WRF Simulations Research Articles

Impact of downscaled rainfall biases on projected runoff changes

Abstract. Realistic projections of changes to daily rainfall frequency and magnitude, at catchment scales, are required to assess the potential impacts of climate change on regional water supply. We show that quantile–quantile mapping (QQM) bias-corrected daily rainfall from dynamically downscaled WRF simulations of current climate produce biased hydrological simulations, in a case study for the state of Victoria, Australia (237 629 km2). While the QQM bias correction can remove bias in daily rainfall distributions at each 10 km × 10 km grid point across Victoria, the GR4J rainfall–runoff model underestimates runoff when driven with QQM bias-corrected daily rainfall. We compare simulated runoff differences using bias-corrected and empirically scaled rainfall for several key water supply catchments across Victoria and discuss the implications for confidence in the magnitude of projected changes for mid-century. Our results highlight the imperative for methods that can correct for temporal and spatial biases in dynamically downscaled daily rainfall if they are to be suitable for hydrological projection.

Rainfall Mechanisms for One of the Wettest Tropical Cyclones on Record in Australia—Oswald (2013)

Abstract Tropical Cyclone Oswald (2013) is considered to be one of the highest-impact storms to make landfall in northern Australia even though it only reached a maximum category 1 intensity on the Australian category scale. After making landfall on the west coast of Cape York Peninsula, Oswald turned southward, and persisted for more than 7 days moving parallel to the coastline as far south as 30°S. As one of the wettest tropical cyclones (TCs) in Australian history, the favorable configurations of a lower-latitude active monsoon trough and two consecutive midlatitude trough–jet systems generally contributed to the maintenance of the Oswald circulation over land and prolonged rainfall. As a result, Oswald produced widespread heavy rainfall along the east coast with three maximum centers near Weipa, Townsville, and Rockhampton, respectively. Using high-resolution WRF simulations, the mechanisms associated with TC Oswald’s rainfall are analyzed. The results show that the rainfall involved different rainfall mechanisms at each stage. The land–sea surface friction contrast, the vertical wind shear, and monsoon trough were mostly responsible for the intensity and location for the first heavy rainfall center on the Cape York Peninsula. The second torrential rainfall near Townsville was primarily a result of the local topography and land–sea frictional convergence in a conditionally unstable monsoonal environment with frictional convergence due to TC motion modulating some offshore rainfall. The third rainfall area was largely dominated by persistent high vertical wind shear forcing, favorable large-scale quasigeostrophic dynamic lifting from two midlatitude trough–jet systems, and mesoscale frontogenesis lifting.

Possible impacts of climate change on fog in the Arctic and subpolar North Atlantic

Abstract. A conventional parameterization of midlatitude warm fog occurrence, based on in situ observations, is employed to estimate marine surface visibility in the Arctic and North Atlantic from three datasets: an ensemble member of the Hadley Earth System (HadGEM2) model and a nested regional WRF simulation that follow historical and future emissions scenarios for 1979–2100, and the ERA-Interim reanalysis for 1979–2004. Over large scales (of an entire year and region), all three gridded datasets agree well in terms of variables like surface air temperature, whose systematic differences seem small by comparison with its predicted change over the course of this century. On the other hand, systematic differences are more apparent in large-scale estimates of relative humidity and visibility. Large differences are attributed to a sensitivity to representation bias that is inherent in the formulation of each individual model and analysis. Two simple linear calibrations are examined, both of which assume that an in situ based parameterization is broadly consistent with the use of marine (ICOADS) observations of air and dew point temperature as an error-free reference. A single-step calibration is considered that takes the mean and variance of ICOADS frequency distributions as a reference. A two-step calibration is also performed in which ICOADS collocations are taken as a reference for the ERA reanalysis, which in turn is taken as a large-scale reference for the 1979–2004 HadGEM2 and WRF simulations. Both linear calibrations are applied (locally in time and space to air and dew point temperature) to the future climate scenarios of HadGEM2 and WRF. Although ICOADS observations are not error-free and parameterized visibility estimates are unlikely to capture much more than half the variance found in observations, attempts are made to present consistent regional changes in the frequency of high relative humidity, as a proxy for warm fog occurrence. The large-scale decrease in visibility over the 21st century is in the range of 8 %–12 % in the Arctic and 0 %–5 % in the North Atlantic.

