Cloud-Resolving Storm Simulator Research Articles

Investigation of an extreme rainfall event during 8–12 December 2018 over central Vietnam – Part 1: Analysis and cloud-resolving simulation

Abstract. An extreme rainfall event occurred from 8 to 12 December 2018 along the coast of central Vietnam. The observed maximum rainfall amount in 72 h was over 900 mm and set a new record, and the associated heavy losses were also significant. The analysis of this event shows some key factors for its occurrence: (1) the interaction between the strong northeasterly winds, blowing from the Yellow Sea into the northern South China Sea (SCS), and easterly winds over the SCS in the lower troposphere (below 700 hPa). This interaction created strong low-level convergence, as the winds continued to blow into central Vietnam against the Truong Son Range, resulting in forced uplift over the coastal plains due to the terrain's barrier effect. Furthermore, the low-level convergence in this event was strong enough, and the air was unstable enough to trigger most of the convection near the shoreline (further inland). As a consequence, heavy rainfall occurred along the coastal zone and coastal sea. (2) The strong easterly wind played an important role in transporting moisture from the western North Pacific (WNP) across the Philippines and the SCS into central Vietnam. (3) The Truong Son Range also contributed to this event due to its barrier effect. (4) In addition to cumulonimbus, the low-level precipitating clouds such as nimbostratus clouds were also major contributors to rainfall accumulation for the whole event. The analyses of local thermodynamics also indicate that the southward movement of the low-level wind convergence zone caused the southward movement of the main heavy rain band during the event. The cloud-resolving storm simulator (CReSS) was employed to simulate this record-breaking event at a grid size of 2.5 km; evaluated results show that the model simulated the surface wind well and captured the southward movement of the low-level wind convergence. The overall rainfall can be captured quite well, not only in quantity but also in its spatial distribution (with a similarity skill score (SSS) ≈0.7 and threat score (TS) >0 at 700 mm for 72 h rainfall). Thus, the CReSS model is shown to be a useful tool for both research and forecasts of heavy rainfall in Vietnam. The model performed better for the rainfall during 9–10 December 2018 but not as good on 11 December 2018. In the sensitivity test without the terrain, the model had poorly simulated the surface wind, which resulted in the model generating insufficient rainfall for this event and also not capturing the spatial distribution of the rainfall. Thus, the test confirms the important role played by the local topography for the occurrence of this event.

Time-Lagged Ensemble Quantitative Precipitation Forecasts for Three Landfalling Typhoons in the Philippines Using the CReSS Model, Part I: Description and Verification against Rain-Gauge Observations

In this study, the 2.5 km Cloud-Resolving Storm Simulator was applied to forecast the rainfall by three landfalling typhoons in the Philippines at high resolution: Mangkhut (2018), Koppu (2015), and Melor (2015), using a time-lagged strategy for ensemble. The three typhoons penetrated northern Luzon, central Luzon, and the middle of the Philippine Archipelago, respectively, and the present study verified the track and quantitative precipitation forecasts (QPFs) using categorical statistics against observations at 56 rain-gauge sites at seven thresholds up to 500 mm. The predictability of rainfall is the highest for Koppu, followed by Melor, and the lowest for Mangkhut, which had the highest peak rainfall amount. Targeted at the most-rainy 24 h of each case, the threat score (TS) within the short range (≤72 h) could reach 1.0 for Koppu at 350 mm in many runs (peak observation = 502 mm), and 1.0 for Mangkhut and 0.25 for Melor (peak observation = 407 mm) both at 200 mm in the best member, when the track errors were small enough. For rainfall from entire events (48 or 72 h), TS hitting 1.0 could also be achieved regularly at 500 mm for Koppu (peak observation = 695 mm), and 0.33 at 350 mm for Melor (407 mm) and 0.46 at 200 mm for Mangkhut (786 mm) in the best case. At lead times beyond the short range, one third of these earlier runs also produced good QPFs for both Koppu and Melor, but such runs were fewer for Mangkhut and the quality of QPFs was also not as high due to larger northward track biases. Overall, the QPF results are very encouraging, and comparable to the skill level for typhoon rainfall in Taiwan (with similar peak rainfall amounts). Thus, at high resolution, there is a fair chance to make decent QPFs even at lead times of 3–7 days before typhoon landfall in the Philippines, with useful information on rainfall scenarios for early preparation.

