Large-scale Orography Research Articles

Mechanical and Thermal Forcings of Asian Large-Scale Orography on Spring Cloud Amount and Atmospheric Radiation Budget over East Asia

Abstract Asian large-scale orography profoundly influences circulation in the North Hemisphere. Considerable spring top-of-the-atmosphere (TOA) radiative cooling over Southeast China (SEC) is very likely related to upstream orography forcing. Here we investigate the mechanical and thermal forcings of Asian large-scale orography, particularly the Tibetan Plateau (TP), on downstream East Asian cloud amount and atmospheric radiation budget during March–April using the Global Monsoons Model Intercomparison Project simulations. The thermal forcing drives significant surface heating and a low-level cyclone over the TP, pumping low-level air to the middle troposphere. Ascent and water vapor convergence triggered by the thermal forcing favor air condensation, low–middle clouds, and resultant strong spring cloud radiative cooling over SEC. Moreover, the thermal forcing moves the position of cloud radiative cooling westward toward the TP. The TP’s blocking role weakens low-level westerlies over SEC, but its deflecting role increases downstream high-level westerlies, dynamically favoring cloud formation over SEC and the eastward ocean. In addition, the TP can force ascent and increase cloud amounts over the western and central TP. The thermal forcing contributes to 57.1% of total cloud amount and 47.6% of TOA cloud radiative cooling induced by the combined orography forcing over SEC while the mechanical one accounts for 79.4% and 95.8% of the counterparts over the ocean to the east of SEC. Our results indicate that Asian large-scale orography shapes the contemporary geographical distribution of spring East Asian cloud amount and atmospheric radiation budget to a large extent. Significance Statement Clouds tied to large-scale topography and circulation exhibit some remarkable geographical distributions. The global strongest cloud radiative cooling, with an intensity of up to −90 W m−2, occurs over Southeast China (SEC) during March–April. The primary purpose of this study is to understand the influences of Asian large-scale orography, particularly the Tibetan Plateau (TP), on this unique climatic phenomenon using the latest climate model simulations. Our results show that Asian large-scale orography forcing significantly increases ascent, low–middle cloud formation, and resultant strong spring cloud radiative cooling over SEC and downstream ocean. The sensible-heat-driven air pump induced by the TP’s thermal forcing maintains strong cloud radiative cooling over SEC. This study provides valuable insights that link Asian large-scale orography forcing to downstream cloud–radiation characteristics.

Revisiting the impact of Asian large-scale orography on the summer precipitation in Northwest China and surrounding arid and semi-arid regions

Revisiting the impact of Asian large-scale orography on the summer precipitation in Northwest China and surrounding arid and semi-arid regions

On the Progressive Attenuation of Finescale Orography Contributions to the Vertical Coordinate Surfaces within a Terrain-Following Coordinate System

AbstractA modified hybrid terrain-following vertical coordinate has recently been implemented within the Global Environmental Multiscale atmospheric model that introduces separately controlled height-dependent progressive decaying of the small- and large-scale orography contributions on the vertical coordinate surfaces. The new vertical coordinate allows for a faster decay of the finescale orography imprints on the coordinate surfaces with increasing height while relaxing the compression of the lowest model levels over complex terrain. A number of tests carried out—including experiments involving Environment and Climate Change Canada’s operational regional and global deterministic prediction systems—demonstrate that the new vertical coordinate effectively eliminates terrain-induced spurious generation and amplification of upper-air vertical motion and kinetic energy without increasing the computational cost. Results also show potential improvements in precipitation over complex terrain.

Testing the Limits and Breakdown of the Nonacceleration Theorem for Orographic Stationary Waves

AbstractThe nonacceleration theorem states that the torque exerted on the atmosphere by orography is exactly balanced by the convergence of momentum by the stationary waves that the orography excites. This balance is tested in simulations with a stationary wave model and with a dry, idealized general circulation model (GCM), in which large-scale orography is placed at the latitude of maximum surface wind speed. For the smallest mountain considered (maximum height H = 0.5 m), the nonacceleration balance is nearly met, but the damping in the stationary wave model induces an offset between the stationary eddy momentum flux (EMF) convergence and the mountain torque, leading to residual mean flow changes. A stationary nonlinearity appears for larger mountains (H ≥ 10 m), driven by preferential deflection of the flow around the poleward flank of the orography, and causes further breakdown of the nonacceleration balance. The nonlinearity grows as H is increased, and is stronger in the GCM than in the stationary wave model, likely due to interactions with transient eddies. The midlatitude jet shifts poleward for H ≤ 2 km and equatorward for larger mountains, reflecting changes in the transient EMFs, which push the jet poleward for smaller mountains and equatorward for larger mountains. The stationary EMFs consistently force the jet poleward. These results add to our understanding of how orography affects the atmosphere’s momentum budget, providing insight into how the nonacceleration theorem breaks down; the roles of stationary nonlinearities and transients; and how orography affects the strength and latitude of eddy-driven jets.

