Asia-Pacific Journal of Atmospheric Sciences最新文献

Abstract

A tornado outbreak occurred in West Bengal (WB), India, about 15–16 h before the landfall of Cyclone Yaas formed in May 2021 over the Bay of Bengal. High-resolution analysis data have been used to investigate the possible tornadoes in terms of environmental conditions connecting to the cyclone. The WB tornado is found as intense as EF2–3 on the tornado scale and is likely associated with a mini-supercell. The total shear of 37 m s⁻¹ from 0–6 km above ground level (AGL) with strong clockwise rotation, the moderate instability (1504 J kg⁻¹) and the energy helicity index of 2.2 are the substantial convective parameters related to the WB tornado. Moreover, the favorable environment owning intense bulk shear, a larger value of storm-relative environmental helicity in the lowest 1 km AGL and high values of significant tornado parameter (STP) urge the potentiality of multiple tornadoes spawning in multi-days accompanying the landfalling Cyclone Yaas. The right-front quadrant of the cyclone is found to be more vulnerable for developing moderate to severe tornadoes within its rainbands. The positive potential vorticity anomalies evidence the cloud-scale cyclonic circulation from surface to 400 hPa with the maximum in the mid-level.

Abstract

In the advent of the new climate normal period (i.e., 1991–2020), questions are raised on what are the recent changes in the observed Philippine climatology. Here we present evidence that the Philippine climate has become warmer (i.e., increased annual surface temperatures) and wetter (i.e., increased annual rainfall) since the mid-1990s while an abrupt increase in tropical cyclone (TC) activity in the Philippines is detected in the mid-2000s. Such regime changes are mainly attributed with the shift of the Atlantic Multidecadal Oscillation (AMO) to its positive phase since the mid-1990s. A positive AMO enhances the Pacific Walker Circulation where the more intense convection center typically shifts towards the western Pacific – this translates to more rainfall, narrowing diurnal temperature range, warmer sea surface temperatures, and more intense TC activity in the Philippines. However, the recent positive AMO phase is reported as externally and possibly driven by anthropogenic warming rather than it is naturally oscillatory, which likely implies that the detected abrupt regime shifts in the Philippine climate, particularly in increased surface temperatures, are also externally driven. Our findings provide new insights on the long-term trends and variability of the Philippine climate in support of its disaster risk reduction preparedness and seasonal forecasting.

Beta gyre and Rossby wave train induced by tropical cyclones were identified from the ERA5 global-reanalysis data of the recent 30 years using a composite method. To pick up the disturbances relevant to the beta gyre and Rossby wave train surrounding tropical cyclones, the disturbances were decomposed into three distinct horizontal scales including small, intermediate, and planetary-scale. Composite map of the disturbances containing small- and intermediate-scale showed a well-organized Rossby wave train. The orientation of wave train was found to depend on the translation direction of tropical cyclones, and also appeared to split into two orientations except for those translating in the west-northwestward direction. The wave energy of the wave train was shown to propagate along the wave train axis, which was inferred from the amplitude change with time within the wave train. The wave train shows a weak upward-westward tilt and increasing amplitude with height, implying the wave energy propagating upward. A dipole circulation cell, bearing a close resemblance to the beta gyre depicted in the theories and numerical models, was found from the Rossby wave train. The strength and orientation of the beta gyres were revealed to vary with the translation direction of the tropical cyclones, with the weakest and strongest amplitudes being found for the westward- and northward-translation cases, respectively. It was shown that the orientation of the beta gyre obtained by a lag-composite method rotates clockwise with time regardless of the translation direction of tropical cyclones.

The development of large-amplitude planetary waves (PWs) in March in the upper stratosphere during the easterly phase of the equatorial quasi-biennial oscillation (QBO) was investigated using ERA-interim reanalysis data for 1979–2019. During the 10-hPa easterly QBO, the amplitude at 3 hPa was significantly larger than that during the westerly QBO for cases of large-amplitude PWs. Case studies were conducted for individual events of the wave number 1 (wave-1) PW growth: an easterly case in 1994 and a westerly case in 1995. During the easterly QBO in March 1994, a developing perturbation at middle latitudes moved rapidly northeastward to replace the decaying high-latitude wave. In the early stage, conversion from the zonal mean to eddy kinetic energy in the subtropical region was crucial for wave development. This energy conversion was dependent on the sign of the meridional shear of the zonal wind in the middle latitudes. Negative shear was produced by the secondary meridional circulation associated with the equatorial QBO. After the perturbation started to develop in the middle latitudes, it moved northeastward over a few days due to potential vorticity flux, and the growth of the high-latitude waves was enhanced. A composite analysis also showed that the meridional shear of the zonal wind in the middle latitudes was negative during the easterly QBO in March. This study improves our understanding of the dynamic mechanism underlying the equatorial-polar relationship in the stratosphere in March.

