Role of baroclinic trough in triggering vertical motion during summertime heavy rainfall events in Korea

The nature of the vertical motion responsible for the summertime (June–September) heavy rainfall events (HREs) in Korea is quantitatively examined. By compositing 318 HREs in 1979–2018, it is found that the synoptic conditions of the HREs are typically characterized by a developing surface cyclone with a southwesterly low-level jet on its southeastern flank and an upper-level trough to the west of the HREs. This baroclinic environment allows for well-organized vertical motion over Korea at the equatorward side of the upper-level jet entrance. The relative importance of dynamic and diabatic forcings in driving the vertical motion is further quantified by solving the quasi-geostrophic omega equation. It turns out that the dynamic forcing, defined as Q-vector convergence, is comparable to the diabatic forcing in the developing stage of the HREs. The diabatic forcing, however, becomes more important in the mature stage as latent heating rapidly increases. The decomposition of Q-vector into the transverse and shearwise components reveals that the dynamic uplift is largely caused by the shearwise Q-vector convergence which is closely related to the developing trough in the upper-to-middle troposphere on the west of the HREs. This result indicates that the HREs in Korea are organized by the baroclinic trough coupled to moist processes, with a minor contribution of the thermally-direct secondary circulation at the entrance region of the upper-level jet.

Dynamical analysis of extreme tropopause folding events in the coastal region of Antarctica

<p>Rapid and deep descent in the tropopause (the so-called tropopause folding; TF) is often observed in the extratropics. Previous studies pointed out that the frequency of deep TF is maximized along the coast of Antarctica. However, the dynamics of TF in the Antarctic region have not yet been studied adequately. In the present study, the extreme TF in the Antarctic are examined using the state-of-art reanalysis data to clarify the uniqueness of TF in the Antarctic.</p><p>First, the distribution of TF frequency in the extra-tropics of the Southern hemisphere is examined. In austral winter, extreme TF often occurs along the coast of Antarctica. Around Syowa Station (69.0S, 39.6E), the frequency of extreme TF is maximized in August while the frequency is small in austral summer. It is interesting that the coast of Antarctica is located to the south of the maximum of the synoptic-scale eddy kinetic energy. This implies that the maximum of TF frequency along the coast of Antarctica cannot be explained only by the geographical distribution of the storm track.</p><p>Next, to examine the dynamics of the extreme TF events along the coast of Antarctica, we perform composite analyses of the extreme TF events at Syowa Station. When the negative anomaly of tropopause height is maximized, the significant downwelling is observed at the location of the extreme TF. From the analyses using the quasi-geostrophic Q-vector, it is found that the divergence of the Q-vector is observed around Syowa Station. The distribution of Q-vector is explained by the local westerly jet and strengthening of the frontal structure associated with a synoptic low-pressure system extending west-east centered at 70°S over Antarctica. The mechanism of the low-pressure system extending along the coast of Antarctica based on ray-tracing theory under the WKB approximation is also discussed.</p>

Modulated Structures, Microstructures and Subsolidus Phase Relations of Labradorite Feldspars

The coupled substitution between Na+Si and Ca+Al, in the plagioclase solid solution, results in a continuous variation in the Al/Si ratio of the composition, which is the reason for the complicated ordering patterns in the intermediate plagioclase feldspars such as labradorite. Both fast-cooled and slow-cooled labradorite feldspars display the incommensurately modulated structures. The ordering pattern in the incommensurately modulated structures of e-plagioclase (characterized by the satellite diffraction peak called e-reflections) is the most complicated and intriguing. The modulated structure has a super-space group symmetry of X(αβγ)0, with a special centering condition of (½ ½ ½ 0), (0 0 ½ ½), (½ ½ 0 ½), and the q-vector has components (i.e., δh, δk, δl) along all three axes in reciprocal space. Displacive modulation, occupational modulation, and density modulation are observed in slowly cooled labradorite feldspars. No density modulation was observed in fast cooled (volcanic) labradorite feldspars. The amplitudes of the modulation waves are new parameters for quantifying the ordering state of labradorite. Iridescent labradorite feldspars display exsolution lamellae with an average periodicity ranging from ~150 nm to ~350 nm. Compositional difference between the lamellae is about 12 mole % in anorthite components. Areas or zones with red (or yellow) iridescent color (i.e., long lamellae periodicity) always contain more Ca (~1 to 3 mole %) than the areas with blue (or green) iridescent color within the same labradorite crystal. We proposed that the solvus for Bøggild intergrowth has a loop-like shape, ranging from ~An44 to ~An63. The Ca-rich side / zone has higher exsolution temperature than the Na-rich side / zone. The shapes of satellite peaks, the distances between e-reflections (modulation periods), and even the intensity of e-reflections may also be used to evaluate the ordering state or cooling rate of the plagioclase feldspar. Both modulated structure and the exsolution lamellae can be used as proxies for quantifying cooling rate of a labradorite and it’s host rock.

