Coastal Precipitation Research Articles

CalWater Field Studies Designed to Quantify the Roles of Atmospheric Rivers and Aerosols in Modulating U.S. West Coast Precipitation in a Changing Climate

Abstract The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions. To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemistry, cloud microphysics, and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on the remaining gaps, a vision was developed to extend these studies offshore over the eastern North Pacific and to enhance land-based measurements from 2014 to 2018 (CalWater-2). The dataset and selected results from CalWater-1 are summarized here. The goals of CalWater-2, and measurements to date, are then described. CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.

Seasonal anomalies of water salinity in the Gelendzhik region of the Black Sea according to shipborne monitoring data

The paper presents results of analyzing the data on the variability of salinity in the upper layer of the coastal zone (including the seasonal thermocline) of the Black Sea during the 2010–2013 warm seasons (April–November). The data for the analysis were obtained from the regularly conducted CTD probing the by R/V Ashamba on a cross section abeam the Golubaya Bay (Gelendzhik). The relationship between salinity anomalies, coastal precipitation, and wind forcing is analyzed. It is shown that the contribution of thermal stratification to the density stratification in the seasonal thermocline is almost always greater than the contribution of salinity stratification, and the ratio of the former to the latter increases from April to November.

Evidence for added value of convection‐permitting models for studying changes in extreme precipitation

AbstractClimate model resolution can affect both the climate change signal and present‐day representation of extreme precipitation. The need to parametrize convective processes raises questions about how well the response to warming of convective precipitation extremes is captured in such models. In particular, coastal precipitation extremes can be sensitive to sea surface temperature (SST) increase. Taking a recent coastal precipitation extreme as a showcase example, we explore the added value of convection‐permitting models by comparing the response of the extreme precipitation to a wide range of SST forcings in an ensemble of regional climate model simulations using parametrized and explicit convection. Compared at the same spatial scale, we find that the increased local intensities of vertical motion and precipitation in the convection‐permitting simulations play a crucial role in shaping a strongly nonlinear extreme precipitation response to SST increase, which is not evident when convection is parametrized. In the convection‐permitting simulations, SST increase causes precipitation intensity to increase only until a threshold is reached, beyond which further SST increase does not enhance the precipitation. This flattened response results from an improved representation of convective downdrafts and near‐surface cooling, which damp the further intensification of precipitation by stabilizing the lower troposphere locally and also create cold pools that cause subsequent convection to be triggered at sea, rather than by the coastal orography. These features are not well represented in the parametrized convection simulations, resulting in precipitation intensity having a much more linear response to increasing SSTs.

Offshore Propagation of Coastal Precipitation

Abstract This work focuses on the seaward propagation of coastal precipitation with and without mountainous terrain nearby. Offshore of India, diurnal propagation of precipitation is observed over the Bay of Bengal. On the eastern side of the bay, a diurnal but nonpropagating signal is observed near the west coast of Burma. This asymmetry is consistent with the inertio-gravity wave mechanism. Perturbations generated by diurnal heating over the coastal mountains of India propagate offshore, amplify in the upwind direction, and dissipate in the downwind direction relative to the steering wind, owing to critical-level considerations. A linear model is applied to evaluate sensitivity to gravity waves, as these affect deep moist convection and propagation. Analyses are performed for various heating depths, mountain widths, stability, Coriolis effect, background mean wind, and friction. Calculations reveal how these factors affect the amplitude, dissipation, initiation phase, and propagation speed of the diurnal disturbance. The propagation of precipitation triggered by land–sea breezes is distinguishable from that triggered by a mountain–plains circulation. Convection resulting purely from mountain heating begins earlier, propagates slower, and damps faster than that of the land–sea breeze. For mountains near a coast, slower propagation and stronger earlier convection result from a resonance-like combination of two dynamical mechanisms. The propagation of precipitation is initially triggered by the mountain breeze near the coastal mountain. Over the open ocean, the dominant signal propagates as that of the land breeze but with stronger convection.