High-resolution time-lagged ensemble prediction for landfall intensity of Super Typhoon Haiyan (2013) using a cloud-resolving model

The prediction of tropical cyclone (TC) intensity at landfall is crucial for regions vulnerable to high winds and storm surges, but its accuracy has experienced only limited improvement. At high resolution with a grid size of 2.5 km, the Cloud-Resolving Storm Simulator (CReSS) is applied to Super Typhoon Haiyan (2013), one of the strongest TCs to ever make landfall (at 85 m s−1 in peak wind speed and 900 hPa in central pressure. Predictions are made every 6 h using a time-lagged strategy so that the computational cost is relatively low. Averaging at 64 m s−1 and 925 hPa, our hindcasts during 4–7 November show large improvements in pre-landfall intensity over global models. Furthermore, when the previous CReSS result that best matches the observed intensity is used as the initial field, the predicted intensity consistently reaches 73–76 m s−1 and below 900 hPa. Thus, this approach is shown to produce more accurate TC intensity forecasts for storm-surge simulations for Haiyan.

Evaluation of Mei-yu heavy-rainfall quantitative precipitation forecasts in Taiwan by a cloud-resolving model for three seasons of 2012–2014

Abstract. In this study, the performance of quantitative precipitation forecasts (QPFs) by the Cloud-Resolving Storm Simulator (CReSS) in Taiwan, at a horizontal grid spacing of 2.5 km and a domain size of 1500×1200 km2, in the range of 1–3 d during three Mei-yu seasons (May–June) of 2012–2014 is evaluated using categorical statistics, with an emphasis on heavy-rainfall events (≥100 mm per 24 h). The categorical statistics are chosen because the main hazards are landslides and floods in Taiwan, so predicting heavy rainfall at the correct location is important. The overall threat scores (TSs) of QPFs for all events on day 1 (0–24 h) are 0.18, 0.15, and 0.09 at thresholds of 100, 250, and 500 mm, respectively, and indicate considerable improvements at increased resolution compared to past results and 5 km models (TS < 0.1 at 100 mm and TS ≤ 0.02 at 250 mm). Moreover, the TSs are shown to be higher and the model more skillful in predicting larger events, in agreement with earlier findings for typhoons. After classification based on observed rainfall, the TSs of day − 1 QPFs for the largest 4 % of events by CReSS at 100, 250, and 500 mm (per 24 h) are 0.34, 0.24, and 0.16, respectively, and can reach 0.15 at 250 mm on day 2 (24–48 h) and 130 mm on day 3 (48–72 h). The larger events also exhibit higher probability of detection and lower false alarm ratio than smaller ones almost without exception across all thresholds. With the convection and terrain better resolved, the strength of the model is found to lie mainly in the topographic rainfall in Taiwan rather than migratory events that are more difficult to predict. Our results highlight the crucial importance of cloud-resolving capability and the size of fine mesh for heavy-rainfall QPFs in Taiwan.

A Modeling Study of Rainbands Upstream from Western Japan during the Approach of Typhoon Tokage (2004)

