High-resolution Regional Atmospheric Model Research Articles

Variability in coastal downwelling circulation in response to high-resolution regional atmospheric forcing off the Pearl River estuary

Abstract. We investigated the variabilities in coastal circulation and dynamics in response to spatiotemporally variable high-resolution atmospheric forcing off the Pearl River estuary during the downwelling wind. Our investigation focused on the dynamics of coastal downwelling circulation in response to variable atmospheric forcing of (1) single-station observation, (2) global reanalysis data, and (3) a high-resolution regional atmospheric model. We found that the high-resolution atmospheric model significantly improved the representations of the near-surface wind and air temperature, and the ocean model driven by the high-resolution and spatially variable atmospheric forcing improved the circulation and associated hydrographic properties in the coastal ocean. Momentum and vorticity analyses further revealed that the cross-isobath water exchange was primarily governed by the along-isobath pressure gradient force (PGF), which was influenced by different components of the atmospheric forcing. The spatial–temporal variability in high-resolution wind forcing determined the strength and structure of coastal circulation and improved estimates of cross-isobath transport and the associated PGF by refining the net stress curl and nonlinear advection of relative vorticity in the simulation. The high-resolution heat forcing can greatly improve the sea surface temperature simulation and adjust the nonlinear advection of relative vorticity, resulting in changes in cross-isobath transport.

Coupled hydrological and hydrodynamic modelling application for climate change impact assessment in the Nemunas river watershed–Curonian Lagoon–southeastern Baltic Sea continuum

Abstract. We analyse the cumulative impacts of climate change in a complex basin–lagoon–sea system continuum, which covers the Nemunas river basin, Curonian Lagoon, and the southeastern part of the Baltic Sea. A unique, state-of-the-art coupled modelling system was developed using hydrological and hydrodynamic models. The results of four regional downscaled models from the Rossby Centre high-resolution regional atmospheric climate model have been bias-corrected using in situ measurements and were used as forcing to assess the changes that the continuum will undergo until the end of this century. Results show that the Curonian Lagoon will be subjected to higher river discharges that in turn increase the outgoing fluxes into the Baltic Sea. Through these higher fluxes, both the water residence time and saltwater intrusion into the lagoon event frequency will decrease. Most of these changes will be more pronounced in the northern part of the lagoon, which is more likely to be influenced by the variations in the Nemunas river discharge. Its delta area may be susceptible to flooding as a result of the elevated discharge during winter. The southern part of the lagoon will experience lesser changes. While water temperatures in the entire lagoon and the southeastern Baltic Sea will steadily increase and salinity will decrease, the foreseen changes in the physical characteristics will not cause significant shifts in the ecosystem functioning but may affect the nutrient retention capacity. However, some ecosystem services such as ice fishing are expected to vanish completely due to the loss of ice cover.

Characteristics of Surface “Melt Potential” over Antarctic Ice Shelves based on Regional Atmospheric Model Simulations of Summer Air Temperature Extremes from 1979/80 to 2018/19

Abstract We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq; %) and intensity (MPI-int; K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23%–35%, MPI-int 1.2–2.1 K), lowest (2%–3%, <0 K) for the Ronne–Filchner and Ross ice shelves, and around 10%–24% and 0.6–1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for their respective region but tied to different large-scale climate forcings. The West Antarctic circulation resembles the central Pacific El Niño pattern with a stationary Rossby wave and a strong anticyclone over the high-latitude South Pacific. By contrast, the East Antarctic circulation comprises a zonally symmetric negative Southern Annular Mode pattern with a strong regional anticyclone on the plateau and enhanced coastal easterlies/weakened Southern Ocean westerlies. Values of MPI are 3–4 times larger for a lower temperature/melt threshold of 271.15 K used in a sensitivity test, as melting can occur at temperatures lower than 273.15 K depending on snowpack properties.