Global Detection and Analysis of Coastline-Associated Rainfall Using an Objective Pattern Recognition Technique

Abstract Coastally associated rainfall is a common feature, especially in tropical and subtropical regions. However, it has been difficult to quantify the contribution of coastal rainfall features to the overall local rainfall. The authors develop a novel technique to objectively identify precipitation associated with land–sea interaction and apply it to satellite-based rainfall estimates. The Maritime Continent, the Bight of Panama, Madagascar, and the Mediterranean are found to be regions where land–sea interactions play a crucial role in the formation of precipitation. In these regions ~40%–60% of the total rainfall can be related to coastline effects. Because of its importance for the climate system, the Maritime Continent is a region of particular interest, with high overall amounts of rainfall and large fractions resulting from land–sea interactions throughout the year. To demonstrate the utility of this study’s identification method, the authors investigate the influence of several modes of variability, such as the Madden–Julian oscillation and the El Niño–Southern Oscillation, on coastal rainfall behavior. The results suggest that during large-scale suppressed convective conditions, coastal effects tend to modulate the rainfall over the Maritime Continent leading to enhanced rainfall over land regions compared to the surrounding oceans. The authors propose that the novel objective dataset of coastally influenced precipitation can be used in a variety of ways, such as to inform cumulus parameterization or as an additional tool for evaluating the simulation of coastal precipitation within weather and climate models.

Exposure age and climate controls on weathering in deglaciated watersheds of western Greenland

Fine-grained sediments deposited by retreating glaciers weather faster than the global average and this weathering can impact the global carbon cycle and oceanic fluxes of nutrients and radiogenic isotopes. Much work has focused on subglacial and proglacial weathering of continental ice sheets, but little is known about weathering and resulting fluxes from deglacial watersheds, which are disconnected from the ice sheets and discharge only annual precipitation and permafrost melt. We investigate the effects of exposure age and precipitation on weathering intensity in four deglacial watersheds on Greenland that form a transect from the coast near Sisimiut toward the Greenland Ice Sheet (GrIS) near Kangerlussuaq based on evaluations of major ion compositions, Sr isotope ratios, and mineral saturation states of waters and sediments. The transect is underlain by Archean orthogneiss and is characterized by gradients in moraine ages (∼7.5–8.0ky inland to ∼10ky at the coast) and water balance (−150mm/yr inland to +150mm/yr at the coast). Anion compositions are generally dominated by HCO3, but SO4 becomes increasingly important toward the coast, reflecting a switch from trace carbonate dissolution to sulfide mineral oxidation. Coastal watersheds have a higher proportion of dissolved silica, higher Na/Cl, Si/Ca, and lower Ca/Sr ratios than inland watersheds, indicating an increase in the relative proportion of silicate weathering and an increase in the extent of weathering toward the coast. More extensive weathering near the coast is also apparent in differences in the 87Sr/86Sr ratios of stream water and bedload (Δ87Sr/86Sr), which decreases from 0.017 inland to 0.005 at the coast, and in increased saturation states relative to amorphous SiO2 and quartz. The steep weathering gradient from inland to coastal watersheds reflects enhanced weathering compared to that expected from the 2 to 3ky difference in exposure age caused by elevated coastal precipitation. The gradient of weathering with exposure age, water budget and distance from the ice sheet indicates that oceanic and atmospheric fluxes will change as continental glaciers retreat, precipitation patterns across the deglacial region readjust, and the relative proportion of deglacial to proglacial runoff increases.

Parasitism of a native Hawaiian stream fish by an introduced nematode increases with declining precipitation across a natural rainfall gradient

AbstractClimate change can promote disease emergence if shifting conditions favour infection of native fauna by introduced parasites and pathogens. In Hawai'i, climate warming is predicted to reduce net precipitation and surface flow in streams, which in turn could increase parasitism of native stream fishes by non‐native parasites. In this study, we utilised a natural precipitation gradient across the Hamakua coast on the island of Hawai'i to assess the relationship between precipitation and infection of Awaous stamineus, a native amphidromous goby, by the introduced nematode Camallanus cotti. We found that the abundance, intensity and prevalence of C. cotti in A. stamineus increased with declining rainfall. Our results also show that parasitism tracks precipitation patterns across the archipelago and that parasitism increases even with moderate decreases in rainfall. As the Hamakua coast precipitation gradient represents a proxy for predicted climate‐driven reductions in precipitation, these findings suggest that infection of native Hawaiian fishes by introduced parasites will increase if climate conditions change as expected. Our findings also suggest that parasitism may be exacerbated by other factors that reduce surface flow, including water extraction for agricultural and urban uses. If so, then adaptive management of minimum flow standards in Hawai'i and elsewhere could improve the well‐being of at‐risk native fishes by alleviating parasitism under current and future climate conditions.