During 19–20 October 2004, a series of spectacular arc-shaped rainbands developed south or southeast of southwestern Japan when Typhoon Tokage (TY0423) approached the region from the southwest. As the typhoon moved closer and the upstream Froude number (Fr) continued to increase, these rainbands first remained quasi-stationary but eventually retreated backward. Using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at 1-km grid size, these rainbands were successfully simulated, and their behavior during the transition period from a relatively low-Fr to a high-Fr regime was investigated and compared with idealized two-dimensional (2D) model results from theoretical studies. In the present case, the rainbands were found to develop along a low-level frontal convergence zone between the southerly flow associated with the typhoon and the northerly flow from the Sea of Japan. The northeasterly winds accelerated through gaps between topography and fed the offshore flow at the backside of the rainbands, producing a strong resistance that allowed the rainbands to remain stationary under significantly higher Fr values (at least 1.2) than predicted by 2D simulations (of about 0.3–0.5) for the retreat to occur in conditionally unstable flow with a convective available potential energy of about 1300 J kg−1. Typically ≤ 500 m in depth with a potential temperature (θ) deficit of 2–4 K across the rainband, the cooler offshore flow was also found to be enhanced by evaporative cooling as in some other events. The cooling effect helped the rainbands to hold their position until Fr of the upstream flow became too large, and the rainband with stronger cooling behind was able to withstand a higher Fr before retreat. Once the retreat started, the offshore layer became thinner and the θ deficit also reduced, and the rainbands were washed back by the strengthening upcoming flow.

Evaluation of Rainfall Forecasts by Three Mesoscale Models during the Mei-yu Season of 2008 in Taiwan. Part I: Subjective Comparison

This study assesses relative merit of three mesoscale models – the Cloud-Resolving Storm Simulator (CReSS), the Central Weather Bureau (CWB) Weather Research and Forecasting (WRF) model, and the CWB Non-hydrostatic Forecast System (NFS) – that participated in the Southwest Monsoon Experiment (SoWMEX) in 2008. The Mei-yu quantitative precipitation forecasts (QPFs) in May–June of the three models are compared against merged data from ~400 rain gauges and the Tropical Rainfall Measuring Mission (TRMM). The three models have a grid size of 3.5 or 5 km, and their QPFs over the forecast range of 12–36 h are verified by subjective means, with particular foci on rainfall spatial pattern, intra-seasonal variability, and diurnal cycle during the entire season, its sub-periods, and Intensive Observation Periods (IOPs). The three models each has advantages and disadvantages. The CReSS performs best in seasonal and diurnal characteristics of rainfall. Despite higher resolution, it suffers from over-forecast and location error problems. Without a data assimilation system, its single domain with a limited size fabricates it more sensitive to errors in initial and boundary conditions (IC/BCs) from the global model. Although the CWB WRF is most stable among three and performs well in simulating spatial distribution of rainfall, it exhibits under-prediction with an inadequate intra-seasonal variation. In both the entire period and IOPs, the CWB NFS simulates rainfall pattern closest to observation with only slight under-prediction. However, the diurnal cycle captured by the CWB NFS has a phase lag of 12 h and thus a timing error that is likely associated with delayed development of convection due to systematic bias.

Evaluation of Rainfall Forecasts by Three Mesoscale Models during the Mei-yu Season of 2008 in Taiwan. Part II: Development of an Object-Oriented Method

This study describes a recently developed object-oriented method suitable for Taiwan for the purpose to verify quantitative precipitation forecasts (QPFs) produced by mesoscale models as a complement to the traditional approaches in existence. Using blended data from the rain-gauge network in Taiwan and the Tropical Rainfall Measuring Mission (TRMM) as the observation, the method developed herein is applied to twice-daily 0–48 h QPFs produced by the Cloud-Resolving Storm Simulator (CReSS) during the South-West Monsoon Experiment (SoWMEX) in May–June 2008. In this method, rainfall objects are identified through a procedure that includes smoothing and thresholding. Various attribute parameters and the characteristics of observed and forecast rain-area objects are then compared and discussed. Both the observed and the QPF frequency distributions of rain-area objects with respect to total water production, object size, and rainfall are similar to chi-distribution, with highest frequency at smaller values and decreased frequencies toward greater values. The model tends to produce heavier rainfall than observation, while the latter exhibits a higher percentage of larger objects with weaker rainfall intensity. The distributions of shape-related attributes are similar between QPF and observed rainfall objects, with more northeast–southwest oriented and fewer northwest–southeast oriented objects. Both observed and modeled object centroid locations have relative maxima over the terrain of Taiwan, indicating reasonable response to the topography. The above results are consistent with previous studies.

