Dual-Doppler Radar Observations on Factors Causing Differences in the Structure of Snow Clouds during Winter Monsoon Surges

Abstract

Various processes of development in a convective cell were investigated in order to clarify the factors causing differences in the structure of snow clouds. Convective cells in snow clouds were observed by two Doppler radars over Ishikari Bay, Hokkaido, Japan, during winter monsoon surges. Seven cells that allowed us to examine structural changes were selected for the investigation. From the time series of maximum reflectivity and vertical mass flux in each cell, the analyzed cells were classified into two groups: ''rapid change'' and ''slow change.'' The former was further divided into two types according to the strength of vertical wind shear in the environment: rapid change in weak shear (Rw), and rapid change in strong shear (Rs). The structure of the Rw-type changed greatly, due to the formation and the descent of a high-reflectivity core (> 30 dBZ), and the subsequent dissipation of the updraft. Since the updraft hardly tilted and collapsed when the core descended in the updraft, loading by graupel particles was suggested to be effective to terminate the updraft. The structure of the Rs-type changed greatly due to the short lifetime of the updraft that was tilted toward the downshear side due to the strong vertical shear. In contrast, the cell of slow-change group (S-type) maintained its structure because of a persistent updraft. Since the updraft, which was tilted hardly, was sustained even when a core (a 30 dBZ) descended in the updraft region, the loading effect was thought to be small due to the small amount of graupel particles. From a comparison of the three types, it was found that the different features of the cell's structure were caused by two factors: the tilt of the updraft, and the effect of loading by graupel particles. Based on these results, we discussed the maintenance process of a snow cloud composed of convective cells. The fact that convective cells in the past observational studies can also be classified in the same way as cells analyzed in the present study, was also discussed. The relationship between the growth of graupel particles, and the temperature in the environment, was also discussed.

Seeking reasons for the differences in size spectra of electrified storms over land and ocean

“…Like other boxes, the NCAPE drops precipitously in the winter months (Figure b), and there is a slight shift to higher vertical velocities (Figure c); however, the shift in RPF size in the winter months is to smaller RPF sizes, unlike the oceanic seasonal progressions shown above. This region experiences most of its lightning during the winter months when there is advection of cold, dry Siberian air masses over the relatively warm water, providing enough instability to create strong convection at low levels, and thereby, increase the probability of lightning [ Kitagawa and Michimoto , ; Yamada et al , ] despite the absence of external forcing (cyclones, fronts, or orographic) [ Magono et al , ]. When we compare the 700 and 850 mbar temperatures for the Northwest Pacific and the Northwest Atlantic (Figure ), both at the same latitude interval, both east of a landmass, both in Northern Hemisphere winter, the temperatures at the same altitude are significantly different.…”

Kinematic and Thermodynamical Structures of Longitudinal-Mode Snow Bands over the Sea of Japan during Cold-Air Outbreaks Part I: Snow Bands in Large Vertical Shear Environment in the Band-Transverse Direction

“…Although there is a large body of research on snow clouds over the Sea of Japan based on observations (Sakakibara et al 1988b;Ishihara et al 1989;Tsuboki et al 1989;Satoh et al 1992;Yamada et al 1994;Yamada et al 1996;Fujiyoshi et al 1998;Yoshimoto et al 2000;Yamada et al 2004;Yoshihara et al 2004;Kawashima and Fujiyoshi 2005), theories (Asai 1970), and numerical model simulations (Ikawa et al 1991;Murakami et al 1994a;Saito et al 1996), most do not treat the kinematic structures of L-mode snow bands under large band-transverse vertical shear. Even for wind-parallel bands occurring during ''lakee¤ect snow storms'' over Lake Michigan, numerous observational studies (e.g., Kelly 1982;Kristovich 1993;Kristovich et al 2003) and model simulations (e.g., Hsu 1987;Hjelmfeld 1990;Ballentine et al 1998) have not discussed band circulations in association with vertical shear.…”

2D Feature Space for Snow Particle Classification into Snowflake and Graupel

“…Sometimes the need of avoiding wrong classification is more important than the classification accuracy, what can be achieved by this system. For example, when we consider to estimate the relationship between radar reflectivity and snowfall rate, we may permit lower particle classification ratio, but the misclassification is inadmissible [16].…”

“…On the other hand, the brightness classification ratio in one-dimensional classification was low -75. 16, what in consequence diminish the roughness -84.48% -performance slightly in the case of J pair. Table 3 presents the classification ratio achieved for DT classifier.…”