The Atlantic ITCZ bias in CMIP5 models

Precipitation over the tropical Atlantic in 24 atmospheric models is analyzed using an object-based approach, which clusters rainy areas in the models as precipitation objects and calculates their properties such as size, amplitude, and location. Based on the distribution of precipitation objects over land and over ocean, two classes of models emerge. The first class of models has a reasonable representation of objects over land but misplaces the ocean object westward, near the coast of Brazil, instead of the central Atlantic as observed. The second class of models show small-sized objects over land with intense precipitation values; for these models, the ocean object is located eastward, near the coast of Guinea. The Atlantic intertropical convergence zone structure in the models exhibits either the West or the East Atlantic bias. No model matches the observed precipitation distribution. The two distinct model behaviors in the mean state are traced to the coastal precipitation bias of the models in boreal spring. In this season, the two model groups place the main precipitation object on opposite coasts—one group puts it at the south coast of Brazil and the other group places it at the Gulf of Guinea. This west–east partitioning of precipitation is sustained in boreal summer, resulting in the West and East Atlantic bias in the annual mean. It is found that models with the East Atlantic bias tend to be high resolution models which rain excessively over the Gulf of Guinea starting from boreal spring.

Maritime climate influence on chaparral composition and diversity in the coast range of central California.

We investigated the hypothesis that maritime climatic factors associated with summer fog and low cloud stratus (summer marine layer) help explain the compositional diversity of chaparral in the coast range of central California. We randomly sampled chaparral species composition in 0.1-hectare plots along a coast-to-interior gradient. For each plot, climatic variables were estimated and soil samples were analyzed. We used Cluster Analysis and Principle Components Analysis to objectively categorize plots into climate zone groups. Climate variables, vegetation composition and various diversity measures were compared across climate zone groups using ANOVA and nonmetric multidimensional scaling. Differences in climatic variables that relate to summer moisture availability and winter freeze events explained the majority of variance in measured conditions and coincided with three chaparral assemblages: maritime (lowland coast where the summer marine layer was strongest), transition (upland coast with mild summer marine layer influence and greater winter precipitation), and interior sites that generally lacked late summer water availability from either source. Species turnover (β-diversity) was higher among maritime and transition sites than interior sites. Coastal chaparral differs from interior chaparral in having a higher obligate seeder to facultative seeder (resprouter) ratio and by being dominated by various Arctostaphylos species as opposed to the interior dominant, Adenostoma fasciculatum. The maritime climate influence along the California central coast is associated with patterns of woody plant composition and β-diversity among sites. Summer fog in coastal lowlands and higher winter precipitation in coastal uplands combine to lower late dry season water deficit in coastal chaparral and contribute to longer fire return intervals that are associated with obligate seeders and more local endemism. Soil nutrients are comparatively less important in explaining plant community composition, but heterogeneous azonal soils contribute to local endemism and promote isolated chaparral patches within the dominant forest vegetation along the coast.

Land–Sea Thermal Contrast and Intensity of the North American Monsoon under Climate Change Conditions