Evaluation of Rainfall Forecasts by Three Mesoscale Models during the Mei-Yu Season of 2008 in Taiwan. Part III: Application of an Object-Oriented Verification Method

In this study, the performances of Mei-yu (May–June) quantitative precipitation forecasts (QPFs) in Taiwan by three mesoscale models: the Cloud-Resolving Storm Simulator (CReSS), the Central Weather Bureau (CWB) Weather Research and Forecasting (WRF), and the CWB Non-hydrostatic Forecast System (NFS) are explored and compared using an newly-developed object-oriented verification method, with particular focus on the various properties or attributes of rainfall objects identified. Against a merged dataset from ~400 rain gauges in Taiwan and the Tropical Rainfall Measuring Mission (TRMM) data in the 2008 season, the object-based analysis is carried out to complement the subjective analysis in a parallel study. The Mei-yu QPF skill is seen to vary with different aspects of rainfall objects among the three models. The CReSS model has a total rainfall production closest to the observation but a large number of smaller objects, resulting in more frequent and concentrated rainfall. In contrast, both WRF and NFS tend to under-forecast the number of objects and total rainfall, but with a higher proportion of bigger objects. Location errors inferred from object centroid locations appear in all three models, as CReSS, NFS, and WRF exhibit a tendency to simulate objects slightly south, east, and northwest with respect to the observation. Most rainfall objects are aligned close to an E–W direction in CReSS, in best agreement with the observation, but many towards the NE–SW direction in both WRF and NFS. For each model, the objects are matched with the observed ones, and the results of the matched pairs are also discussed. Overall, though preliminarily, the CReSS model, with a finer grid size, emerges as best performing model for Mei-yu QPFs.

Multifractal Analysis of Rainfall-Rate Datasets Obtained by Radar and Numerical Model: The Case Study of Typhoon Bolaven (2012)

AbstractTyphoon Bolaven caused significant damage with severe rainfall all over South Korea, including Cheju Island, which received more than 250 mm in 2 days in August 2012. It was regarded as the most powerful storm to strike the Korean Peninsula in nearly a decade. The rainfall-rate datasets were obtained from S-band radar operated by the Korea Meteorological Administration to be analyzed and compared with the mesoscale Cloud Resolving Storm Simulator (CReSS) model simulation. Multifractal analysis was conducted to understand the structure of the rainfall rate with height in the typhoon system. The radar rainfall data presented with strong intermittency across scales at lower altitudes (1 and 2 km) and a more homogeneous rainfall field at high altitude (5 km) with two parameters (fractal codimension and multifractality index). The statistical scaling moment function and maximal singularities show clear significant differences between radar and the CReSS model.

Evaluation of the 2.5-km Cloud-Resolving Storm Simulator in Predicting Local Afternoon Convection during the Summer in Taiwan

The forecast of summertime afternoon convection by numerical weather prediction models is highly challenging because of its weak dynamical forcing, small scale, and low predictability. To assess such an ability for future improvement, we evaluated the performance of the 2.5-km Cloud-Resolving Storm Simulator (CReSS) in predicting afternoon convection in Taiwan under weak synoptic conditions during the summers (May–October) of 2011 and 2012. For a total of 89 target days, daily CReSS forecasts in three ranges, starting at 0000 UTC (0800 LST) on the same day (D0), the day before (D-1), and 2 days before (D-2), were examined. With regard to the occurrence of afternoon convection anywhere in Taiwan, the predictive skill of the model was considerably high, as evidenced by the threat score (TS) and post-agreement (PA) of D0 (D-1) forecasts reaching 0.75 and 0.90 (0.50 and 0.79), respectively. While the score values decrease when Taiwan is divided into four regions (i.e., the forecast must be in the correct region to be considered a hit), the TS and PA for D0 (D-2) forecasts remain respectable at 0.44 and 0.73 (0.29 and 0.59). Among the four regions, the TS (0.48–0.65) and PA (0.77–0.83) for Central Taiwan are the highest with the best predictive skill. Overall, while the prediction of afternoon convection in the correct region is challenging, the 2.5-km CReSS model has considerable skill (TS ~ 0.30) even 2 days in advance, and can provide useful guidance for afternoon convection in Taiwan.

