FIG 1 - uploaded by Joseph Egger
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Rotating earth and the components i i M i of the global angular momentum. The TP is hatched. Also given are the poles Z i of the ( ˜ 1 , ˜ i ) systems adapted to the axes i i. In particular, Z 1 (Z 2 ) is the "Greenwich" pole (90°E pole).
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The interaction of large-scale wave systems with the Tibetan Plateau (TP) is investigated by regressing pressure, potential temperature, winds, precipitation, and selected fluxes in winter onto the three components Toi of this massif’s mountain torque on the basis of the 40-yr ECMWF reanalysis (ERA-40) data. Events with respect to the equatorial “G...
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... torque events represent somewhat different flow situa- tions. For this reason we include in our analysis the mountain torques with respect to the equatorial com- ponents M 1 and M 2 of the global angular momentum M 3 i1 M i i i as basic parameters for data stratifica- tion. The equatorial component M 1 i 1 points toward the Greenwich meridian ( Fig. 1) and there is also a 90°E component M 2 i 2 . The axial component M 3 i 3 is, of course, aligned with the earth's rotation ...
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... the formulas for the equatorial mountain torques are fairly complicated when expressed in stan- dard spherical (, ) coordinates, they become rather simple when a rotated coordinate system ( ˜ i , ˜ i , z) is introduced for each angular momentum axis where the basic vector i i is the "polar" axis, that is, if the point Z i as given in Fig. 1 is the "North Pole" of this system. The other unit vectors i j are then embedded in the "equa- torial" plane˜iplane˜plane˜i 0. The mountain torque with respect to this axis is where˜iwhere˜ where˜i is "longitude," p s is the surface pressure, h is the topography, and F is a selected area for which the torque is evaluated. The actual ...
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... that the pressure fall in the lee is mainly due to warm-air advection from the south ahead of the upper-level trough. The growth of the Japan high is also mainly due to advection of cold air aloft related to the high's westward tilt with height. As in EH06, the triple terms have a rich spatial struc- ture and vary strongly with lag. We show in Fig. 10 the horizontal flux covariance C[T 2 , (v 2 )|] at 2 days when these fluxes are strong. It is seen that the fluxes at that time are directed mainly opposite to what one might expect. For example, there is anticyclonic flux around the "Japan high," which drifts eastward at that moment. A naive guess would have been to assume that the ...
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... "Japan high," which drifts eastward at that moment. A naive guess would have been to assume that the northerlies east of the high transport anomalously cool air so that the fluxes would have to be directed northward. However, the same result obtains for nega- tive torques so that these contributions drop out in the covariance calculations. Thus, Fig. 10 tells us that the Japan high (T 2 0) has weaker fluxes than the corre- sponding Japan low for T 2 ...
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... of pressure anomalies at the northern edge of the TP and the following southward extension at the eastern slope. Indeed, a high pressure cell is found northeast of the TP at 5 days (not shown), which approaches the TP slowly during the following days. At 3 days (Fig. 11a), anomalously high pressure is found everywhere north of the TP but the pressure maximum is still located northwest of the TP. There are hardly any pressure anomalies at the lee slope. South- ward protrusion begins at 2 days and a broad tongue of high pressure extends along the lee slope at 0 ( Fig. 11b). A low emerges near Japan. The ...
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... during the following days. At 3 days (Fig. 11a), anomalously high pressure is found everywhere north of the TP but the pressure maximum is still located northwest of the TP. There are hardly any pressure anomalies at the lee slope. South- ward protrusion begins at 2 days and a broad tongue of high pressure extends along the lee slope at 0 ( Fig. 11b). A low emerges near Japan. The simi- larity to Fig. 4b is obvious. The further development of the T 1 events departs somewhat from the course of the T 2 events because the Japan low extends now toward the northwest (Fig. 11c). Compo et al. (1999) related surface pressure perturbations at leeside points to winds and pressure at various ...
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... lee slope. South- ward protrusion begins at 2 days and a broad tongue of high pressure extends along the lee slope at 0 ( Fig. 11b). A low emerges near Japan. The simi- larity to Fig. 4b is obvious. The further development of the T 1 events departs somewhat from the course of the T 2 events because the Japan low extends now toward the northwest (Fig. 11c). Compo et al. (1999) related surface pressure perturbations at leeside points to winds and pressure at various levels and found patterns quite similar to ours but for lags shifted backward by 1-2 days. The leeside protrusion appears to be more shallow than in the T 2 series. For example, there is hardly any indication of a ...
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... leeside points to winds and pressure at various levels and found patterns quite similar to ours but for lags shifted backward by 1-2 days. The leeside protrusion appears to be more shallow than in the T 2 series. For example, there is hardly any indication of a southward-protruding high pressure tongue above the lee slope at z 5500 m and 3 days (Fig. 11e). The pressure field at z 9500 m exhibits a few special features not seen at the lower levels. In particular, a high is growing above India and the southern TP with peak intensity at 1 day, which is still clearly visible in Fig. 11d ( 0). More- over, that the Japan low is linked to a low pressure system above the TP that is first seen ...
