While the terrain-following (sigma) system of representing topography in atmospheric models has been dominant for about the last 60 years, already half a century ago problems using the system were reported in areas of steep topography. A number of schemes had been proposed to address these problems. However, when topography steepness exceeds a given limit all these schemes except the vertical interpolation of the pressure gradient begin to use model information that for physical reasons they should not use.

A radical departure from the system was that of the step-topography eta; but its attractiveness was reduced by the discovery of the corner separation problem. The shaved-cell scheme, nowadays referred to as cut-cell, was free of that problem, and was tested subsequently in idealized as well as real case experiments with encouraging results. The eta discretization has lately been refined to make it also a cut-cell scheme. Another method referred to usually as immersed boundary method enabling treatment of terrain as complex as urban landscape came from computational fluid dynamics. It was made available coupled to the atmospheric Weather Research and Forecasting model.

Results of recent experiments of the cut-cell Eta driven by European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble members are analyzed. In these experiments, all cut-cell Eta members achieved better verification scores with respect to 250 hPa wind speed than their ECMWF driver members. This occurred when an upper-tropospheric trough was crossing the Rocky Mountains barrier. These results are considerably less favorable for the Eta when switched to use sigma, i.e., Eta/sigma, pointing to the benefits of using topography intersecting as opposed to terrain-following systems. But even so the Eta/sigma shows an advantage over its driver members, suggesting that its other features deserve attention.

This study compares the regional characteristics of heavy rain clouds in terms of Cloud Top Height (CTH) and Storm Height (SH) from long-term Tropical Rainfall Measuring Mission (TRMM) observations. The SH is derived from Precipitation Radar reflectivity, and the CTH is estimated, using visible and infrared scanner brightness temperature (10.8 µm) and reanalysis temperature profiles. As the rain rate increases, the average CTH and average SH increase, but by different degrees in different regions. Heavy rainfall in continental rainfall regimes, such as Central Africa and the United States, is characterized by high SH, in contrast to oceanic rainfall regions, such as the northwestern Pacific, Korea, and Japan; the increased atmospheric instability in dry environments is interpreted as a continental flood mechanism. Conversely, heavy rain events in Korea and Japan occur in a thermodynamically near-neutral environment with large amounts of water vapor, which are characterized by the lowest CTH, SH, and ice water content. The northwestern Pacific exhibits the lowest SH in humid environments, similar to Korea and Japan; however, this region also characteristically exhibits the highest convective instability condition, as well as high CTH and CTH–SH values, in contrast to Korea and Japan. The observed CTH and SH characteristics of heavy rain clouds should be useful for evaluating and improving satellite-based precipitation estimates and numerical model cloud parameterization.

　平成27年９月９日から11日にかけて、関東及び東北地方で非常に激しい降水が発生した。この豪雨においては、日本付近の二つの台風、第17号(Kilo)と第18号(Etau)の接近が総観場の大きな特徴であった。Etauが東海地方に上陸し温帯低気圧に変わった後、南北に延びた線状降水帯が関東地方に発生、12時間にわたって停滞し、特に栃木県で激しい降水となった。また、この時Kiloは関東地方の東方の洋上に接近していた。本研究では、本事例におけるKiloの役割について、Stretch-NICAM（最小解像度約5.6km）による数値シミュレーションによって調べた。コントロール実験では、観測とほぼ同じ位置、時刻に強い降水帯を再現することができたが、より早い時刻を初期値とした実験においては再現されなかった。Kilo中心部の水蒸気を除去することでKiloを弱化させる感度実験を早い初期時刻の下で行った。一般に水蒸気を除去すると台風が弱化し降水も弱まると考えられるが、この感度実験では、関東域の降水は５％少ないものの降水帯を再現した。さらに関東地方を含む台風第18号のアウターバンド域における降水はコントロール実験と比較して約10％増加した。この実験によって、Kiloと関東地方の間に位置していた高圧部（リッジ）に伴う南東風が、Kiloとオホーツク高気圧との間で形成された東風よりも、関東地方への水蒸気供給に重要な役割を持つことが見いだされた。本事例においては、弱化したKiloがリッジに関する北西向きの水蒸気フラックスを強め、それによってより強い降水がもたらされた。