Comparative study on WRF model simulations from the viewpoint of optimum ship routing

In addition to other studies focused on the WRF performance of rainfall, temperature, and precipitation, the present study provides practical suggestions for ship operators on choosing grid point value (GPV) datasets and WRF modeling approaches to help make better decisions on weather routing by estimating strong wind with a higher accuracy. Two different GPV datasets, the NCEP-FNL and ERA-Interim, are used for WRF modeling. Various modeling approaches, which include applying different physics options and choosing whether to adopt the four-dimensional-data-assimilation, are employed for numerical simulation. Then, on-board measurements of a bulk carrier has been used to validate the calculated wind of eight rough-sea navigational cases. This occurs over a wide spatial range, covering ocean regions such as the Northwest Pacific Ocean, South of Australia, Tasman Sea, Southeast of Africa, and Southeast of America, as well as a wide temporal range of four years. Results of 64 WRF simulations show that ERA-Interim is suitable for longer periods, while NCEP-FNL can analyze extremely high winds. Additionally, grid nudging can also reduce the wind strength due to its smoothing effect. Finally, selections of these different GPV datasets and modeling approaches for ship weather routing are discussed, particularly for rough-sea navigations.

Front–orography interactions during landfall of the 1992 New Year's Day Storm

Abstract. Although following a common synoptic evolution for this region, the 1992 New Year's Day Storm was associated with some of the strongest winds observed along the Norwegian west coast. The narrow wind band along its bent-back front became famous as the “poisonous tail” and paved the way towards today's sting jet terminology. This article re-examines the storm's landfall with a particular focus on the interaction with the orography. Sensitivity analyses based on WRF simulations demonstrate that the formation and the evolution of the warm-air seclusion and its poisonous tail are largely independent of orography. In contrast, the warm sector of the storm undergoes considerable orographically induced modifications. While moving over the orography, both warm and cold fronts are eroded rapidly. This development fits neither the cold-air-damming paradigm nor the passive-advection paradigm describing front–orography interactions. The warm sector is lifted over the orography, thereby accelerating the occlusion process. The insensitivity of the warm-air seclusion to the orographic modifications of the warm sector raises the question of to which extent these entities are still interacting after the onset of the occlusion process.

Characteristics of the upstream flow patterns during PM2.5 pollution events over a complex island topography

Meteorological and geographic conditions play important roles in the occurrence of air pollution. This is especially true in island countries like Taiwan, where the atmospheric flow plays a key role in determining air quality. The atmospheric flows of 334 air pollution events between 2013 and 2017 were analyzed via radiosonde data and WRF simulations to elucidate how the spatial distribution of PM2.5 concentrations in Taiwan is correlated to its terrain and environmental wind fields. The variables included in this analysis were the wind directions of the environmental wind field, Froude number (Fr), pitching angle (PA) of the surface-level and atmospheric boundary layer (ABL)-level airflows through the terrain, differences in wind speed between surface-level and ABL-level airflows, and the formation of lee-side vortices. Thus, we examined how the areas of air pollutant accumulation are related to the passage of environmental airflows through mountain ranges. Based on a composite analysis, we determined that the air pollution events can be divided into 10 categories, depending on the direction of the environmental airflows and the geographic location of the air pollution events. Fr was less than 0.25 in 87% air pollutant events, which indicates that the environmental airflows had a high stability and tended to split around the terrain. Each category corresponded to different environmental wind directions and PAs. It was found that air pollution events occurred the most frequently when PA was close to the north, and these events were accompanied by obvious differences in wind speed between surface-level and ABL-level. During the analysis of airflow characteristics via the WRF simulation, it was found that lee-side vortices were formed on the orthographic lee-side of terrain in seven synoptic flow categories. The results of this study provide evidence and variables to demonstrate how atmospheric flow, terrain, and pollutant accumulation areas are linked to each other in island.