The Global/Regional Integrated Model system (GRIMs) is upgraded to version 4.0, with the advancement of the moisture advection scheme and physics package, focusing on the global model program (GMP) for seasonal simulation and climate studies. Compared to the original version 3.1, which was frozen in 2013, the new version shows no Gibbs phenomenon in the moisture and tracer fields by implementing the semi-Lagrangian advection scheme with a better computational efficiency at higher resolution. The performance of the seasonal ensemble simulation (June–August 2017 and December 2016–February 2017) is significantly improved by new physics and ancillary data. The advancement is largest in the stratosphere, where the cold bias is dramatically reduced and the wind bias of the polar jets is alleviated, especially for the winter hemisphere. Noticeable improvements are also found in tropospheric zonal mean circulation, eddy transport, precipitation, and surface air temperature. This allows GRIMs version 4.0 to be used not only for long-term climate simulations, but also for subseasonal-to-seasonal climate prediction.

The National Institute of Environmental Research, under the Ministry of Environment of Korea, provides two-day forecasts, through AirKorea, of the concentration of particulate matter with diameters of ≤ 2.5 μm (PM_2.5) in terms of four grades (low, moderate, high, and very high) over 19 districts nationwide. Particulate grades are subjectively designated by human forecasters based on forecast results from the Community Multiscale Air Quality (CMAQ) and artificial intelligence (AI) models in conjunction with weather patterns. This study evaluates forecasts from the long short-term memory (LSTM) algorithm relative to those from CMAQ-solely and AirKorea using observations from 2019. The skills of the one-day PM_2.5 forecasts over the 19 districts were 39–70% for CMAQ, 72–79% for LSTM, and 73–80% for AirKorea; the AI forecasts showed comparable skills to the human forecasters at AirKorea. The one-day forecast skill levels of high and very high PM_2.5 pollution grades are 31–98%, 31–74%, and 39–81% for the CMAQ-solely, the LSTM, and the AirKorea forecasts, respectively. Despite good skills for forecasting the high and very high events, CMAQ-solely forecasts also generate substantially higher false alarm rates (up to 86%) than the LSTM and AirKorea forecasts (up to 58%). Hence, applying only the LSTM model to the CMAQ forecasts can yield reasonable forecast skill levels comparable to the operational AirKorea forecasts that elaborately combine the CMAQ model, AI models, and human forecasters. The present results suggest that applications of appropriate AI models can greatly enhance PM_2.5 forecast skills for Korea in a more objective way.

Air pollution modeling and forecasting over a national level scale for a country as large as India is a very challenging task due to the large amount of data involved in a limited spatial frequency. Often the air pollution and pollutant dispersion process depend on underlying meteorological conditions. Recently, Graph Neural Networks emerged as an effective deep learning model for discovering spatial patterns for various classification and regression tasks. This study proposes to employ a cluster-based Local Hybrid-Graph Neural Network (HGNN) methodology instead of using a single global Graph Neural Network for monitoring station-wise multi-step PM_2.5 concentration forecasting across India’s states. This methodology respects sudden changes in PM(_{2.5}) concentration due to the local meteorological variations. However, the local Hybrid GNN models consist of two parts: a spatio-temporal unit containing a Graph Neural Network layer along with a Gated Recurrent Unit layer to model the influence of wind speed and other meteorological variables on PM_2.5 concentration. The other part is a station wise feature extraction unit to extract station-wise meteorological feature impact on PM_2.5 concentration, along with the temporal dependency between historical records. The results from the two units are fused in step-wise manner for multi-step PM_2.5 forecasting. The proposed methodology was used to develop separate PM_2.5 concentration forecasting models, +24, +48 and +72 hours ahead. Subsequently, a detailed analysis is carried out to unfold the advantages of the proposed methodology. Results demonstrate the proposed models perform better than the state-of-the-art with significantly lesser computation time.

The x and y components of wind (U and V, respectively) are widely used as control variables in radar assimilation; therefore, it is common to choose (U, V) as the control variables for multi-scale data assimilation (DA) in convective-scale. When the model resolution reaches the convective scale, whether (U, V), as the momentum control variables, are still more suitable than the stream function (ψ) and unbalanced velocity potential (χ_u), it needs to be studied further examination. This study uses 3-km resolution forecast samples to calculate the background error covariance (B) with two different pairs of momentum control variables ((ψ, χ_u) and (U, V)) by the National Meteorology Center (NMC) method. In single-observation experiments, the analysis wind field is most sensitive to the two pairs of B, and the temperature is insensitive. When using (U, V) as the control variables, the local characteristic is more evident according to vertical and horizontal wind increments. The study assimilates low- resolution conventional observations to compare different momentum control variables, (ψ, χ_u) and (U, V), in numerical simulation experiments of the torrential rainfall in North China. In addition, the impacts of the two control variables options are also compared in terms of the 15 continuous days of cases in flood season. The main results are as follows: (1) the wind field is the critical difference between the two assimilation experiments at the analysis time. Using (U, V) as the control variables, the analysis field of wind from both the surface and different vertical levels is superior. The analysis field closer fits the wind observation; (2) the use of (U, V) control variables improves the short term (0 ~ 3-h) in surface wind prediction; and (3) the use of (U, V) control variables enhances the 24-h TS (threat score) in moderate rain and heavy rain.