Impact of the extremely warm Gulf Stream on heavy precipitation induced by Hurricane Sandy (2012) during its extratropical transition

This study investigated the role of the extremely warm sea surface temperature (EWSST) over the Gulf Stream (GS) in heavy precipitation induced by Hurricane Sandy in October 2012 during its extratropical transition (ET). A control (CTL) run with observed SST and a climate (CLM) run with climatological SST over the GS were performed using a high-resolution regional atmospheric model. Sandy-induced precipitation over the Mid-Atlantic coastal region was larger by approximately 40% in the CTL run than in the CLM run during its ET phase. The western sector of Sandy in the CTL run exhibited the westward-tilting lower-to-middle tropospheric front and the pronounced convectively unstable layer created by large surface fluxes of sensible and latent heat from the EWSST region. The intensified convective instability was released by the frontal deep updrafts, enhancing frontal precipitation in the western part of Sandy. A backward trajectory analysis demonstrated that the abundant moisture evaporated from the GS was imported into the western frontal rainbands by the low-level easterlies passing the northern sector of Sandy. The EWSST facilitated the moistening process of air parcels transported by the easterlies due to the active heat and moisture supply from the underlying current, contributing to the increase in moisture influx toward the western front. Another trajectory analysis indicated that the formation of low-level easterlies north of Sandy originated from the combination of Sandy and high-latitude synoptic-scale systems such as an anomalous high over the Labrador Sea and an extratropical cyclone east of Sandy. These results corroborate the importance of the extremely warm midlatitude ocean and complex high-latitude weather systems to the enhancement of Sandy-induced heavy precipitation.

Signals of northward propagating monsoon intraseasonal oscillations (MISOs) in the RegCM4.7 CORDEX-CORE simulation over South Asia domain

Northward propagating summer monsoon intraseasonal oscillations (MISOs) in the Indian Ocean region remain poorly understood and difficult to predict. Here we examine a free-running high-resolution regional atmospheric model (RegCM4.7 with 25 km resolution), forced distantly at the boundaries by atmospheric observations (ERA-Interim, 0.75 $$^{\circ }$$ ) and forced locally by observed sea-surface temperature (SST) over the period 1979–2016, to assess its ability to reproduce key aspects of these MISOs. We find that the model MISO exhibits spatial structures and northward propagation characteristics broadly similar to observed MISO when confining the analysis to the 25–90 day period band. The MISO precipitation anomalies are then shown to be consistent with previously known observed relationships to broad-scale sea-level pressure patterns, Inter-Tropical Convergence Zone (ITCZ) positioning, and changes in the regional Hadley Cell component. The total simulated seasonal (JJAS) rainfall anomalies over India are not significantly correlated with observations, indicating that intrinsic variations in the regional model atmosphere dominate most of the precipitation variability. However, the bandpass-filtered MISO anomalies surprisingly exhibit a significant correlation (0.61) with observations. This suggests that instabilities in the regional broad-scale atmospheric circulation, e.g., linked to the ITCZ position or strength, may be partly controlled by the large-scale atmospheric flows specified at the domain boundaries and/or that specified local SST anomalies may help to guide some fraction of the developing model MISO to follow observations. This result motivates further research on MISO initiation and development using this type of regional atmospheric model.

Circulation and stratification drivers during the summer season in the upwelling bay of Paracas (Peru): A modelling study

The circulation and stratification in the shallow semi-enclosed bay of Paracas located downstream of the main upwelling cell off the Peruvian coast were studied during the summer season using a regional circulation model and in situ observations. A downscaling strategy based on a series of three embedded grids, from 10 km to 500 m resolution in the bay allows to take into account the influence of remote perturbations on the bay dynamics. Debiased surface winds from a high-resolution regional atmospheric model were used to force the model. The shortwave absorption depth was parameterized using satellite measurements of surface chlorophyll.Sensitivity experiments to the model forcing and parameterizations were performed to investigate the impact of the wind diurnal variability, tidal forcing, freshwater discharge from a nearby river and shortwave absorption depth on the bay stratification. Results show that: debiasing the wind intensity reduced the model cold bias in the bay and increase the stratification; a shallow shortwave absorption depth induced a cooling of the subsurface water, increasing the stratification; freshwater discharge from the Pisco river north of the bay increased slightly the stratification in the bay during days of weak wind. The high sensitivity of the bay stratification to the atmospheric forcing calls for the need to use more realistic wind forcing products.The circulation in the bay under strong (>5.5 m s−1) and weak (<3 m s−1) winds was also examined. The summer circulation during strong upwelling-favorable wind conditions was characterized by northward surface currents transporting the bay surface waters outward and subsurface currents transporting cold deeper waters into the bay along its western shore. During weak wind conditions, the current is outward in the bottom layer and a surface southward current related to the poleward undercurrent flowing over the continental slope and shelf transported warm waters into the bay, generating a cyclonic circulation in the bay.