On the Forecast Sensitivity of MCS Cold Pools and Related Features to Horizontal Grid Spacing in Convection-Allowing WRF Simulations

Abstract While the implementation of convection-allowing models has improved the representation of convective features, a consensus is lacking regarding what horizontal grid spacing most appropriately resolves convective structures, is computationally feasible, and provides the most useful output to forecasters. The present study evaluates 14 simulated MCSs with 3-, 1- and 0.333-km horizontal grid spacing in order to understand sensitivity in simulated MCS forward propagation speeds and cold pool behavior with decreased grid spacing. MCS cold pools were found to be significantly larger in runs using finer grid spacing. In addition, a greater similarity in solutions occurred when grid spacing was refined to 1 km and less, with 1- and 0.333-km MCS cold pools more similar in magnitude, depth, length, and areal coverage, than 3-km cold pools. The 1-km simulations demonstrated a small increase in forecast skill for 3-h QPF throughout MCS evolution compared to 3-km runs. The 1-km MCS 9-h precipitation swaths were also better aligned with observations compared to 3-km simulations. When evaluating MCS forward propagation speeds, however, 3-km simulated MCS speeds were more similar to observations compared to 1 km.

Characterizing precipitation in high altitudes of the western Tibetan plateau with a focus on major glacier areas

AbstractSolid water resources such as glaciers and snow cover are widely distributed in the western Tibetan plateau (WTP), and precipitation is the key supply of them. However, the characteristics of precipitation over the WTP remain unclear due to sparse observations. Using observation‐based gridded data (APHRODITE, GPCC), satellite products (TRMM, GPM), reanalysis data (ERA5, ERA‐Interim, JRA‐55) and weather research and forecasting model (WRF)‐simulation data (HAR10, HAR30), this study investigated the precipitation characteristics in high mountain areas (altitude >2,500 m a.s.l.) of the WTP (26–44°N, 70–85°E) from 2001 to 2013. Results show that annual GPM precipitation (336 mm) is much lower than that of other data sets (570–800 mm) for the WTP and appears to be an outlier; meanwhile, the other data sets show increasing precipitation as grid resolution decreases (becomes coarser). In contrast, high spatial resolution WRF simulations (HAR10, HAR30) yielded considerably higher precipitation in four strongly glacierized alpine areas (Western Himalaya, Karakorum, Tajikistan, and west Kunlun) than the other data sets. When compared to precipitation estimated using the water balance in nine glacierized catchments of the Upper Indus River Basin, the HAR data sets more reasonably represent the amount of precipitation in high mountain areas than the other data sets. The surface albedo, snow cover extent, and snow cover duration observed by satellites further indicate that the HAR data sets yield reasonable spatial variability of precipitation. This implies that the commonly used data sets (such as observation data and satellite products) underestimate the precipitation in high mountain areas. According to the HAR10 data, the four major glacierized areas on the WTP have similar precipitation amounts in summer and autumn, and the winter–spring contribution to annual precipitation is at least 70% in three of these areas (Western Himalaya, Karakorum, and Tajikistan). This pattern indicates that precipitation over the high‐altitude areas in WTP is mainly controlled by the mid‐latitude westerlies. The annual precipitation in these areas is over 1,000 mm, which is comparable to that of the southeast TP but much larger than that on the central plateau. Therefore, the spatial precipitation pattern across the TP is ‘wet east and west, dry middle’.

Analog Site Experiment in the High Andes-Atacama Region: Surface Energy Budget Components on Ojos del Salado from Field Measurements and WRF Simulations.