　第5次結合モデル相互比較プロジェクト（CMIP5）の将来予測では、夏季東アジアモンスーンの南風指数（SWI）は強まる。しかし、そのモデル間の差は平均に比べてはるかに大きい。SWIを定義する夏季東アジアの地表面気圧配置の将来変化に対して経験直交関数（EOF）解析を適用し、アンサンブル平均と５つの気圧配置モードおよびこれに伴う降水量変化を同定し、さらに考えられる要因を調査した。

　夏季アジア太平洋域における気圧配置のCMIP5アンサンブル平均の将来変化は、第１モードから第３モードの特徴を持つ。その第１モード成分と第２モード成分は正のSWI将来変化に貢献するが、これは第３モード成分によっておおむね打ち消される。第１モードは、アジア太平洋域の低い海面水温上での高気圧偏差である。第２モードは、北半球大陸の温度上昇と赤道太平洋域での降水増加と関係している。SWIの大きなモデル依存性は第３モードによって作り出されるが、これは東アジア北部の弱い太平洋高気圧を表し、北半球インド洋上および太平洋上での鉛直流の抑制が特徴である。第４モードはオホーツク海高気圧を表している。第５モードは、東アジア南部における地表面気圧の東西コントラストを表し、北半球の海面水温と関係している。第４モードと第５モードは、夏季東アジアの現在気候再現性がよい10のモデル平均の将来予測を特徴づける。

　地球温暖化時に生じる各モードに関係した鉛直流の抑制は、現在気候における鉛直流（すなわち降水量）分布を反映しているため、山岳近辺を除く熱帯海洋上で, 現在気候のアジア太平洋モンスーンの降水量が比較的少ない(多い)モデルは、将来のSWI増加(減少)を予測する傾向を示す。

Accurate forecast of global horizontal irradiance (GHI) is one of the key issues for power grid managements with large penetration of solar energy. A challenge for solar forecasting is to forecast the solar irradiance with a lead time of 1–8 hours, here termed as intra-day forecast. This study investigated an algorithm using a long short-term memory (LSTM) model to predict the GHI in 1–8 hours. The LSTM model has been applied before for inter-day (> 24 hours) solar forecast but never for the intra-day forecast. Four years (2010–2013) of observations by the National Renewable Energy Laboratory (NREL) at Golden, Colorado were used to train the model. Observations in 2014 at the same site were used to test the model performance. According to the results, for a 1–4 hour lead time, the LSTM-based model can make predictions of GHIs with root-mean-square-errors (RMSE) ranging from 77 to 143 W m⁻², and normalized RMSEs around 18.4–33.0 %. With five-minute inputs, the forecast skill of LSTM with respect to smart persistence model is 0.34–0.42, better than random forest forecast (0.27) and the numerical weather forecast (−0.40) made by the Weather Research and Forecasting (WRF) model. The performance levels off beyond 4-hour lead time. The model performs better in fall and winter than in spring and summer, and better under clear-sky conditions than under cloudy conditions. Using adjacent information from the reanalysis as extra inputs can further improve the forecast performance.

The impact of assimilating thermodynamic profiles measured with lidars into the Weather Research and Forecasting (WRF)-Noah-Multiparameterization model system on a 2.5-km convection-permitting scale was investigated. We implemented a new forward operator for direct assimilation of the water vapor mixing ratio (WVMR). Data from two lidar systems of the University of Hohenheim were used: the water vapor differential absorption lidar (UHOH WVDIAL) and the temperature rotational Raman lidar (UHOH TRL). Six experiments were conducted with 1-hour assimilation cycles over a 10-hour period by applying a 3DVAR rapid update cycle (RUC): 1) no data assimilation 2) assimilation of conventional observations (control run), 3) lidar–temperature added, 4) lidar–moisture added with relative humidity (RH) operator, 5) same as 4) but with the WVMR operator, 6) both lidar–temperature and moisture profiles assimilated (impact run). The root-mean-square-error (RMSE) of the temperature with respect to the lidar observations was reduced from 1.1 K in the control run to 0.4 K in the lidar–temperature assimilation run. The RMSE of the WVMR with respect to the lidar observations was reduced from 0.87 g kg⁻¹ in the control run to 0.53 g kg⁻¹ in the lidar-moisture assimilation run with the WVMR operator, while no improvement was found with the RH operator; it was reduced further to 0.51 g kg⁻¹ in the impact run. However, the RMSE of the temperature in the impact run did not show further improvement. Compared to independent radiosonde measurements, the temperature assimilation showed a slight improvement of 0.71 K in the RMSE to 0.63 K, while there was no conclusive improvement in the moisture impact. The correlation between the temperature and WVMR variables in the static-background error-covariance matrix affected the improvement in the analysis of both fields simultaneously. In the future, we expect better results with a flow-dependent error covariance matrix. In any case, the initial attempt to develop an exclusive thermodynamic lidar operator gave promising results for assimilating humidity observations directly into the WRF data assimilation system.