Impacts of high-resolution atmospheric forcing and air-sea coupling on coastal ocean circulation off the Pearl River Estuary

We investigated the impacts of high-resolution atmospheric forcing and ocean-atmosphere coupling on the estuary-shelf ocean simulation off the Pearl River Estuary. We conducted process and dynamics analyses of the wind-driven coastal ocean circulation under atmospheric flux forcing from (1) global reanalysis data, (2) a high-resolution regional atmospheric model, and (3) an air-sea coupled model during an upwelling-favorable wind. The results revealed that the high-resolution atmospheric model significantly improved the representations of the near-surface wind field and air temperature. The air-sea coupled model outperformed the uncoupled model in simulating coastal currents, water temperature, and salinity. The high-resolution uncoupled model strengthened the surface wind stress and along-isobath pressure gradient force (PGF), resulting in an intensified cross-isobath transport. The improved wind forcing from the air-sea coupled model modulated the spatial variation of the net stress curl and vorticity advection and enhanced the along-isobath PGF for a stronger cross-isobath transport. The lower sea surface temperature (SST) in the air-sea coupled model reduced the air temperature and wind stress. Adjusting the SST in the air-sea coupled model improved the momentum fluxes and the associated ocean transport dynamics.

Projection of upwelling-favorable winds in the Peruvian upwelling system under the RCP8.5 scenario using a high-resolution regional model

The Peruvian upwelling system (PUS) is the most productive Eastern Boundary Upwelling System (EBUS) of the world ocean. Contrarily to higher latitude EBUSs, there is no consensus yet on the response of upwelling-favorable winds to regional climate change in this region. Global climate models are not able to reproduce the nearshore surface winds, and only a few downscaling studies have been performed by using relatively coarse-grid atmospheric models forced by idealized climate change scenarios. In the present study, the impact of climate change on the PUS upwelling-favorable winds was assessed using a high resolution regional atmospheric model to dynamically downscale the multi-model mean projection of an ensemble of 31 CMIP5 global models under the RCP8.5 worst-case climate scenario. We performed a 10-year retrospective simulation (1994–2003) forced by NCEP2 reanalysis data and a 10-year climate change simulation forced by a climate change forcing (i.e. differences between monthly-mean climatologies for 2080–2100 and 1989–2009) from CMIP5 ensemble added to NCEP2 data. We found that changes in the mean upwelling-favorable winds are weak (less than 0.2 m s−1). Seasonally, summer winds weakly decrease (by 0–5%) whereas winter winds weakly increase (by 0–10%), thus slightly reinforcing the seasonal cycle. A momentum balance shows that the wind changes are mainly driven by the alongshore pressure gradient, except in a local area north of the Paracas peninsula, downstream the main upwelling center, where wind increase in winter is driven by the shoreward advection of offshore momentum. Sensitivity experiments show that the north–south sea surface temperature gradient plays an important role in the wind response along the north and central coasts, superimposed onto the South Pacific Anticyclone large-scale forcing. A reduction (increase) of the gradient induces a wind weakening (strengthening) up to 15% (25%) off the northern coast during summer. This local mechanism is not well represented in global climate models projections, which underlines the strong need for dynamical downscaling of coastal wind in order to study the impact of climate change on the Peruvian upwelling ecosystem.