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... is hardly any indication of a southward-protruding high pressure tongue above the lee slope at z 5500 m and 3 days (Fig. 11e). The pressure field at z 9500 m exhibits a few special features not seen at the lower levels. In particular, a high is growing above India and the southern TP with peak intensity at 1 day, which is still clearly visible in Fig. 11d ( 0). More- over, that the Japan low is linked to a low pressure system above the TP that is first seen at 3 days west of the TP. Low pressure prevails above the plateau till the end of this period. The Japan low becomes weaker but does not move eastward at this height. The winds at z 2500 m begin to blow southward over the China Sea at 1 ...
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... similar to that in Fig. 5 to be presented. Given the overall similarity of T 1 and T 2 cases there is no need to display the temperature anomalies where cool air is filling the western Pacific much the same (but of oppo- site sign) as in Fig. 6. The vertical motion near the lee slope shows an in- teresting feature barely resolved by the analysis (Fig. 12). There is upward motion in a narrow stripe near the lee slope related to the southward protrusion of cool air. In this case, then, upward motion contributes to the cooling. This effect is, however, restricted to this nar- row domain. Otherwise, there is descent almost all around the TP. Rainfall is below normal at the southwestern ...
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... This effect is, however, restricted to this nar- row domain. Otherwise, there is descent almost all around the TP. Rainfall is below normal at the southwestern slopes at all lags whereas the precipitation maximum forms at the end of the cold air protrusion in the less. At 2 days, the southern part of the Japan low has less rainfall than normal (Fig. ...
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... for the leeside region where the linearity criterion (2.6) is satisfied for 4 1 day (not shown). Linear forecasts are fairly useless in the lee and the Pacific for 0. The actual computation on the basis of (2.4) show, that the contribution by horizontal temperature advection L 2 gives at least a qualitatively correct answers for 0. We show in Fig. 14 quite strong in this case. The predicted pressure changes for the Japan low and for the high northeast of the TP are almost identical to those in Fig. 14b and only slightly smaller in the ...
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... Pacific for 0. The actual computation on the basis of (2.4) show, that the contribution by horizontal temperature advection L 2 gives at least a qualitatively correct answers for 0. We show in Fig. 14 quite strong in this case. The predicted pressure changes for the Japan low and for the high northeast of the TP are almost identical to those in Fig. 14b and only slightly smaller in the ...
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... corresponding equations will not be written down but the result as displayed in Fig. 15 can be understood even so. The kinetic energy K grows monotonously from 5 days till 1 day to attain its maximum of 0.8 10 18 J. Such a value corresponds with a mean velocity of 1.2 m s 1 . There is decay for 1 day. Inspection of the equations shows that K can be af- fected by conversions R 1 from the kinetic energy of the mean flow. As ...
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... understood even so. The kinetic energy K grows monotonously from 5 days till 1 day to attain its maximum of 0.8 10 18 J. Such a value corresponds with a mean velocity of 1.2 m s 1 . There is decay for 1 day. Inspection of the equations shows that K can be af- fected by conversions R 1 from the kinetic energy of the mean flow. As can be seen from Fig. 15, R 1 is positive but relatively small. The main conversion term is R 2 CT 2 , v 2 | · CT 2 , p | dV, 4.1 which peaks at 1 day. This term describes conver- sions from the available potential energy and also those due to interactions with the atmosphere outside the analysis domain. It is large enough to approximately explain the observed ...
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... which peaks at 1 day. This term describes conver- sions from the available potential energy and also those due to interactions with the atmosphere outside the analysis domain. It is large enough to approximately explain the observed increase of K so that a conven- tional picture of baroclinic growth appears to emerge. However, the decay of K in Fig. 15 cannot be due to R 2 because R 2 is positive and important at all lags. There is a further conversion term involving triple correlations which has not been evaluated. We speculate, that the decay of the kinetic energy K in Fig. 15 is at least partly due to this turbulence term. Surface friction does not stop the growth for 0 and should ...
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... increase of K so that a conven- tional picture of baroclinic growth appears to emerge. However, the decay of K in Fig. 15 cannot be due to R 2 because R 2 is positive and important at all lags. There is a further conversion term involving triple correlations which has not been evaluated. We speculate, that the decay of the kinetic energy K in Fig. 15 is at least partly due to this turbulence term. Surface friction does not stop the growth for 0 and should be not much more important for 0 so that the decay of the perturba- tions must reflect the impact of triple terms similar to what has been found near Greenland (EH06). Such conclusions must be seen with caution because of the ...
Citations
Tibetan Plateau snow cover (TPSC) has subseasonal variations and rapidly influences the atmosphere. In this study, we present the rapid response of the East Asian trough (EAT) within a week to subseasonal variations in TPSC during the boreal winter. Using snow cover analysis obtained from the daily interactive multisensor snow and ice mapping system and the ERA‐Interim reanalysis, a considerable relationship between TPSC and 500‐hPa geopotential height anomalies over the downstream EAT region is found. Significant negative (positive) 500‐hPa geopotential height anomalies originating from the Tibetan Plateau and moving into the EAT region appear within a week following anomalous positive (negative) TPSC events, which lead to changes in EAT strength. Thus, a significantly enhanced (reduced) intensity of the EAT occurs approximately 5–6 days after increased (decreased) TPSC. Numerical experiments confirm the causality of this relationship. Further analysis of the quasi‐geostrophic geopotential height tendency equations in numerical experiments indicates that such EAT variations result from anomalous thermal advection from the Tibetan Plateau forced by TPSC.