下層雲場が同じ構造を持つ複数の雲セルで構成されているという仮定をもとに、下層雲被覆率に関する新しいエネルギー平衡モデルを構築した。広大な計算領域で実行した高解像度数値シミュレーションの結果を用いて、エネルギー収支解析を行った。その結果、計算された下層雲場は、海面から供給されるエネルギーフラックスと放射フラックスおよび一様流による移流が近似的に平衡状態にあることが示された。次に計算結果から抽出した雲セルを解析したところ、各セルの構造が類似していることが分かった。この点を基に、エネルギー保存式から雲被覆率を診断するシンプルなモデルを構築した。構築したモデルを用いて計算結果の雲被覆率を診断したところ、シミュレーション結果から直接計算した値に近い結果が得られた。

This study examined the effects of an upper-level anticyclonic circulation and a lower-level cyclonic circulation on tropical cyclone (TC) genesis. We ran idealized simulations using the Advanced Research Weather Research and Forecasting (WRF-ARW) model. The simulation results show that the upper-level anticyclonic circulation makes a negative contribution to TC genesis, while the lower-level cyclonic circulation makes a positive contribution. The upper-level anticyclonic circulation results in slower TC genesis due to a substantial vertical zonal wind shear that shifts the upper-level vortex eastward from its initial position. This shift is unfavorable for the vortex's vertical alignment and warm core maintenance. This substantial vertical zonal wind shear is associated with the asymmetric vertical motion and associated diabatic heating, induced by the lower-level beta gyre. The upper-level anticyclonic circulation increases the westerly wind north of the vortex, resulting in a substantial vertical westerly wind shear. Thus, the initial upper-level anticyclonic circulation is unnecessary for TC genesis. The strong upper-level anticyclonic circulation, generally observed with a strong TC, should be considered a result of deep convection. The strong lower-level winds induce large surface heat fluxes and vorticity due to the superposition of the large-scale lower-level cyclonic circulation and vortex. These conditions lead to strengthened convection and diabatic heating and a quick build-up of positive vorticity, resulting in rapid TC genesis.

Uncertainty in numerical weather forecasts arising from an imperfect knowledge of the initial condition of the atmospheric system and the discrete modeling of physical processes is addressed with ensemble prediction systems. The breeding method allows the creation of initial condition perturbations in a simple and computationally inexpensive way. This technique uses the full nonlinear dynamics of the system to identify fast-growing modes in the analysis fields, obtained from the difference between control and perturbed runs rescaled at regular time intervals. This procedure is more suitable for the high-resolution ensemble forecasts required to reproduce small-scale high-impact weather events, as the complete nonlinear model is employed to generate the perturbations. The underdispersion commonly observed in ensemble forecasts emphasizes the need to develop methods that increase ensemble spread and diversity at no cost to forecast skill. In this sense, we investigate the benefits of different breeding techniques in terms of ensemble diversity and forecast skill for a mesoscale ensemble over the Western Mediterranean region. In addition, we propose a new method, Bred Vectors Tailored Ensemble Perturbations, designed to control the scale of the perturbations and indirectly the ensemble spread. The combination of this method with orthogonal bred vectors shows significant improvements in terms of ensemble diversity and forecast skill with respect to the current arithmetic methods.