Quantifying sources of Brazil's CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; emissions between 2010 and 2018 from satellite data

Abstract. Brazil's CH4 emissions over the period 2010–2018 were derived for the three main sectors of activity: anthropogenic, wetland and biomass burning. Our inverse modelling estimates were derived from GOSAT (Greenhouse gases Observing SATellite) satellite measurements of XCH4 combined with surface data from Ragged Point, Barbados, and the high-resolution regional atmospheric transport model NAME (Numerical Atmospheric-dispersion Modelling Environment). We find that Brazil's mean emissions over 2010–2018 are 33.6±3.6Tgyr-1, which are comprised of 19.0±2.6Tgyr-1 from anthropogenic (primarily related to agriculture and waste), 13.0±1.9Tgyr-1 from wetlands and 1.7±0.3Tgyr-1 from biomass burning sources. In addition, between the 2011–2013 and 2014–2018 periods, Brazil's mean emissions rose by 6.9±5.3Tgyr-1 and this increase may have contributed to the accelerated global methane growth rate observed during the latter period. We find that wetland emissions from the western Amazon increased during the start of the 2015–2016 El Niño by 3.7±2.7Tgyr-1 and this is likely driven by increased surface temperatures. We also find that our estimates of anthropogenic emissions are consistent with those reported by Brazil to the United Framework Convention on Climate Change. We show that satellite data are beneficial for constraining national-scale CH4 emissions, and, through a series of sensitivity studies and validation experiments using data not assimilated in the inversion, we demonstrate that (a) calibrated ground-based data are important to include alongside satellite data in a regional inversion and that (b) inversions must account for any offsets between the two data streams and their representations by models.

High-Resolution Precipitation Gridded Dataset on the South-Central Zone (34° S–41° S) of Chile

Having a distributed gridded precipitation product responds to a multitude of scientific field’s needs: climatology, hydrology, biology, and socioeconomic, among many others. When the insufficient spatial distribution of the rainfall network does not allow (for either lack of quality, spatial or temporal resolution) the correct study of hydro climatic variability, grilled data resulting from different sources (satellite, in situ, models, et al.) are used. Although there are many precipitation grids worldwide they do not respond to high spatial-temporal variability in regions with complex topography, and even more so, along western South America, and central Chile, where the hydroclimate and topography is modulated by Coastal and Andes cordillera. To provide and improve the accessibility of meteorological data on the area, we developed a dynamic geostatistical method to build a high resolution (~800 m) monthly gridded precipitation product for the central-southern zone of Chile for the period 2000-2011. The product involves data from high resolution regional atmospheric modeling, global precipitation grid sets, and 136 in-situ observation stations. The central-southern zone of Chile ($34\cc S-41\cc S$) encompasses from O'Higgins to Los Rios regions, including the Rapel, Mataquito, Maule, Itata, BioBío, Imperial, Toltén and Valdivia hydrologic basins which have different topographies and distinct climates. Thus, for its development, varying spatio-temporal multiple linear regression techniques are applied over different geographical domains where topographic descriptors variables are used as independent variables (elevation, slope, exposure and continentality). Results show that most of the precipitation spatial-temporal variability is well-captured by the model in the north and central regions, from O'Higgins to Biobío, with goodness-of-fit (R^2) fluctuating around 0.86 and 0.82 respectively. Toward the South, Araucanía and Los Ríos, goodness-of-fit (R^2) decrease to values around 0.74 and 0.72 respectively. Both the modified Willmott coefficient (d) and the nse indicated a good model skill, with values over 0.8 and 0.7 respectively. Meanwhile, the $\sigma_e$, $nrmse$ and $pbias$ changed between 0.04-0.2, 0.35-0.52, and 12\%-22\%, respectively. This database is freely available to different regional or national institutions and will help the development of better understanding and management of local and regional hydrology.

Wave modeling of a reef-sheltered coastal zone in the Red Sea

The coastal areas of the Red Sea are characterized by shallow banks of fringing and barrier reefs that provide protection against coastal hazards and erosion by dissipating wave energy. This study investigates the wave climate and extremes of a reef-protected coastal zone in the Red Sea using a high-resolution coupled wave and circulation model, ADCIRC + SWAN, configured on an unstructured grid forced with the meteorological fields from high-resolution regional atmospheric model. Our simulations suggest that the relatively narrow offshore reefs with steep fore-reef slopes dissipate 40–50% of the wave energy propagating towards the shoreline, and this is more pronounced during extremes. The impact of the coupling on determining the wave climate is negligible, but is significant for storms with ~10 cm higher significant wave height (Hs) during the observed period. The back-reef wave climatology computed from 30-year model simulations shows that the mean Hs distribution is uniform throughout the year, and extremes occur more often from February to May. Different return levels of Hs in the sheltered areas are estimated using extreme value analysis. Our results emphasize that preserving the complex offshore reefs is crucial for mitigating the coastal hazards of high-energy waves which are projected to increase with climate change.