Abstract The hypothesis that global warming during the twenty-first century will increase the land–sea thermal contrast (LSTC) and therefore the intensity of early season precipitation of the North American monsoon (NAM) is examined. To test this hypothesis, future changes (2075–99 minus 1979–2004 means) in LSTC, moisture flux convergence (MFC), vertical velocity, and precipitation in the region are analyzed using six global climate models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) under the representative concentration pathway 8.5 (RCP8.5) emission scenario. A surface LSTC index shows that the continent becomes warmer than the ocean in May in the North American Regional Reanalysis (NARR) and ECMWF Interim Re-Analysis (ERA-Interim) and in June in the mean ensemble of the GCMs (ens_GCMs), and the magnitude of the positive LSTC is greater in the reanalyses than in the ens_GCMs during the historic period. However, the reanalyses underestimate July–August precipitation in the NAM region, while the ens_GCMs reproduces the peak season surprisingly well but overestimates it the rest of the year. The future ens_GCMs projects a doubling of the magnitude of the positive surface LSTC and an earlier start of the continental summer warming in mid-May. Contrary to the stated hypothesis, however, the mean projection suggests a slight decrease of monsoon coastal precipitation during June–August (JJA), which is attributed to increased midtropospheric subsidence, a reduced midtropospheric LSTC, and reduced MFC in the NAM coastal region. In contrast, the future ens_GCMs produces increased MFC and precipitation over the adjacent mountains during JJA and significantly more rainfall over the entire NAM region during September–October, weakening the monsoon retreat.

Long‐term variation of winter precipitation linked to sea‐surface heat fluxes around the Japan/East Sea

AbstractWe revealed long‐term variation of winter precipitation at the northwest coast of Japan and its link to decadal variation of autumn‐to‐winter sea‐surface fluxes from the Japan/East Sea with observational data of sea‐surface fluxes, sea‐surface temperature (SST), and ocean heat content (OHC) data. Much precipitation was observed after 1995 because both SST and OHC were significantly higher in the Japan/East Sea. In a numerical experiment forced by the climatological SST that excluded the decadal change, the decadal variation of the land precipitation did not appear, suggesting that the decadal variation of the coastal precipitation is sensitive to SST.

Atlantic warm and cold water events and impact on African west coast precipitation

ABSTRACTVariability of sea surface temperatures (SSTs) in the tropical southeast Atlantic Ocean has previously been shown to significantly contribute to changes in summer rainfall along the West African as well as the Angolan coast. This study examines links between southeast Atlantic SST and African west coast precipitation variability for an extended 60‐year period from 1951 to 2010. In contrast to earlier studies, our analyses cover the whole Atlantic coast from Guinea to South Africa and are not limited to specific seasons. In addition to the analyses of the total variability, pronounced anomalies in terms of warm and cold water events and their impact on African west coast precipitation are analyzed. By using for the first time a new comprehensive classification of Atlantic Niño and Niña events, consistent results are achieved for a larger region of Africa, also considering Atlantic cold water events which have rather been neglected so far.Results show that, depending on the particular region, southeast Atlantic SSTs play an important role for coastal rainfall variability throughout the year. Furthermore, the rainfall response to Atlantic cold and warm water events appears to be asymmetric in season and magnitude. Atlantic cold events can cause a stronger decrease in rainfall along the West African and Gabon coast than the increase is in warm events. In addition, not all seasons show a significant rainfall response to both warm and cold water events.

Impacts of Aerosol Particle Size Distribution and Land Cover Land Use on Precipitation in a Coastal Urban Environment Using a Cloud-Resolving Mesoscale Model

Urban environments influence precipitation formation via response to dynamic effects, while aerosols are intrinsically necessary for rainfall formation; however, the partial contributions of each on urban coastal precipitation are not yet known. Here, the authors use aerosol particle size distributions derived from the NASA aerosol robotic network (AERONET) to estimate submicron cloud condensation nuclei (CCN) and supermicron CCN (GCCN) for ingestion in the regional atmospheric modeling system (RAMS). High resolution land data from the National Land Cover Database (NLCD) were assimilated into RAMS to provide modern land cover and land use (LCLU). The first two of eight total simulations were month long runs for July 2007, one with constant PSD values and the second with AERONET PSDs updated at times consistent with observations. The third and fourth runs mirrored the first two simulations for “No City” LCLU. Four more runs addressed a one-day precipitation event under City and No City LCLU, and two different PSD conditions. Results suggest that LCLU provides the dominant forcing for urban precipitation, affecting precipitation rates, rainfall amounts, and spatial precipitation patterns. PSD then acts to modify cloud physics. Also, precipitation forecasting was significantly improved under observed PSD and current LCLU conditions.