Remote sensing data are abundant, whereas surface in situ verification of atmospheric conditions is rare on Mars. Earth-based analogs could help gain an understanding of soil and atmospheric processes on Mars and refine existing models. In this work, we evaluate the applicability of the Weather Research and Forecasting (WRF) model against measurements from the Mars analog High Andes-Atacama Desert. Validation focuses on the surface conditions and on the surface energy budget. Measurements show that the average daily net radiation, global radiation, and latent heat flux amount to 131, 273, and about 10 W/m2, respectively, indicating extremely dry atmospheric conditions. Dynamically, the effect of topography is also well simulated. One of the main modeling problems is the inaccurate initial soil and surface conditions in the area. Correction of soil moisture based on in situ and satellite soil moisture measurements, as well as the removal of snow coverage, reduced the surface skin temperature root mean square error from 9.8°C to 4.3°C. The model, however, has shortcomings when soil condition modeling is considered. Sensible heat flux estimations are on par with the measurements (daily maxima around 500 W/m2), but surface soil heat flux is greatly overestimated (by 150-500 W/m2). Soil temperature and soil moisture diurnal variations are inconsistent with the measurements, partially due to the lack of water vapor representation in soil calculations.

Impact of Different Meteorological Initializations on WRF Simulation During the KORUS-AQ Campaign

Impact of Different Meteorological Initializations on WRF Simulation During the KORUS-AQ Campaign

Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super Typhoon Rammasun (2014)

Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of Typhoon Rammasun. We show that the basic difference stems from subtle differences in initializations of (a) 500–850-hPa environmental winds, and (b) midlevel moisture and ventilation. We then describe how these differences impact on the evolving convective organization, storm structure, and the timing of RI. As expected, ascent, diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI. The evolution of vortex cloudiness from this member is similar to the actual imagery, with the development of an inner cloud band wrapping inwards to form the eyewall. We present evidence that this structure, and hence the enhanced diabatic heating, is related to the tilt and associated dynamics of the developing inner-core in shear. For the most accurate ensemble member: (a) inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air, which is transported to the down-shear sector to feed a developing convective asymmetry; (b) with minimal ventilation, undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; and (c) this process seems related to a critical down-shear tilt of the vortex from midlevels, and the vertical phase-locking of the circulation over up-shear quadrants. For the member that forecasts a much-delayed RI, these processes are inhibited by stronger vertical wind shear, initially resulting in poor vertical coherence of the circulation, lesser moisture and larger ventilation. Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear, moisture and vortex inner-structure.

Accuracy evaluation of atmospheric stability at offshore area by WRF simulation

Accuracy evaluation of atmospheric stability at offshore area by WRF simulation

Offshore wind resource assessment using MCP combing short-term Lidar observations and long-term WRF simulations

Offshore wind resource assessment using MCP combing short-term Lidar observations and long-term WRF simulations

A Numerical Study of Convection Initiation Associated With a Gust Front in Bohai Bay Region, North China

AbstractConvection initiation (CI) associated with a cold pool gust front (GF) of a mesoscale convective system (MCS) occurred in the Bohai Bay region, North China, is investigated using a real‐data, high‐resolution Weather Research and Forecasting model (WRF) simulation. Observations revealed that many CIs occurred successively near the GF, most of which developed significantly in both size and intensity and eventually merged with the MCS. The WRF simulation captured the general features of the CIs, GF, and MCS. Lagrangian vertical momentum budgets were conducted along the backward trajectories of air parcels within a convective cell initiated near the GF. The total vertical acceleration was decomposed into dynamic and buoyant components. The results showed that the dynamic acceleration played a decisive role for the CI, which can be further decomposed into five terms. The horizontal curvature term was rather small, with the height variation of air density almost zero. Although both vertical and horizontal extension terms played a positive role during the CI process, the vertical twisting term had the greatest contribution to the CI. This reflects the vertical tilting (to the forward of the GF) of an elevated updraft which was generated by the convergence of a forward‐tilting updraft at the GF leading edge and another airflow coming from ahead of the GF.