Comparison of methodologies for generation of future weather data for building thermal energy simulation

In the last decade, building energy simulation (BES) became a central component in building energy systems’ design and optimization. For each building location, BES requires one year of hourly weather data. Most buildings are designed to last 50+ years, consequently, the building design phase should include BES with future weather files considering climate change. This paper presents a comparative study of two methods to produce future climate hourly data files for BES: Morphing and typical meteorological year of future climate (F-TMY). The study uses data from a high-resolution (9 km) regional climate atmospheric model simulation of Iberia, spanning 10 years of historical and future hourly data. This study compares both methods by analyzing anomalies in air temperature, and the impact in BES predictions of annual and peak energy consumption for space heating, cooling and ventilation in 4 buildings. Additionally, this study performs a sensitivity analysis of morphing method. The analysis shows that F-TMY is representative of the multi-year simulation for BES applications. A high-quality Morphed TMY weather file has a similar performance compared to F-TMY (average difference: 8% versus 7%). Morphing based on different baseline climates, low-grid resolution and/or outdated climate projections leads to BES average differences of 16%-20%.

Climate and surface mass balance of coastal West Antarctica resolved by regional climate modelling

ABSTRACTWest Antarctic climate and surface mass balance (SMB) records are sparse. To fill this gap, regional atmospheric climate modelling is useful, providing that such models are employed at sufficiently high horizontal resolution and coupled with a snow model. Here we present the results of a high-resolution (5.5 km) regional atmospheric climate model (RACMO2) simulation of coastal West Antarctica for the period 1979–2015. We evaluate the results with available in situ weather observations, remote-sensing estimates of surface melt, and SMB estimates derived from radar and firn cores. Moreover, results are compared with those from a lower-resolution version, to assess the added value of the resolution. The high-resolution model resolves small-scale climate variability invoked by topography, such as the relatively warm conditions over ice-shelf grounding zones, and local wind speed accelerations. Surface melt and SMB are well reproduced by RACMO2. This dataset will prove useful for picking ice core locations, converting elevation changes to mass changes, for driving ocean, ice-sheet and coupled models, and for attributing changes in the West Antarctic Ice Sheet and shelves to changes in atmospheric forcing.

Impacts of spectral nudging on the simulated surface air temperature in summer compared with the selection of shortwave radiation and land surface model physics parameterization in a high-resolution regional atmospheric model

The impact of a spectral nudging technique for the dynamical downscaling of the summer surface air temperature in a high-resolution regional atmospheric model is assessed. The performance of this technique is measured by comparing 16 analysis-driven simulation sets of physical parameterization combinations of two shortwave radiation and four land surface model schemes of the model, which are known to be crucial for the simulation of the surface air temperature. It is found that the application of spectral nudging to the outermost domain has a greater impact on the regional climate than any combination of shortwave radiation and land surface model physics schemes. The optimal choice of two model physics parameterizations is helpful for obtaining more realistic spatiotemporal distributions of land surface variables such as the surface air temperature, precipitation, and surface fluxes. However, employing spectral nudging adds more value to the results; the improvement is greater than using sophisticated shortwave radiation and land surface model physical parameterizations. This result indicates that spectral nudging applied to the outermost domain provides a more accurate lateral boundary condition to the innermost domain when forced by analysis data by securing the consistency with large-scale forcing over a regional domain. This consequently indirectly helps two physical parameterizations to produce small-scale features closer to the observed values, leading to a better representation of the surface air temperature in a high-resolution downscaled climate.