North American Climate in CMIP5 Experiments. Part II: Evaluation of Historical Simulations of Intraseasonal to Decadal Variability

This is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pacific. The frequency and mean amplitude of ENSO are generally well reproduced, although teleconnections with North American climate are widely varying among models and only a few models can reproduce the east and central Pacific types of ENSO and connections with U.S. winter temperatures. The models capture the spatial pattern of Pacific decadal oscillation (PDO) variability and its influence on continental temperature and West Coast precipitation but less well for the wintertime precipitation. The spatial representation of the Atlantic multidecadal oscillation (AMO) is reasonable, but the magnitude of SST anomalies and teleconnections are poorly reproduced. Multidecadal trends such as the warming hole over the central–southeastern United States and precipitation increases are not replicated by the models, suggesting that observed changes are linked to natural variability.

Climatic variability and trends in the surface waters of coastal British Columbia

Multi-decadal records of monthly sea surface temperature (SST) and sea surface salinity (SSS) collected at a set of lighthouse stations are used to examine climatic variability and trends in the coastal waters of British Columbia. Particular attention is given to relations between the water property anomalies and variability in coastal freshwater discharge and alongshore wind stress. Within the Strait of Georgia, SSS anomalies are closely related to Fraser River discharge anomalies. Along the Pacific coast, anomalies in alongshore wind stress and freshwater runoff have the characteristics of white noise processes. A cross-correlation analysis demonstrates that SST and SSS variability along the open west coast is consistent with the response of a first-order autoregressive process driven by anomalous alongshore wind stress and coastal freshwater discharge, respectively. Thus climatic variability of SST and SSS along the Pacific coast of British Columbia occurs, in part, through the integration of noisy atmospheric forcing and coastal precipitation. Seasonal correlations show that SST is strongly related to wind stress during winter and fall. Conversely, SSS is relatively weakly related to the alongshore wind during spring, suggesting that variability in upwelling makes only a modest contribution to variability of SSS in the nearshore environment. Consistent with previous studies, secular trends indicate long-term warming and freshening of the coastal ocean at most stations. It is shown that long-term SST trends can be obscured by the pronounced climatic variability of these waters, requiring that time series extend for several decades to be reliably detected.

A Nonlinear Response of Sahel Rainfall to Atlantic Warming

Abstract The response over West Africa to uniform warming of the Atlantic Ocean is analyzed using idealized simulations with a regional climate model. With warming of 1 and 1.5 K, rainfall rates increase by 30%–50% over most of West Africa. With Atlantic warming of 2 K and higher, coastal precipitation increases but Sahel rainfall decreases substantially. This nonlinear response in Sahel rainfall is the focus of this analysis. Atlantic warming is accompanied by decreases in low-level geopotential heights in the Gulf of Guinea and in the large-scale meridional geopotential height gradient. This leads to easterly wind anomalies in the central Sahel. With Atlantic warming below 2 K, these easterly anomalies support moisture transport from the Gulf of Guinea and precipitation increases. With Atlantic warming over 2 K, the easterly anomalies reverse the westerly flow over the Sahel. The resulting dry air advection into the Sahel reduces precipitation. Increased low-level moisture provides moist static energy to initiate convection with Atlantic warming at 1.5 K and below, while decreased moisture and stable thermal profiles suppress convection with additional warming. In all simulations, the southerly monsoon flow onto the Guinean coast is maintained and precipitation in that region increases. The relevance of these results to the global warming problem is limited by the focus on Atlantic warming alone. However, confident prediction of climate change requires an understanding of the physical processes of change, and this paper contributes to that goal.