Assessment of offshore wind farm characteristics with the cloud resolving storm simulator: A case study in Japan

AbstractWind conditions and output power characteristics of a wind farm in Japan are evaluated with highly resolved weather predictions from the so‐called cloud resolving storm simulator. One year of 30‐hour‐ahead predictions with 2‐km spatial resolution and 1‐hour time resolution are evaluated against 10‐minute averaged measurements (averaged to hourly data) from the wind farm. Also, extremely detailed shorter‐term predictions with 200‐m spatial resolution and 1‐second time resolution are evaluated against 1‐Hz measurements. For the hourly data, wind speeds are predicted with an RMSE of 3.0 to 3.5 m/s, and wind power with about 0.3 per unit. Wind direction is predicted with a standard deviation of errors of 16° to 28° for hourly data, and generally below 10° for the 1‐Hz data. We show that wind power variability—here in terms of increments—can be assessed on the timescale of several hours. The measured and predicted wind spectra are found similar on both short and long timescales.

A Multimodel Intercomparison of an Intense Typhoon in Future, Warmer Climates by Four 5-km-Mesh Models

Intense tropical cyclones (TCs) sometimes cause huge disasters, so it is imperative to explore the impacts of climate change on such TCs. Therefore, the authors conducted numerical simulations of the most destructive historical TC in Japanese history, Typhoon Vera (1959), in the current climate and a global warming climate. The authors used four nonhydrostatic models with a horizontal resolution of 5 km: the cloud-resolving storm simulator, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, the Japan Meteorological Agency (JMA) operational nonhydrostatic mesoscale model, and the Weather Research and Forecasting Model. Initial and boundary conditions for the control simulation were provided by the Japanese 55-year Reanalysis dataset. Changes between the periods of 1979–2003 and 2075–99 were estimated from climate runs of a 20-km-mesh atmospheric general circulation model, and these changes were added to the initial and boundary conditions of the control simulation to produce the future climate conditions. Although the representation of inner-core structures varies largely between the models, all models project an increase in the maximum intensity of future typhoons. It is found that structural changes only appeared around the storm center with sudden changes in precipitation and near-surface wind speeds as the radius of maximum wind speed (RMW) contracted. In the future climate, the water vapor mixing ratio in the lower troposphere increased by 3–4 g kg−1. The increased water vapor allowed the eyewall updrafts to form continuously inside the RMW and contributed to rapid condensation in the taller and more intense updrafts.

A numerical study of convection in rainbands of Typhoon Morakot (2009) with extreme rainfall: roles of pressure perturbations with low-level wind maxima

Abstract. This paper investigates the formation and evolution of deep convection inside the east–west oriented rainbands associated with a low-level jet (LLJ) in Typhoon Morakot (2009). With the typhoon center to the northwest of Taiwan, the westerly LLJ occurred as a result from the interaction of typhoon circulation with the southwest monsoon flow, which supplied the water vapor for the extreme rainfall (of ~ 1000 mm) over southwestern Taiwan. The Cloud-Resolving Storm Simulator with 1 km grid spacing was used to simulate the event, and it successfully reproduced the slow-moving rainbands, the embedded cells, and the dynamics of merger and back-building (BB) on 8 August as observed. Our model results suggest that the intense convection interacted strongly with the westerly LLJ that provided reversed vertical wind shear below and above the jet core. Inside mature cells, significant dynamical pressure perturbations (p'd) are induced with positive (negative) p'd at the western (eastern) flank of the updraft near the surface and a reversed pattern aloft (> 2 km). This configuration produced an upward-directed pressure gradient force (PGF) to the rear side and favors new development to the west, which further leads to cell merging as the mature cells slowdown in eastward propagation. The strong updrafts also acted to elevate the jet and enhance the local vertical wind shear at the rear flank. Additional analysis reveals that the upward PGF there is resulted mainly by the shearing effect but also by the extension of upward acceleration at low levels. In the horizontal, the upstream-directed PGF induced by the rear-side positive p'd near the surface is much smaller, but can provide additional convergence for BB development upstream. Finally, the cold-pool mechanism for BB appears to be not important in the Morakot case, as the conditions for strong evaporation in downdrafts do not exist.