Projected Future Changes of Tropical Cyclone Activity over the Western North and South Pacific in a 20-km-Mesh Regional Climate Model

AbstractA high-resolution regional atmospheric model is employed to project the late twenty-first-century changes of tropical cyclone (TC) activity over the western North Pacific (WP) and southwest Pacific (SP). The model realistically reproduces the basic features of the TC climatology in the present-day simulation. Future projections under the representative concentration pathway 4.5 (RCP45) and 8.5 (RCP85) scenarios are investigated. The results show no significant change of TC genesis frequency (TCGF) in the WP by RCP45 due to the cancellation of the reduction over the western part and the increase over the eastern part together with a considerable decrease of TCGF by RCP85 due to the excessive TCGF reduction in the western part. The TCGF over the SP consistently decreases from RCP45 to RCP85. Despite the fact that the simulated maximum surface wind speeds are below 52 m s−1, the change with more strong TCs and fewer weak TCs is robust. The future changes in the TC genesis locations and translational speeds modulate the TC lifetime and frequency of occurrence. The TC genesis potential index (GPI) is used to evaluate the projected TCGF changes. The results show that low-level vorticity and midtropospheric vertical velocity largely contribute to the reduction of GPI in the western part of the WP, while vertical wind shear and midtropospheric vertical velocity mainly contribute to the decrease of GPI over the SP. The weakening of the monsoon trough is found to be responsible for the decreases of GPI and TCGF over the western part of the WP.

Characteristics of the modelled meteoric freshwater budget of the western Antarctic Peninsula

Rapid climatic changes in the western Antarctic Peninsula (WAP) have led to considerable changes in the meteoric freshwater input into the surrounding ocean, with implications for ocean circulation, the marine ecosystem and sea-level rise. In this study, we use the high-resolution Regional Atmospheric Climate Model RACMO2.3, coupled to a firn model, to assess the various contributions to the meteoric freshwater budget of the WAP for 1979–2014: precipitation (snowfall and rainfall), meltwater runoff to the ocean, and glacial discharge. Snowfall is the largest component in the atmospheric contribution to the freshwater budget, and exhibits large spatial and temporal variability. The highest snowfall rates are orographically forced and occur over the coastal regions of the WAP (>2000mm water equivalent (w.e.) y−1) and extend well onto the ocean up to the continental shelf break; a minimum (∼500mmw.e.y−1) is reached over the open ocean. Rainfall is an order of magnitude smaller, and strongly depends on latitude and season, being large in summer, when sea ice extent is at its minimum. For Antarctic standards, WAP surface meltwater production is relatively large (>50mmw.e.y−1), but a large fraction refreezes in the snowpack, limiting runoff. Only at a few more northerly locations is the meltwater predicted to run off into the ocean. In summer, we find a strong relationship of the freshwater fluxes with the Southern Annular Mode (SAM) index. When SAM is positive and occurs simultaneously with a La Niña event there are anomalously strong westerly winds and enhanced snowfall rates over the WAP mountains, Marguerite Bay and the Bellingshausen Sea. When SAM coincides with an El Niño event, winds are more northerly, reducing snowfall and increasing rainfall over the ocean, and enhancing orographic snowfall over the WAP mountains. Assuming balance between snow accumulation (mass gain) and glacial discharge (mass loss), the largest glacial discharge is found for the regions around Adelaide Island (10Gty−1), Anvers Island (8Gty−1) and southern Palmer Land (12Gty−1), while a minimum (<2Gty−1) is found in Marguerite Bay and the northern WAP. Glacial discharge is in the same order of magnitude as the direct freshwater input into the ocean from snowfall, but there are some local differences. The spatial patterns in the meteoric freshwater budget have consequences for local productivity and carbon drawdown in the coastal ocean.

Temperature and Wind Climate of the Antarctic Peninsula as Simulated by a High-Resolution Regional Atmospheric Climate Model

Abstract The latest polar version of the Regional Atmospheric Climate Model (RACMO2.3) has been applied to the Antarctic Peninsula (AP). In this study, the authors present results of a climate run at 5.5 km for the period 1979–2013, in which RACMO2.3 is forced by ERA-Interim atmospheric and ocean surface fields, using an updated AP surface topography. The model results are evaluated with near-surface temperature and wind measurements from 12 manned and automatic weather stations and vertical profiles from balloon soundings made at three stations. The seasonal cycle of near-surface temperature and wind is simulated well, with most biases still related to the limited model resolution. High-resolution climate maps of temperature and wind showing that the AP climate exhibits large spatial variability are discussed. Over the steep and high mountains of the northern AP, large west-to-east climate gradients exist, while over the gentle southern AP mountains the near-surface climate is dominated by katabatic winds. Over the flat ice shelves, where katabatic wind forcing is weak, interannual variability in temperature is largest. Finally, decadal trends of temperature and wind are presented, and it is shown that recently there has been distinct warming over the northwestern AP and cooling over the rest of the AP, related to changes in sea ice cover.

