPCZ), which forms downstream of the Korean Highlands, extends southeastward to Honshu, and generates a mesoscale band of precipitation. Mesoscale polar vortices (MPVs) ranging in horizontal scale from tens (i.e., meso-β-scale cyclones) to several hundred kilometers (i.e., “polar lows”) are also common during CAOs and often interact with the JPCZ. Here we use satellite imagery and Weather Research and Forecast model (WRF) simulations to examine the formation, thermodynamic structure, and airflow of a JPCZ that formed in the wake of an MPV during a CAO from 2–7 February 2018. The MPV and its associated warm seclusion and bent-back front developed in a locally warm, convergent, and convective environment over the SOJ near the base of the Korean Peninsula. The nascent JPCZ was structurally continuous with the bent-back front and lengthened as the MPV migrated southeastward. Trajectories illustrate how flow splitting around the Korean Highlands, channeling through low passes and valleys along the Asian coast, and air-sea interactions affect the formation and thermodynamic structure of the JPCZ. Contrasts in airmass origin and thermodynamic modification over the SOJ affect the cross-JPCZ temperature gradient, which reverses in sign along the JPCZ from the Asian coast to Honshu. These results provide new insights into the thermodynamic structure of the JPCZ, which is an important contributor to hazardous weather over Japan.

High moisture confluence in Japan Sea polar air mass convergence zone captured by hourly radiosonde launches from a ship

“…Previous numerical simulations showed that JPCZ initiation is caused by the confluence of two sets of the airflows that go northward and southward around Mt. Paektu 9 . However, no direct observations of this confluence or the strength of this confluence have been made.…”

“…This current is referred to as the Tsushima Warm Current. The behaviour of the Tsushima Warm Current is also expected to play an important role in the development of the JPCZ because the warm sea supplies the overlying cold airmass with large amounts of water vapour and heat 9,[12][13][14][15][16] . The heated surface airmass has the potential to weaken atmospheric vertical static stability because cold air with high density overlies the warm surface air with low density.…”

Formation of maritime convergence zones within cold air outbreaks due to the shape of the coastline or sea ice edge

“…We investigate the effect of a difference in the fetch for the formation of a convergence zone, where the shape of a coastline or sea‐ice edge is a primary factor determining a difference in the fetch. Although topography can also cause an asymmetry in the fetch (West and Steenburgh, 2022), we will focus on the role of the shape of a coastline or sea‐ice edge in the formation of a convergence zone to isolate this effect. Other factors, such as topography, are of interest for future work.…”

Using a Pollution-to-Risk Method to Evaluate the Impact of a Cold Front: A Case Study in a Downstream Region in Southeastern China

“…In the frontal zone, cold air advances into warm air, thereby creating convergence between the two air masses [21]. This convergence forces the warm air to ascend in the frontal region and noticeably enhance condensation rates [22]. With a southward-moving cold front, surface wind in the frontal zone is governed by the convergence of cold air from the north and warm air from the south.…”