Improved Quantitative Precipitation Forecasts by MHS Radiance Data Assimilation with a Newly Added Cloud Detection Algorithm

Abstract Satellite microwave humidity sounding data are assimilated through the gridpoint statistical interpolation (GSI) analysis system into the Advanced Research core of the Weather Research and Forecasting (WRF) model (ARW) for a coastal precipitation event. A detailed analysis shows that uses of Microwave Humidity Sounder (MHS) data from both NOAA-18 and MetOp-A results in GSI degraded precipitation threat scores in a 24-h model forecast. The root cause for this degradation is related to the MHS quality control algorithm, which is supposed to remove cloudy radiances. Currently, the GSI cloud detection is based on the brightness temperature differences between observations and the model background state at two MHS window channels. It is found that the GSI quality control algorithm fails to identify some MHS cloudy radiances in cloud edges where the ARW model has no cloud and the water vapor amount is low. A new MHS cloud detection algorithm is developed based on a statistical relationship between three MHS channels and the Geostationary Operational Environmental Satellite (GOES) imager channel at 10.7 μm. The 24-h quantitative precipitation forecast is improved rather than degraded by MHS radiance data assimilation when the new cloud detection algorithm is added to the GSI MHS quality control process. The temporal evolution of 3-h accumulative rainfall distributions compared favorably with that of multisensor NCEP observations and GOES-12 imager observations. The precipitation threat scores are increased by more than 50% after 3–6 h of model forecasts for 3-h rainfall thresholds exceeding 1.0 mm.

Inter-specific variation in salinity effects on germination in Pacific Northwest tidal wetland plants

Environmental stressors such as salinity may affect plant germination and early growth, eventually impacting the distribution and abundance of more mature individuals. In a lab study we evaluated germination sensitivity to salinity in 13 tidal wetland species found in the Pacific Northwest and then compared germination responses with the distributions of established plants along a soil salinity gradient. For two species we also tested whether seeds from wetlands with different salinity regimes varied in their tolerance of higher salinity. All species examined, except Sarcocornia perennis and Symphyotrichum subspicatum, showed maximum germination and seedling lengths under fresh to oligohaline (0–5ppt) conditions. Most species, including those commonly distributed in more saline wetland soils as adults, had reduced germination at salinities ≥10ppt. Sensitivity to elevated salinity in Triglochin maritima and Hordeum brachyantherum did not differ markedly between sampled populations. Our results show a mismatch between germination sensitivity and adult tolerance for about half of the species tested. The occurrence of low salinity in time or space may be necessary for optimal germination rates in these species. Future increases in estuarine salinity, perhaps in response to sea-level rise or reduced coastal precipitation, may alter germination patterns in tidal wetland plants and thereby shift plant composition.

