Orographically Anchored El Niño Effect on Summer Rainfall in Central China
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Abstract:
Year-to-year variations in summer precipitation have great socioeconomic impacts on China. Historical rainfall variability over China is investigated using a newly released high-resolution dataset. The results reveal summer-mean rainfall anomalies associated with ENSO that are anchored by mountains in central China east of the Tibetan Plateau. These orographically anchored hot spots of ENSO influence are poorly represented in coarse-resolution datasets so far in use. In post–El Niño summers, an anomalous anticyclone forms over the tropical northwest Pacific, and the anomalous southwesterlies on the northwest flank cause rainfall to increase in mountainous central China through orographic lift. At upper levels, the winds induce additional adiabatic updraft by increasing the eastward advection of warm air from Tibet. In post–El Niño summers, large-scale moisture convergence induces rainfall anomalies elsewhere over flat eastern China, which move northward from June to August and amount to little in the seasonal mean.Keywords:
Anticyclone
Orographic lift
Orography
Subtropical ridge
Extratropical cyclone
Geopotential height
Abstract The accuracy with which parametrizations of orographic blocking and orographic gravity wave drag (OGWD) are able to reproduce the explicitly resolved impacts on flow over complex terrain is investigated in two models: the Met Office Unified Model (MetUM) and the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System (ECMWF IFS). To this end, global and limited area short‐range forecast experiments across a range of horizontal resolutions, and their model errors relative to analyses, are assessed over two complex mountainous regions: the Himalayas and the Middle East. The impact of resolved orography on the circulation is deduced by taking the difference between high‐resolution experiments with a high (4 to 9 km) and low‐resolution (125 to 150 km) orography. This is then compared with the impact of parametrized orographic drag, deduced from global low‐resolution experiments with and without parametrized orographic drag. At resolutions ranging from tens to hundreds of kilometres, both the MetUM and ECMWF IFS exhibit too strong zonal winds relative to analyses in the lower stratosphere in the region of maximum resolved orographic gravity wave breaking, indicative of some deficiency in the parametrization of OGWD. Diagnosis of the parametrized physics and resolved dynamics tendencies across a range of OGWD parameter values reveal that this error is partly due to the manner in which the resolved dynamics interacts with the parametrized OGWD. This work introduces a method for quantifying the impacts of resolved versus parametrized orographic drag in models and highlights the importance of physics‐dynamics interactions.
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Orographic lift
Parametrization (atmospheric modeling)
Wave drag
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Abstract An evaluation of the performance of the subgrid orography parametrization scheme in the ECMWF Integrated Forecast System is made by examining the variation with resolution of the orographic torques (resolved and parametrized) in short‐range global forecasts. As the spectral resolution is increased from T95 to T511, the magnitudes of the resolved torques increase while those of the parametrized torques decrease. In general the changes in the total orographic torques are reasonably small, but an important exception is between 20 ° N and 50 ° N in winter. Here the total orographic torque decreases significantly with increasing resolution. Through examination of errors in the modelled angular momentum budgets, it is suggested that the problems lie primarily at the low resolution end, where the parametrized orographic torques seem to be excessive. Furthermore, recent work with the NCEP reanalysis data suggests that this result may not be unique to the ECMWF system. Copyright © 2004 Royal Meteorological Society
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A numerical model with one layer is used to study the dynamics of frontal motion near orography. First frontal propagation along a vertical wall of infinite height is examined and it is found that an orographic jet forms in the one-layer model which bears good resemblance to jets observed in the laboratory. The jet's speed of propagation is found to be fairly insensitive with respect to grid resolution. However the shape of the jet depends strongly on the structure of the grid. Next frontal propagation near mountains is studied. The formation of an orographic jet is strongly influenced by the representation of the orography in the grid. Jet formation is most pronounced for vertical mountain walls.
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The orographic influence of the Alps on 500 mb northerly or southerly flow has been studied, using the vorticity equation and a numerically evaluated mountain profile. The effects on trajectory deflection from orography and from the variation in the coriolis parameter have opposite signs for northerly flow but the same sign for southerly flow. Using the most frequent wind speed at 500 mb it is found that in the majority of cases a northerly current will pass the Alps without any significant deflection.