Extreme Precipitation and Climate Gradients in Patagonia Revealed by High-Resolution Regional Atmospheric Climate Modeling

Abstract This study uses output of a high-resolution (5.5 km) regional atmospheric climate model to describe the present-day (1979–2012) climate of Patagonia, with a particular focus on the surface mass balance (SMB) of the Patagonian ice fields. Through a comparison with available in situ observations, it is shown that the model is able to simulate the sharp climate gradients in western Patagonia. The southern Andes are an efficient barrier for the prevalent atmospheric flow, generating strong orographic uplift and precipitation throughout the entire year. The model suggests extreme orographic precipitation west of the Andes divide, with annual precipitation rates of &amp;gt;5 to 34 m w.e. (water equivalent), and a clear rain shadow east of the divide. These modeled precipitation rates are supported qualitatively by available precipitation stations and SMB estimates on the ice fields derived from firn cores. For the period 1979–2012, a slight atmospheric cooling at upper ice field elevations is found, leading to a small but insignificant increase in the ice field SMB.

Simulations of the Present and Late-Twenty-First-Century Western North Pacific Tropical Cyclone Activity Using a Regional Model

Abstract A high-resolution regional atmospheric model is used to simulate present-day western North Pacific (WNP) tropical cyclone (TC) activity and to investigate the projected changes for the late twenty-first century. Compared to observations, the model can realistically simulate many basic features of the WNP TC activity climatology, such as the TC genesis location, track, and lifetime. A number of spatial and temporal features of observed TC interannual variability are captured, although observed variations in basinwide TC number are not. A relatively well-simulated feature is the contrast of years when the Asian summer monsoon trough extends eastward (retreats westward), more (fewer) TCs form within the southeastern quadrant of the WNP, and the corresponding TC activity is above (below) normal over most parts of the WNP east of 125°E. Future projections with the Coupled Model Intercomparison Project phase 3 (CMIP3) A1B scenario show a weak tendency for decreases in the number of WNP TCs, and for increases in the more intense TCs; these simulated changes are significant at the 80% level. The present-day simulation of intensity is limited to storms of intensity less than about 55 m s−1. There is also a weak (80% significance level) tendency for projected WNP TC activity to shift poleward under global warming. A regional-scale feature is a projected increase of the TC activity north of Taiwan, which would imply an increase in TCs making landfall in north China, the Korean Peninsula, and parts of Japan. However, given the weak statistical significance found for the simulated changes, an assessment of the robustness of such regional-scale projections will require further study.

The terraced thermal contrast among the Tibetan Plateau, the East Asian plain, and the western North Pacific and its impacts on the seasonal transition of East Asian climate

The heating sources over the Tibetan Plateau (TP), the East Asian plain, and the western North Pacific (WNP) form a terraced thermal contrast in the west-east direction. Over East Asia and the WNP, this zonal thermal contrast contributes as high as 45 % to the seasonal variance based on the EOF analysis and exerts a significant impact on the seasonal transition of the East Asian climate through the enhancement of the year-round southerly to the southeast of the TP in late March and early April. This effect is investigated in this study using a high-resolution regional atmospheric model by doubling the surface sensible heat flux, respectively, over the TP, the East Asian plain, and the WNP in three sensitivity experiments. Comparisons among the experiments reveal that doubling the surface sensible heat flux over the WNP has little upstream response over East Asia. The increased zonal thermal contrast between the TP and the East Asian plain due to doubled heat flux over the TP would induce anomalous northerly over the region with year-round southerly to the southeast of the TP and weaken its seasonal enhancement. Doubling the surface sensible heat flux over the East Asian plain decreases the zonal thermal contrast and leads to southerly anomaly over the region with year-round southerly to the southeast of the TP and South China, which is favorable for the enhancement of the year-round southerly and its eastward extension.