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Pakistan experienced two major flood events in July 2010 and July–August 2022, resulting in over 1,500 fatalities. The 2010 floods primarily affected Northern Pakistan, while the 2022 floods severely impacted Southern Pakistan. This study examines rainfall patterns, moisture transport, and their connection to European heatwaves and blocking highs. In 2022, unprecedented rainfall in Southern Pakistan contrasted with the northern-focused rainfall in 2010. An intense cycle of low-pressure systems in 2022 transported moist air from the Indian Ocean, facilitated by southeasterly and southerly winds, and triggered deep convection, while in July 2010, southerly winds were dominant. Rossby wave breaking events in late July 2010 and late August 2022 caused the southward movement of upper-level vorticity perturbations, leading to heavy rainfall. Additionally, the heavy rainfall in Southern Pakistan in late August 2022 was influenced by concurrent intense low-pressure systems.

Original Paper

Ogata, T., Komori, N., Doi, T., Yamamoto, A., & Nonaka, M. (2024). Seasonal prediction system using CFES and comparison with SINTEX-F2. SOLA, 20, 92-101.

The Mozambique Channel trough (MCT) is one of the weather systems that affect Southern Africa including Mozambique, but has not received much attention. Through the empirical orthogonal function (EOF) analysis applied for austral summer-mean sea level pressure (SLP), interannual variability of the MCT is categorized into that of its intensity and zonal shift. The MCT intensity is significantly correlated to ENSO but is not correlated to regional precipitation in Mozambique. In contrast, the zonal shift of the MCT is not significantly correlated to ENSO but is correlated to regional precipitation there. A westward shift of the MCT is accompanied by the positive subtropical Indian Ocean dipole (SIOD) and strengthened Mascarene High (MH) that enhance moisture convergence over the Southern Indian convergence zone, inducing increased precipitation in southern and central regions of Mozambique. An eastward shift of the MCT is not necessarily accompanied by the SIOD but accompanied by weak suppression of the MH and precipitation there. However, if the eastward shift occurs simultaneously with the negative SIOD, it accompanies El Niño and prominent weakening of the MH, which should lead to stronger precipitation decrease in southern and central regions and increase in the northeast region of Mozambique.

This study investigates the dependency of the shortwave three-dimensional radiative transfer (3D RT) effects upon the grid width when simulating an isolated cumulus. A meteorological model coupled with a 3D RT model enables us to investigate the dependency through simulations which consider feedback of the 3D RT to the atmosphere. Using the coupled model, simulations of the isolated cumulus were conducted for grid widths ranging between 25 m and 500 m. The same series of simulations were also conducted using a conventional one-dimensional (1D) radiative transfer model and were compared with the results by the 3D RT model. The comparison clarifies that the discrepancy in liquid water path between the two RT schemes becomes more prominent with smaller grid widths. The comparison also indicates that the difference in the radiative heating is negligible for grid spacing of 500 m but was substantial for finer grid spacings. These results suggest that the 3D RT model is required to accurately simulate the feedback between clouds and shortwave radiation for an isolated cumulus with grid widths of at least 250 m or smaller, when the lateral edge of the cloud is spatially resolved.

In this study, we detected and analyzed cold air damming (CAD) and precipitation occurrences spanning 40 years from 1980 to 2019 in the Kanto region. We explored the climatological relationship between CAD and heavy precipitation. Of the 13.5 CAD events per year, 4.0 were associated with 24-h precipitation of 100 mm or more. In the southern Kanto region except for the western mountainous area, more than 25% of heavy precipitation was related to CAD, reaching over 40% in parts of the Boso Peninsula. CAD events with heavy precipitation exhibited consistent characteristics throughout seasons, including (1) a pronounced pressure gradient between the well-developed low-pressure off the southern coast of western Japan and the high-pressure on the northeast side of Japan, and (2) a strong moisture influx toward the Kanto region. These features highlight the crucial and general mechanisms of heavy precipitation formation through significant moisture ascent over the cold air dammed over the Kanto region.

In this study, we examine the characteristics of drop size distributions (DSDs) in the midlatitude maritime environment of the Sea of Japan. This study is based on two years of observational data collected using a disdrometer installed on Hegura-jima Island, a remote island in the Sea of Japan. The analysis revealed that there are two primary types of DSD regimes over the midlatitude maritime region: “larger drop size type” (D-type) and “larger number density type” (N-type). The D- and N-type regimes are characterized by larger raindrop sizes and a higher number density of raindrops at a given precipitation intensity, respectively. Stratification with reference to the satellite-derived brightness temperature indicates that N-type regime is associated with warm rain processes, whereas D-type precipitation events occur through ice-phase processes. In addition, this study offers a methodology for better comprehending the holistic view of precipitation processes by integrating satellite and ground-based observational data.