둥지격자기법에 따른 영동지방의 겨울철 강수량에 대한 WRF 모델의 민감도: 사례 연구

기후변화로 인해 전 세계적으로 홍수, 폭염, 한파, 가뭄의 피해가 빈번하게 발생하고 있으며, 특히, 강원도 지역은 태백산맥과 동해의 복합적인 영향으로 다른 지역에 비해 재해기상이 빈번히 발생하는 지역으로 2011년 2월에 발생한 대설로 인해 농 어업시설 파괴, 도로 등을 마비시켜 막대한 경제적 피해를 초래한바 있다. 따라서 기상재해를 유발시키는 겨울철 강원 영동지역의 대설사례를 대상으로 둥지격자기법(One-way nesting 기법과 Two-way nesting 기법)을 각각 적용해 WRF 수치모델을 구동하여 그 결과를 분석해 봄으로써, 영동지역의 정확도 높은 대설 예측을 위해 어떠한 둥지격자기법을 사용하여야 하는지 알아보고 자 하였다. 먼저, 영동지역의 대표적인 두 가지 형태의 강수사례, 즉 해안형 강수사례(event A)와 산악형 강수사례(event B)를 각각 선정하여, 지표바람장과 발산장 그리고 누적 강수량의 분포를 분석하였다. 그리고 강수 예측의 정확성을 알아보고자 oneway nesting 기법과 two-way nesting 기법을 사용하여 모의된 3시간 누적 강수량에 대한 Threat Score를 계산하여 분석하였다. 먼저, one-way nesting 기법과 two-way nesting 기법의 사용에 따른 수치모의 결과의 차이를 상호 비교해보면, 해안형 강수사례(event A)의 경우, two-way nesting 기법을 사용한 모의가 one-way nesting 기법을 사용한 모의보다 영동 해안지역에서 서풍 계열의 바람이 부는 영역을 보다 넓게, 그리고 서풍 계열의 바람을 다소 강하게 모의하여 one-way nesting 기법을 사용한 모의보다 two-way nesting 기법을 사용한 모의가 해상쪽(동쪽)으로 갈수록 누적 강수량을 많이 모의하였다. 결과적으로 two-way nesting 기법이 one-way nesting 기법보다 영동지역에 모의되는 강수분포를 다소 동쪽으로 치우치는 경향을 보였다. 산악형 강수사례(event B)의 경우, two-way nesting 기법이 one-way nesting 기법보다 산악지역에서 동풍기류를 상대적으로 약하게 모의하였고 이에 따라, 산악지형에 의한 강제 상승효과가 상대적으로 작았기 때문에 산악지역에서 two-way nesting 기법을 이용하여 모의한 누적 강수량이 one-way nesting 기법을 사용하여 모의한 누적 강수량보다 적게 나타났다. 한편, 수치모의 결과에 대한 상호비교가 아닌 실제 관측과 비교하기 위하여, event A와 event B사례에 대하여 one-way nesting 기법과 two-way nesting 기법을 각각 적용하여 얻어진 수치모의 결과자료와 AWS 관측자료를 비교한 Threat score를 구하였다. 비록 one-way nesting 기법을 사용한 경우보다 two-way nesting 기법을 사용한 경우, 강수역이 보다 해상쪽(동쪽)으로 치우쳐 모의되는 경향을 보였지만, 두 기법에 대한 TS값의 차이(평균 약 0.01 정도)는 매우 작게 나타나, 어느 기법을 사용하든 강수 예측의 정확성은 거의 비슷한 수준이라고 볼 수 있었다. WRF (the Weather Research and Forecasting) model was run with one-way grid nesting or two-way grid nesting, respectively for Yeongdong province's two typical precipitation cases of the coastal precipitation case (event A) and the mountain precipitation case (event B) which were synoptically under the influence of the northeasterly to examine whether the choice of nesting method can have a significant impact on the precipitation distribution. For the coastal precipitation case (event A), the average accumulated-precipitation from simulation using two-way nesting was approximately 7.78 mm bigger than that using one-way nesting in the Yeongdong coast area. However, that from simulation using two-way nesting was approximately 4.23 mm smaller than that using one-way nesting in the mountain area. Similarly, for the mountain precipitation case (event B), the average accumulated-precipitation from simulation using two-way nesting was approximately 1.56 mm bigger than that using one-way nesting in the Yeongdong coast area. In addition, that from simulation using two-way nesting was approximately 4.91 mm smaller than that using one-way nesting in the mountain area. That is, for both the coastal precipitation case (event A) and the mountain precipitation case (event B), the average accumulated-precipitation from simulation using two-way nesting was bigger than that using one-way nesting in the coast area, but in the mountain area that was the reverse.

The influence of the North Sea on coastal precipitation in the Netherlands in the present-day and future climate

Coastal precipitation in the Netherlands for present-day and future climate using observations and simulations with the KNMI regional climate model RACMO2 is studied. Overall a good correspondence between RACMO2 and the observations is found for the present-day climate. Yet, the model generally simulates too sharp gradients in precipitation amounts near the coast line, and the maximum in precipitation is shifted towards the sea compared to the observations. The seasonal cycle of coastal precipitation (here the difference between precipitation near the coast and inland) is well linked to the land sea temperature contrast. Yet, this is not a one-to-one relation and other effects (like atmospheric stability) are likely to be important as well. Variations from year to year in monthly coastal precipitation correlate well with variations in strength of the cyclonic westerly circulation in the periods August to November. A moderate sensitivity to sea surface temperatures (≈6 mm month−1 K−1) has also been found. In simulations of climate change a small increase in coastal precipitation is generally found, except for the spring period. Using a more realistic prescription of North Sea temperatures (from a slab ocean model) only marginally changes these results. The increase in coastal precipitation is tempered by a decrease in land sea temperature contrast, and considerably larger increases of coastal precipitation could be expected if the temperature rise over sea equalled the temperature rise over land. Despite the fact that the overall changes in coastal effect appear relatively moderate, impact on more extreme events could be considerable, increasing the probability of exceeding extreme thresholds by a factor two or more.