The More Rain, the Better the Model Performs—The Dependency of Quantitative Precipitation Forecast Skill on Rainfall Amount for Typhoons in Taiwan

Abstract A strong dependency of model performance in quantitative precipitation forecasts (QPFs) as measured by scores such as the threat score (TS) on rainfall amount (i.e., the better the model performs when there is more rain), is demonstrated through real-time forecasts by the 2.5-km Cloud-Resolving Storm Simulator (CReSS) for 15 typhoons in Taiwan in 2010–12. Implied simply from the positive correlation between rain-area sizes and scores, this dependency is expected to exist in all regions, models, and rainfall regimes, while for typhoon QPFs in Taiwan it is also attributed to the model’s capability to properly handle (within 72 h) the processes leading to more rain, which are largely controlled by the typhoon’s track, size, structure, and environment, and the island’s topography. Because of this dependency, the performance of model QPFs for extreme events can be assessed accurately only when forecasts targeted for periods of comparable rainfall magnitude are included for averaging. For the most-rainy 24 h of the top-5 typhoons, the 0–24-h QPFs by CReSS have mean TS of 0.67, 0.67, 0.58, 0.51, and 0.32 at thresholds of 25, 50, 130, 200, and 350 mm, and 0.64, 0.57, 0.37, 0.33, and 0.22 from 48–72-h QPFs, respectively, suggesting superior performance even 2–2.5 days in advance. These scores are strikingly high, precisely because Taiwan can receive extreme rainfall from typhoons. For smaller (nonhazardous) events, the mean scores are progressively lower, but also unimportant and less representative statistically. Therefore, it is inappropriate to average scores over multiple forecasts as those for less-rainy periods would contaminate the result for key periods. The implication for forecasters is also discussed.

Effect of an isolated elliptical terrain (Jeju Island) on rainfall enhancement in a moist environment

A series of idealised experiments using a cloud-resolving storm simulator (CReSS) was performed to investigate the effects of the isolated elliptically shaped terrain of Jeju Island (oriented east–west), southern Korea, on the enhancement of pre-existing rainfall systems under the influence of prevailing southwesterly moist flows. Control parameters were the low-altitude wind speed (Froude numbers: 0.2, 0.4, 0.55) and the initial location of the elongated (oriented north–east) rainfall system (off the northwestern or western shores of the island). Simulations were conducted for all combinations of initial location and wind regime. Overall, results indicate that weak southwesterlies flowing around the steep mountain on the island (height, 2 km) generate two local convergences, on the northern lateral side and on the lee side of the island, both in regions of moist environments, thus producing conditions favourable for enhanced rainfall. As an eastward-moving rainfall system approaches the northwestern shore of the island, the southwesterlies at low altitudes accelerate between the system and the terrain, generating a local updraft region that causes rainfall enhancement onshore in advance of the system's arrival over the terrain. Thus, the prevailing southwesterlies at low altitudes that are parallel to the terrain are a crucial element for the enhancement. Relatively weak southwesterlies at low altitudes allow system enhancement on the lee side by generating a convergence of relatively weak go-around northwesterlies from the northern island and relatively strong moist southwesterlies from the southern island, thus producing a relatively long-lived rainfall system. As the southwesterlies strengthen, a dry descending air mass intensifies on the northeastern downwind side of the terrain, rapidly dissipating rainfall and resulting in a relatively short-lived rainfall system. A coexisting terrain-generated local convergence, combined with the absence of dry descending air on the downwind side of the terrain, prolongs the lifetime of the rainfall system.