Based on previously reported Asian-Australian Monsoon indices, this study characterizes the intensity of the southwest (SW) monsoon associated rain fall and its variability in different subregions of the western Philippines. Reanalysis and satellite-based datasets are utilized to derive these monsoon indices, which include the southerly and westerly wind shear indices, and outgoing longwave radiation-, and mean sea level pressure-based indices, spanning from 1991 to 2020. Subsequently, these indices were integrated to develop a local SW monsoon index (LSWMI) in the Philippines, which was compared and assessed with climatological gridded- and ground-based rainfall datasets to quantitatively describe the spatiotemporal dynamics of the SW monsoon season over the 30-year period. Results show that the proposed LSWMI can sufficiently capture the occurrences of heavy rainfall events in western Philippines. Moreover, the LSWMI is also capable in describing the evolution (onset, peak and decay) and distinct spatiotemporal characteristics of the SW monsoon, as it propagates from northern to southern Philippines. Overall findings demonstrate the significance of utilizing the LSWMI in characterizing and quantifying the SW monsoon, which ultimately provides new insights on advancing the monsoon monitoring and forecasting capabilities in the country.

This study examines the seasonal characteristics of the inter-annual stratospheric variability that impact the polar tropospheric climate in the northern hemisphere winter, herein referred to as the Arctic Stratospheric Oscillation (ASO). The westerly wind anomalies associated with the ASO begin in the middle stratosphere around 60°N in early winter, gradually strengthen to reach their maximum in the upper stratosphere in January, and then move downwards with a decrease in intensity to the lower stratosphere in March. The seasonal progression of the ASO is found to be associated with increasing negative sea level pressure anomalies at the polar cap, reaching their maximum in March. It has been determined that the main driving force for ASO is planetary waves, with a major contribution from non-stationary waves. It is suggested that the ASO could have an impact on the occurrence of Sudden Stratospheric Warmings and Vortex Intensifications until midwinter. November signal of the ASO is found to be a promising candidate for predicting the polar climate for the subsequent winter season in both the troposphere and the stratosphere.

In this study, we analyzed lightning distribution in the Malacca Strait area from 2009 to 2021 and explored its underlying physical mechanisms. Lightning activity in this area was higher in the strait than over the land. Both land and sea areas exhibited distinct diurnal cycles. Lightning at sea was prevalent during the night and early morning, whereas lightning over land was prevalent during the late afternoon and evening. Seasonal disparities in lightning occurrences were also noted, with the movement of the Intertropical Convergence Zone influencing the relatively high lightning occurrences in Southeast Asia during Mar–May and Sep–Nov. Furthermore, lightning in the eastern terrestrial area of the strait preceded the western terrestrial area by approximately 1 to 2 h. Additionally, lightning tends to initiate earlier in the more southward parts around the strait. Lightning occurrences strongly correlated with surface airflow convergence, highlighting the diurnal land–sea breeze cycle as the primary mechanism underlying the lightning formation of in the Malacca Strait.

To examine how the Kuroshio large meander (KLM) changes the intensity of a distant tropical cyclone (TC), we conducted a control simulation of Typhoon Neoguri, which approached the Kuroshio during the KLM event in October 2019, and sea surface temperature (SST) sensitivity experiments regarding the SST warming near the Tokai District and SST cooling south of the Kii Peninsula (KP) during KLM periods. Comparisons between the control and SST sensitivity runs revealed that the KLM can impact the intensity of Neoguri, even being far away from the Kuroshio. KLM-induced SST warming (cooling) enhanced (weakened) the moisture influx toward Neoguri through changes in the surface evaporation in the KP–Tokai area, indicating the penetration of wetter (drier) air parcels into the TC inner core region. The relatively wet (dry) environment in the inner core modulated the latent heating around the TC center, leading to enhancing (weakening) the distant TC. In October 2019, the KLM acted in the direction of strengthening Neoguri because the warm (cool) SST anomalies near Tokai (KP) were remarkable (insignificant). This study suggests that the KLM could have contradictory impacts on the intensity of a distant TC, depending on the KLM-induced SST anomaly patterns in the KP–Tokai area.