A multiscale analysis of the tornadoes of 30–31 May 2019 in south-central Chile

Abstract On 30 and 31 May 2019, tornadoes occurred in the cities of Los Angeles and Talcahuano/Concepcion in south-central Chile, in a region where tornado activity is not common. The main goal of this study was to analyze these tornadoes across multiple scales: synoptic, mesoscale, and subseasonal. On the synoptic scale, the tornadoes were associated with an anomalous 500-hPa trough and associated surface cyclone to the west of Chile. A strong (20+ m s−1) low-level jet accompanied this trough, potentially enhanced by flow blocking by the Andes. A relatively warm and saturated surface layer combined with cold upper-level temperatures in the trough to yield 200–500 J kg−1 of CAPE on both days. This CAPE was accompanied by high levels of both deep-layer and low-level shear. Storm motions inferred by lightning swaths and GOES-IR imagery, along with estimates of storm motion and updraft helicity from a high-resolution WRF simulation, suggested this CAPE-shear combination was sufficient for the tornadic thunderstorms to be supercells. Finally, anomalies of sea level pressure, 500-hPa height, and surface dew point temperature from 27 to 31 May 2019 resembled long-term composite anomalies for MJO phases 1 and 2, suggesting a subseasonal link between the extreme event in Chile and convection in the tropics.

Observed and Simulated Precipitation over Northeastern North America: How Do Daily and Subdaily Extremes Scale in Space and Time?

AbstractThe characterization of extreme precipitation at fine spatiotemporal scale represents a paramount challenge in hydroclimate sciences due to large uncertainties affecting the precipitation estimation from existing datasets. Comparing the spatiotemporal structure of precipitation extremes estimated from different datasets thus represents an essential step for climate model evaluation, as it provides insight into a model’s ability to simulate atmospheric processes occurring at different scales. This study compares the probability distributions and the annual and diurnal cycles of occurrence of daily and subdaily precipitation annual maxima (AM) estimated over northeastern North America from five observed and simulated datasets: meteorological station series, the bias-corrected (CRT) satellite CMORPH, version 1.0, and the Multi-Source Weighted-Ensemble Precipitation (MSWEP), version 2, gridded datasets, various Canadian RCM, version 5 (CRCM5), simulations, and a 13-yr convection-permitting WRF, version 3.4.1, simulation. ERA-Interim-driven CRCM5 and WRF simulations well reproduced subdaily extreme quantiles and the AM annual and diurnal cycles observed at stations, while CMORPH and MSWEP displayed good performance only for daily and longer extreme statistics. The spatiotemporal statistical structure of precipitation extremes is then assessed considering the variation of AM quantiles across various spatial scales and durations. The results suggest that a two-parameter analytical relationship well describes the AM spatiotemporal structure at the regional scale, allowing us to approximate some crucial properties of point precipitation extremes from gridded datasets. Averaging the estimates from various members of the initial-condition CRCM5 Large Ensemble (CRCM5-LE) also made it possible to reduce the sampling errors and robustly estimate the AM spatiotemporal structure at the local scale of each model grid box.

Warm-Sector Heavy Rainfall in Southern China and Its WRF Simulation Evaluation: A Low-Level-Jet Perspective

Abstract Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within the warm sector hundreds of kilometers south of a front or without a front during April–June, characterized by poor predictability and a close relationship with low-level jets (LLJs). Based on 45 warm-sector heavy rainfall episodes in 2013 and 2014 in southern China, this study examines their general characteristics and evaluates the performance of convection-permitting WRF Model simulations from an LLJ perspective. The results show that 64% of the warm-sector heavy rainfall episodes are associated with an LLJ (LLJ type) and 36% are not (no-LLJ type). The LLJ type is distinct from the no-LLJ type, with large rainfall accumulation along the coastal area. It is more common for LLJs to occur at both 800 and 925 hPa in the LLJ type, where there is a wide 800-hPa LLJ west of Guangdong Province and two 925-hPa LLJs over Beibu Gulf and the South China Sea (SCS). The coastal convergence associated with the terminus of the LLJ on 925 hPa is conducive to the coastal rainfall. WRF generally presents lower QPF skill in the LLJ type than in the no-LLJ type, due to the severe underestimation of coastal rainfall. The QPF skill of the LLJ type is significantly correlated with the forecast accuracy of LLJs, especially at 925 hPa. The north bias of the simulated LLJ on 925 hPa over the SCS and the associated overestimation of wind speed below ~900 hPa over the inland region weaken the coastal convergence and eventually lead to the underestimation in coastal precipitation.