The evolution of the two components of the equatorial mountain torque (EMT) applied by mountains on the atmosphere is analyzed in the NCEP reanalysis. A strong lagged relationship between the EMT component along the Greenwich axis TM1 and the EMT component along the 90°E axis TM2 is found, with a pronounced signal on TM1 followed by a signal of opposite sign on TM2. It is shown that this result holds for the major massifs (Antarctica, the Tibetan Plateau, the Rockies, and the Andes) if a suitable axis system is used for each of them. For the midlatitude mountains, this relationship is in part associated with the development of cold surges.
Following these results, two hypotheses are made: (i) the mountain forcing on the atmosphere is well measured by the regional EMTs and (ii) this forcing partly drives the cold surges. To support these, a purely dynamical linear model is proposed: it is written on the sphere, uses an f-plane quasigeostrophic approximation, and includes the mountain forcings. In this model, a positive (negative) peak in TM1 produced by a mountain massif in the Northern (Southern) Hemisphere is due to a large-scale high surface pressure anomaly poleward of the massif. At a later stage, high pressure and low temperature anomalies develop in the lower troposphere east of the mountain, explaining the signal on TM2 and providing the favorable conditions for the cold surge development.
It is concluded that the EMT is a good measure of the dynamical forcing of the atmospheric flow by the mountains and that the poleward forces exerted by mountains on the atmosphere are substantial drivers of the cold surges, at least in their early stage. Therefore, the EMT time series can be an important diagnostic to assess the representation of mountains in general circulation models.
1] Previous studies have shown (1) that the Tibetan Plateau produces a significant fraction of the two components of the Equatorial Mountain Torque (EMT) in winter, (2) that these torques are in part related to the East Asian cold surges, and (3) that the cold surges affect the convection over the maritime continent. We show here that these relations are strong enough for the convection over the Equatorial South China Sea to be associated with significant signals on the two components of the EMT that can precede by a few days and more the convection. These signals are associated to surface pressure and temperature patterns that are strongly reminiscent of the East Asian cold surges. Our results therefore show that the Tibetan Plateau couple dynamically the midlatitudes and the tropical region, and that the vectors of this dynamical coupling are the cold surges. This coupling also influences the convection over the northern Bay of Bengal, mainland southeast Asia, and Indonesia.
Theories of topographic instability predict growth of perturbations of mean flow and wave modes due to their interaction with mountains under favorable conditions. Mountain torques form an important part of this interaction. It has been suggested that topographic instabilities contribute significantly to the subseasonal variability of the atmosphere but observational tests of topographic instability mechanisms have not yet been performed. Greenland is selected as a test bed because of its isolation, simple shape, and appropriate size. The observed flow development during mountain torque events is investigated in terms of a regression analysis. Changes of axial angular momentum and zonal mean wind with respect to the torques are monitored for domains covering Greenland since the acceleration (deceleration) of the regional zonal flow in response to a positive (negative) torque is a key feature of topographic instability. In particular, southern and northern analysis domains are considered separately in order to test “dipole” instability theories in addition to “monopole” situations where the meridional extent of the pressure perturbations is similar to that of Greenland. Moreover, zonal bands are used as analysis domains. It is found that the response of the zonal wind to the torques is quite small and not systematic. There is no evidence of monopole or dipole topographic instability. A less detailed analysis for the Tibetan Plateau leads to the same result. Reasons for these negative outcomes are discussed as are shortcomings of the tests.
The mountains surrounding the Mediterranean exert torques T during the passage of North Atlantic systems which affect the angular momentum of the airflow passing over and around the massifs. The Alps, the Atlas range and the
orographic block of Asia Minor are selected to investigate the typical flow conditions during torque events. These mountain ranges are small enough to justify a local angular momentum analysis. Both the zonal and the meridional components of a mountain’s torque (Tλ and Tϕ) are used as stratification parameters in a statistical investigation of the interaction of large-scale perturbations with this mountain. How are these flows affected by the obstacle? A simple scheme is tested which attempts to interpret results.
The torque analysis singles out eastward-moving large-scale systems. Their isobars are oriented from southwest (northwest) to northeast (southeast) near the mountain in zonal torque Tλ (Tϕ) cases. The massifs tend to generate a low-level distortion of the pressure field such that the angular momentum of the flow over the mountain is reduced. These results can be explained within the framework of the scheme. The influence of the mountains on the pressure field is seen only at heights ≤4000 m. The low-level distortions of the pressure field contribute positively to the total torque for lags τ ≤ 0 in the Alps and for all lags −2 ≤ τ ≤ 2 days in Asia Minor, where only Tλ is evaluated. The impact of the Atlas mountains is seen only at τ = 0.