Troposphere delays from space geodetic techniques, water vapor radiometers, and numerical weather models over a series of continuous VLBI campaigns

Continuous, very long baseline interferometry (VLBI) campaigns over 2 weeks have been carried out repeatedly, i.e., CONT02 in October 2002, CONT05 in September 2005, CONT08 in August 2008, and CONT11 in September 2011, to demonstrate the highest accuracy the current VLBI was capable at that time. In this study, we have compared zenith total delays (ZTD) and troposphere gradients as consistently estimated from the observations of VLBI, Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) at VLBI sites participating in the CONT campaigns. We analyzed the CONT campaigns using the state-of-the-art software following common processing strategies as closely as possible. In parallel, ZTD and gradients were derived from numerical weather models, i.e., from the global European Centre for Medium-Range Weather Forecasts (ECMWF) analysis fields, the High Resolution Limited Area Model (European sites), the Japan Meteorological Agency-Operational Meso-Analysis Field (MANAL, over Japan), and the Cloud Resolving Storm Simulator (Tsukuba, Japan). Finally, zenith wet delays were estimated from the observations of water vapor radiometers (WVR) at sites where the WVR observables are available during the CONT sessions. The best ZTD agreement, interpreted as the smallest standard deviation, was found between GNSS and VLBI techniques to be about 5–6 mm at most of the co-located sites and CONT campaigns. We did not detect any significant improvement in the ZTD agreement between various techniques over time, except for DORIS and MANAL. On the other hand, the agreement and thus the accuracy of the troposphere parameters mainly depend on the amount of humidity in the atmosphere.

Microphysical structures associated with enhancement of convective cells over Mt. Halla, Jeju Island, Korea on 6 July 2007

An intense precipitation system that developed during the rainy season generated an enhanced rainfall event over the northern and lee sides of an isolated elliptical-shaped area (Mt. Halla: height 1950m, width 35km, length 78km) of Jeju Island on 6 July 2007. In this study, we investigate the microphysical structures associated with the developed convective cells interior of the system, which was shown in the vicinities of a terrain, using a cloud-resolving storm simulator (CReSS) that successfully reproduced the intensification of the convective cells. A control experiment simulated the terrain-modified go-around airflow (i.e., the flow of air around Mt. Halla rather than over it) and two localized moist regions (relative humidity >96%) on the northwestern side and southeastern slope of the island. Regarding the intensification of the convective cell off the northwestern shore of the island, an increase in updraft was seen from the sea surface to an altitude of 1.5km (maximum at 4.5km), where the moisture content was high (17gkg−1). Due to the increased updraft and plentiful moisture supply at low altitudes, cloud water increased from the sea surface to reach a maximum at an altitude of 2.7km. Consequently, the microphysical processes that convert cloud water into raindrops (e.g., collection and conversion) were significant below 4.3km, and an increase in rainfall was seen between heights of 1.0 and 4.2km; i.e., mainly below the freezing level (4.9km). With regard to the enhancement of the convective cell on the lee side of the island, an increase in updraft was seen above 1km (maximum at 5.5km). The plentiful moisture supply, which was localized on the southeastern slope of the island, was supplied to the convective cell by the southwesterly upslope wind at low altitudes. Driven by the moist updraft above 1.0km, cloud water increased above 1.5km, and the microphysical processes that produce raindrops were significantly activated in the region between 1.5 and 5.3km in altitude, which extended above the freezing level. Consequently, above the freezing level, the melting rate of graupel decreased while cloud water increased during the intensification of convective cell on the lee side. Off the northwestern shore of the island, most of the hydrometeors increased significantly from the sea surface to below the freezing level as a result of the warm rain process. In contrast, on the lee side, the hydrometeors increased below and above the freezing level due to the action of both stationary terrain-generated convergence, and the southwesterly upslope wind in a moist environment; consequently, both of the active warm and cold rain processes contributed to the generation of the secondary enhancement.