Optimized wind and wave energy resource assessment and offshore exploitability in the Mediterranean Sea

A mapping of wind and wave potentials of the whole Mediterranean basin, aimed at identifying the areas where wind and wave events are not time-correlated, was performed in order to plan offshore coupled wind-wave energy extraction. For this purpose, meteorological and marine conditions of the whole Mediterranean basin for the period 1979–2016 were obtained by means of WRF and WaveWatchIII simulations. More promising areas where to plan a hybrid installation for the joint exploitation of wind and wave energy resources were thus selected on the basis of a suitable statistical index. Such an index, combines information on the availability of wind and waves together with the degree of correlation of the occurrence of the two resources. The index will efficiently select the most interesting area in the Mediterranean Sea characterized by non-concomitant-in-time availability of wind and wave thus ensuring an optimized time-availability of the combined resources. Finally the power production of an hypothetical hybrid plant composed by different models of wind rotors and wave energy converters is evaluated for four test sites. The results of this analysis shows that the described index represents a useful tool for preliminary evaluation of areas where combined wind-wave energy extraction could be more profitable.

Nonlinear Zonal Propagation of Organized Convection in the Tropics

Abstract A wide range of the observed variability in the ITCZ is frequently explained in terms of equatorially trapped modes arising from Matsuno’s linear shallow-water model. Here, a series of zonally constant, meridionally symmetric aquachannel WRF simulations are used to study the propagation of tropical cloud clusters (CCs; patches of deep cloudiness and precipitation) in association with eastward-moving super cloud clusters (SCCs), also called convectively coupled Kelvin waves (CCKWs). Two independent but complementary methods are used: the first, from a local approach, involves a CC-tracking algorithm, while the second uses Lagrangian trajectories in a nonlocal framework. We show that the large-scale flow in low to midlevels advects the CCs either eastward or westward depending on model climatology, proximity to the CCKW axis, and latitude. Moreover, for most analyzed cases, sequences of CCs oscillate, describing qualitatively sinusoidal-like paths in longitude–time space, although with sharp transitions from westward to eastward motion due to westerly wind burst activity associated with the CCKWs. We also find that the discrete precipitation elements (CCs) are embedded in continuous tracks of positive moisture anomalies, which are parallel to the Lagrangian trajectories themselves. A conceptual model of the nonlinear SCC–CC interaction is presented.

Accuracy Comparison of Coastal Wind Speeds between WRF Simulations Using Different Input Datasets in Japan

In order to improve the accuracy of the wind speed simulated by a mesoscale model for the wind resource assessment in coastal areas, this study evaluated the effectiveness of using the Japan Meteorological Agency (JMA)’s latest and finest (2 km × 2 km) grid point value (GPV) data, produced from the local forecast model (LFM) as input data to the mesoscale model. The evaluation was performed using wind lidar measurements at two sites located on the coasts of the Sea of Japan and Pacific Ocean. The accuracy of the LFM–GPV was first compared with that of two products from the JMA Meso Scale Model (MSM) (5 km × 5 km): MSM-GPV and mesoscale analysis (MANAL). Consequently, it was shown that LFM–GPV exhibited the most accurate wind speeds against lidar measurements. Next, dynamical downscaling simulations were performed using the weather research and forecasting model (WRF) forced by the three datasets above, and their results were compared. As compared to the GPVs, it was found that the WRF dynamical downscaling simulation using them as input can improve the accuracy of the coastal wind speeds. This was attributed to the advantage of the WRF simulation to improve the negative bias from the input data, especially for the winds blowing from the sea sectors. It was also found that even if the LFM–GPV is used as an input to the WRF simulation, it does not always reproduce more accurate wind speeds, as compared to the simulations using the other two datasets. This result is partly owing to the tendency of WRF to overestimate the wind speed over land, thus obscuring the higher accuracy of the LFM–GPV. It was also shown that the overestimation tendency cannot be improved by only changing the nudging methods or the planetary boundary layer schemes in WRF. These results indicate that it may be difficult to utilize the LFM–GPV in the WRF wind simulation, unless the overestimation tendency of WRF itself is improved first.