Inertia-Gravity Wave Generation Near Upper Jets in the Australian Region

<p>During rapid extratropical cyclogenesis in the Australian region, distinctive striated, triangular-shaped clouds commonly form on the poleward side of the jet exit near the axis of inflection between an upstream trough and downstream ridge. These clouds are called striated deltas and the striations are shown here to be caused by radiating inertia-gravity waves. An analysis of 28 striated delta clouds shows that the striated deltas have a mean length of 1034 km and width of 537 km, and a striation wavelength of 74 km. Large parcel accelerations, nonlinear flow imbalance and active convection within the striated delta are features of the composite upper-tropospheric environment. Patterns of Q-vector illustrates the unique shape of striated delta clouds to be coincident with a delta-shaped forcing of adiabatic ascent in the poleward jet exit. One of the extratropical cyclones analysed and its associated striated delta is simulated with WRF-ARW both with and without diabatic heating. Both simulations produce pronounced gravity wave packets along the surface cold front, along the downstream upper jet axis and a delta-shaped packet in the lower stratosphere above the jet exit. Ray tracing identifies the source region of the waves in the stratosphere to be the upper jet. Vertically-radiating gravity waves originating in the vicinity of the jet during rapid extratropical cyclogenesis, propagate both upwards and downwards, imprinting striations onto the cloud in the jet exit and setting the spacing between the convective bands.</p>

The Motion of Mesoscale Snowbands in Northeast U.S. Winter Storms

The spatial distribution of snowfall accumulation accompanying winter storms is a product of both snowfall rate and duration. Winter storms are commonly associated with mesoscale snowbands that can strongly modulate snowfall accumulation. Although the development of mesoscale snowbands can usually be anticipated, snowband residence time at a fixed location is often a forecasting challenge. However, given that snowband residence time is related to characteristics of band motion, an improved understanding of band motion presents an opportunity to improve snowfall-accumulation forecasts. This study investigates environmental features associated with specific snowband motion characteristics. Using radar reflectivity data, snowband events in the northeast United States spanning a 6-yr period are categorized according to a band-motion classification scheme, with this scheme consisting of laterally translating, hybrid, laterally quasi-stationary, and pivoting snowbands. On the basis of this classification, composite analysis is performed to identify common environmental features associated with particular band-motion categories. Results indicate that snowband motion is related to cyclone-relative band position, the confluence/diffluence and curvature of midlevel streamlines, and the distribution of horizontal temperature advection. Snowband motion is also related to hodograph shape, as well as to the across- and along-isotherm components of the Q vector. Composite results are supplemented with case studies, which suggest that laterally quasi-stationary and pivoting snowbands can favor distinct gradients in snowfall accumulation. The present study proposes that snowband motion warrants consideration during the forecasting process and, to that end, conceptual models are presented to synthesize key findings for operational application.

Y3Ru2−x—A Representative of a Composite Modulated Family of Intermetallics

The compound Y3Ru2−x was synthesized from the elements and the structure was solved from single crystal synchrotron data. The high quality of the data allowed the determination of the incommensurate ordering of the compound, previously reported as disordered, with respect to the second subsystem. The compound crystallizes in the super space group X-3(00γ)0 with the q-vector axial along c*, q = 00, λ = 0.4276(7) and the centering vectors (1/3 2/3 0 1/3), (2/3 1/3 0 2/3).

The role of hydrogen bonds in order-disorder transition of a new incommensurate low temperature phase β-[Zn-(C7H4NO4)2]·3H2O

The room temperature structure withP21/csymmetry of the zinc(II) complex of pyridine-2,6-dicarboxylic acid was published by Okabe and Oya (N. Okabe, N. Oya, Copper(II) and zinc(II) complexes of pyridine-2,6-dicarboxylic acid.Acta Crystallogr. C.2000,56, 305). Here we report crystal structure of the low temperature phaseβ-[Zn(pydcH)2]·3H2O, pydc=C7H3NO4, resulting from the phase transition around 200K. The diffraction pattern of the low temperature phase revealed satellite reflections, which could be indexed with q-vector 0.4051(10)b* corresponding to (3+1)Dincommensurately modulated structure. The modulated structure was solved in the superspace groupX21/c(0b0)s0, whereXstands for a non-standard centring vector (½, 0, 0, ½), and compared with the room temperature phase. It is shown that hydrogen bonds are the main driving force of modulation.