The tropical oceans spawn hundreds of tropical disturbances during the tropical cyclone (TC) peak season every year, but only a small fraction eventually develop into TCs. In this study, using observations from the Global Precipitation Measurement (GPM) satellite, tropical disturbances over the western North Pacific (WNP) from July to October during 2014–2016 are categorized into developing and nondeveloping groups to investigate the differences between satellite-retrieved convective and stratiform precipitation properties in both the inner-core (within 200 km of the disturbance center) and outer-core (within 200–400 km of the disturbance center) regions. The developing disturbances experience a remarkably more oscillatory process in the inner-core region than in the outer-core region. The large areal coverage of strong rainfall in the inner-core region of the disturbance breaks into scattered remnants and then reorganizes and strengthens near the disturbance center again. Contrarily, the precipitation characteristics in the nondeveloping group evolve more smoothly. It can be summarized that disturbances prone to developing into a TC over the WNP satisfy two essential preconditions in terms of precipitation characteristics. First, a large fraction of stratiform precipitation covers the region that is within 400 km from the disturbance center. The mean vertically integrated unconditional latent heating rate of stratiform and convective precipitation in the developing group above 5.5 km is 6.6 K h⁻¹ and 2.4 K h⁻¹, respectively; thus, the stratiform rainfall makes a major contribution to the warming of the upper troposphere. Second, strong convective precipitation occurs within the inner-core region. Compared with stratiform precipitation, which plays a critical role in warming the mid-to-upper levels, the most striking feature of convective precipitation is that it heats the mid-to-lower troposphere. Overall, the formation of TCs evolving from parent disturbances can be regarded as an outcome of the joint contribution from the two distinct types (convective and stratiform) of precipitation clouds.

　水の氷相過程タイムスケールは液相のそれと比べて何桁も大きいことが知られている。本研究では、乱流スキーム内で用いられている飽和調節的な氷相過程の取り扱いが上層雲の温暖化応答に与えるインパクトについて、放射対流実験に基づいて解析した。数値実験は地球サイズの計算領域を用いて一様な温度の海を下部境界条件とし、陽に雲微物理を計算する非静力学モデルを用いて実施した。

　乱流スキーム内の飽和調節的な氷相過程に伴う浮力生成を抑制する感度実験を実施した結果、標準実験に比べておよそ20%の雲量が減少したほか、標準実験と異なる上層雲の温暖化応答がみられた。こうした違いは乱流スキーム内の氷相過程が上層雲近傍での静的安定度を減少させることによるものである。

　氷相過程タイムスケールは液相のそれと比べて何桁も大きく、氷に対する過飽和状態も一般にみられることから、雲解像モデル・全球気候モデルのいずれにおいても、乱流スキーム中で飽和調節的な氷相過程を用いることは乱流拡散の過大評価とバイアスをもたらすと考えられる。これは乱流スキーム内での現実的な氷相過程の再現が、現在気候の再現性のみならず気候予測精度向上にとって重要であることを示している。

A method is proposed to gain insight into ozone recovery over Antarctica. The following metrics relating to the ozone hole are considered: minimum total column ozone (TCO₃) within the hole, TCO₃ at the South Pole, area of the ozone hole, mass of ozone loss within the hole, and density of loss per unit area. The daily metric values, based on the Royal Netherlands Meteorological Institute archives of the ozone hole, are averaged for each year over the period 1979–2019 for the following intervals: 1–30 September, 15 September–15 October, 1–31 October, 15 October–15 November, and 1–30 November. The following indicators of the ozone hole recovery are examined: the metric recovery rate by 2019 (i.e., the change between its extreme and its 2019 level divided by the change between the extreme year and 1980) and the year of metric recovery. The recovery year is derived by forward-in-time extrapolation of the metric linear trend found for the period 2000–2019. The uncertainties in these indicators are obtained using a bootstrap approach analyzing statistics of the synthetic time series of the metrics. A comparison of the proposed ozone hole healing indicators with the indicators inferred from the equivalent effective stratospheric chlorine (EESC) loading over Antarctica (22.1 % and year 2076) shows to what extent recovery of the ozone layer is associated with EESC effects. For the mass and density of ozone loss in the periods 1–30 September and 15 September–15 October, the metric recovery rate by 2019 is ∼ 2 times larger and the recovery year is at least 20–30 years earlier than the corresponding indicators of the EESC changes. Therefore, the ozone hole is recovering faster during these periods than expected based on the stratospheric halogen loading alone.