Numerical simulation of the heavy rainfall caused by a convection band over Korea: a case study on the comparison of WRF and CReSS

This study investigates the capability of two numerical models, namely the Weather Research and Forecasting (WRF) and Cloud Resolving Storm Simulator (CReSS), to simulate the heavy rainfall that occurred on September 21, 2010 in the middle of the Korean peninsula. This event was considered part of the typical rainfall caused by intense quasi-stationary convection band, leading to a large accumulated rainfall amount within a narrow area. To investigate the relevant characteristics of this heavy rainfall and the feasibility of the numerical models to simulate them, the experiments using both numerical models were designed with a focus on Korea with a horizontal grid spacing of 2 km. The initial and later boundary conditions were interpolated using the output of the mesoscale model of Japan Meteorological Agency and integration spanned the 24-h period from 2100 UTC on September 20, 2010 when the rainfall started in the Yellow Sea. Generally, the spatial distribution and temporal evolution of the rainfall simulated by CReSS are closer than those of the WRF to the in situ observations (655 stations). The WRF simulation reveals the deficiency in capturing the unusual stagnant behavior of this event. The spatial and vertical patterns of reflectivity are consistent with the rainfall pattern, supporting that strong reflectivity coincides with the convective activity that accompanies excessive rainfall. The thermodynamic structure is the main driver of the different behavior between both simulations. The higher equivalent potential temperature, deep moist absolutely unstable layer and strong veering wind shear seen in the CReSS simulation play a role in the development of a favorable environment for inducing convection.

Sensitivity of typhoon track to asymmetric latent heating/rainfall induced by Taiwan topography: A numerical study of Typhoon Fanapi (2010)

Typhoon Fanapi (2010) traveled westward across the Central Mountain Range of Taiwan on 19 September and its rainfall shifted from a symmetric to an asymmetric pattern with convection mostly to the south and southeast. Meanwhile, the storm slowed down from 22 to 14 km h−1 for 12 h upon leaving Taiwan, and led to heavy rainfall (>800 mm) and serious flooding over the low‐lying southwestern plains. Through simulation and sensitivity tests using the Cloud‐Resolving Storm Simulator at 3 km grid size, this study shows that the sudden and temporary speed reduction was caused by the asymmetric latent heating (LH), not the environmental flow. Specifically, over a 9 h period, the model storm moved westward at 16 km h−1 in the control run, but increasingly faster and more toward the northwest when the moisture (and thus the LH effect and its asymmetry) is gradually reduced. Steering flow analysis and estimation using model results suggest an eastward motion vector of about 8 km h−1, consistent with the observation, is produced by the asymmetric LH effect, when the effects from the vertical wind shear and beta‐drift are both taken into account. This result is further supported by the diagnosis on storm motion based on potential vorticity tendency. Although important, such feedback to typhoon track from rainfall asymmetry that is induced by the blocking effect of topography have not been reported or studied.

Development of an arc-shaped precipitation system during the pre-monsoon period in Bangladesh

A numerical simulation is performed to understand the features and development processes of the arc-shaped precipitation system that dominates in Bangladesh during the pre-monsoon (March–May) period. An arc-shaped precipitation system of 26 April 2002 is simulated using the Cloud Resolving Storm Simulator (CReSS) with a horizontal grid increment of 2 km. The Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model is used for downscaling. Hourly outputs of the finest domain (grid increment of 5 km) of MM5 and National Oceanic and Atmospheric Administration Reynolds weekly mean sea surface temperature data are used as the initial and boundary conditions for CReSS. Younger and more intense cells are formed in the southwestern end of the system. These cells move northeastward and merge with the system producing intense rainfalls. Simulation results indicate that low-level southwesterly or southerly wind brings warm moist air from the Bay of Bengal and helps develop new cells. The propagation speed of the system is 8 m/s, and the northeastern end moves faster than the southwestern end, creating clockwise rotation of the system. The propagation speed and the rotation of the simulated system coincide well with radar observations. The clockwise rotation of the system can be explained by the stronger (weaker) outflow and weaker (stronger) inflow in the northeastern (southwestern) end. The propagation of the system is attributable to the weak (≤7 m/s, storm relative) rear-to-front flow in the moist environment. Thus, the arc-shaped precipitation system common to the pre-monsoon period in Bangladesh develops through a balance of strong southwesterly or southerly moist inflow in the low altitudes below 2 km and relatively weak outflow in the rear of the system.