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Relative contributions of common ingredients to heavy orographic rainfall associated with the passage of Hurricanes Hugo (1989) and Isabel (2003) over the Appalachian Mountains are examined using a numerical weather prediction model. It is found that the key ingredients for producing local heavy orographic rainfall were: high precipitation efficiency, strong low-level flow, strong orographically forced upward motion associated with strong low-level flow over relatively gentle upslope, concave geometry providing local areas of convergence, high moist flow upstream, a relatively large convective system associated with both tropical cyclones (TCs), and relatively slower movement. In addition, neither conditional instability nor potential (convective) instability is found to play essential roles in producing strong upward motion leading to heavy orographic TC rain. A modified Orographic Rain Index (ORI) is proposed as a predictor for heavy orographic TC precipitation, which includes the upstream incoming horizontal wind speed normal to the local orography, the steepness of the mountain, the relative humidity, the TC moving speed, and the horizontal scale of the TC. It is found that the ORI estimated in regions of local maximum rainfall by using fine-resolution numerically simulated results correlate well with rainfall rates for both hurricanes, indicating that it may serve as a predictor for heavy orographic TC rainfall.
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Orographic influence on the formation of clouds and its associated precipitation amount and distribution is dramatic. The influence of orography was well recognized very early in human history and documented in numerous meteorological and climatological literatures. When a moist airflow impinges on a mountain, the dynamical and cloud microphysical characteristics of the airflow are modified by orographic lifting and blocking which may modify and/or trigger cloud and precipitation systems in the vicinity of the mountain. Figure 11.1 shows the mean annual precipitation for the period 1971–1990 over Western Europe. Areas of heavy precipitation are concentrated on the Alpine mountains. Note that precipitation over the Alps is produced by weather systems coming from different directions, in particular, from the northern and southern sides.
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In recent years, the study of numerical weather prediction (NWP) in complex orographic areas has attracted a great deal of attention. Complex orography plays an important role in the occurrence and development of extreme rainfall events. In this study, the Yin–He Global Spectrum Model (YHGSM) was used, and the wave number truncation method was employed to decompose the orographic data to different resolutions. The obtained orographic data with different resolutions were used to simulate the extreme rainfall in Zhengzhou, Henan Province, China, to discuss the degree of influence and mechanism of the different orographic resolutions on the extreme rainfall. The results show that the simulation results of the YHGSM with high-resolution orography are better than those of the low-resolution orography in terms of the rainfall intensity and range. When the rainfall intensity is higher, the results of the low-resolution orography simulated the rainfall range of big heavy rainfall better. The orography mainly affected the rainfall by affecting the velocity of the updraft, but it had a limited influence on the maximum height that the updraft could reach. A strong updraft is one of the key factors leading to extreme rainfall in Henan Province. When the orographic resolution changes, the sensitivity of the vertical velocity of the updraft to the orographic resolution is the greatest, the sensitivity of the upper-air divergence and low-level vorticity to the orographic resolution is lower than that of the vertical velocity. In conclusion, the high-resolution orography is helpful in improving the model’s prediction of extreme rainfall, and when predicting extreme rainfall in complex orographic areas, forecasters may need to artificially increase rainfall based on model results.
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The orographic influence of the Alps on 500 mb northerly or southerly flow has been studied, using the vorticity equation and a numerically evaluated mountain profile. The effects on trajectory deflection from orography and from the variation in the coriolis parameter have opposite signs for northerly flow but the same sign for southerly flow. Using the most frequent wind speed at 500 mb it is found that in the majority of cases a northerly current will pass the Alps without any significant deflection. DOI: 10.1111/j.2153-3490.1964.tb00141.x
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Orography strongly interacts with the atmospheric circulation, especially during frontal events, generating an enhanced spatial variability of the rainfall field. Regional models of extreme rainfall have to deal with these circumstances in order to provide good spatial estimation of the regionalized variable. We present a model for the regional estimation of the mean of the probability distribution of the annual daily rainfall maxima in a region (Campania, Southern Italy) with complex orography. In a recent work, we found that areas with enhanced variability of extreme rainfall, in the same region, correspond to a particular set of orographic objects, which had been classified through an automatic, GIS-based geomorphological procedure. Here, we propose an approach that considers the same orographic objects as building blocks for a regional model that is able to capture the amplification of extreme rainfall caused by orography. The regional model is then the product of a basic stationary random spatial process and an amplification factor, whose values are related to the topographic features of the orographic objects. This approach represents a step towards the improvement of the predictive ability of regional models of extreme rainfall within orographically complex areas.
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