The Impact of Large-Scale Orography on Northern Hemisphere Winter Synoptic Temperature Variability

Abstract The impact of large-scale orography on wintertime near-surface (850 hPa) temperature variability on daily and synoptic time scales (from days to weeks) in the Northern Hemisphere is investigated. Using a combination of theory, idealized modeling work, and simulations with a comprehensive climate model, it is shown that large-scale orography reduces upstream temperature gradients, in turn reducing upstream temperature variability, and enhances downstream temperature gradients, enhancing downstream temperature variability. Hence, the presence of the Rockies on the western edge of the North American continent increases temperature gradients over North America and, consequently, increases North American temperature variability. By contrast, the presence of the Tibetan Plateau and the Himalayas on the eastern edge of the Eurasian continent damps temperature variability over most of Eurasia. However, Tibet and the Himalayas also interfere with the downstream development of storms in the North Pacific storm track, and thus damp temperature variability over North America, by approximately as much as the Rockies enhance it. Large-scale orography is also shown to impact the skewness of downstream temperature distributions, as temperatures to the north of the enhanced temperature gradients are more positively skewed while temperatures to the south are more negatively skewed. This effect is most clearly seen in the northwest Pacific, off the east coast of Japan.

Two types of summertime heating over Asian large-scale orography and excitation of potential-vorticity forcing II. Sensible heating over Tibetan-Iranian Plateau

Based on analysis and simulation, the interaction of thermal forcing between the Tibetan Plateau (TP) and Iranian Plateau (IP) in summer is investigated. Associated influences on water vapor transport in the Asian subtropical monsoon region and the formation of a cold center in the lower stratosphere over Eurasia are also investigated. Results show that surface sensible heating (SH) over the two plateaus not only have mutual influences but also feedback to each other. SH over the IP can reduce the SH and increase the LH over the TP, whereas the SH over the TP can increase surface heating over the IP, thereby reaching quasi-equilibrium among the SH and LH over the TP, IP SH and atmosphere vertical motion. Therefore, the so-called Tibetan-Iranian Plateau coupling system (TIPS) is constructed, which influences atmosphere circulation. In the TIPS system, interaction between surface SH and LH over the TP plays a leading role. SH of the IP and TP influences on other regions not only have superimposed effects but also mutually offset. Accounting for contributions to the convergence of water vapor transport in the Asian subtropical monsoon region, TP SH contributes more than twice that of the IP. The combined influence of SH over TP and IP represents the major contribution to the convergence of water vapor transport in that region. In addition, the heating effect of TIPS increases the upper tropospheric temperature maximum and lifts the tropopause, cooling the lower stratosphere. Combined with large-scale thermal forcing of the Eurasian continent, the TIPS produces a strong anticyclonic circulation and the South Asian High that warms the upper troposphere and cools the lower stratosphere, thereby affecting regional and global weather and climate.

Two types of summertime heating over the Asian large-scale orography and excitation of potential-vorticity forcing I. Over Tibetan Plateau

Based on numerical simulation, this study explored the characteristics and interactions of surface sensible heating and atmospheric latent heating over the main part of the Tibetan Plateau, i.e., terrain at elevations >2 km in summer. The impacts of these two types of heating on local vertical motion and monsoonal meridional circulation were compared. Theoretical analysis and numerical experimentation demonstrated that by changing the configuration of the upper-tropospheric air temperature and circulation, the two types of heating could generate both minimum absolute vorticity and abnormal potential vorticity forcing near the tropopause, enhance the meridional circulation of the Asian summer monsoon, and produce an eastward- propagating Rossby wave train within the mid-latitude westerly flow. Consequently, the manifestations of these features were shown to influence the circulation of the Northern Hemisphere.

Impacts of the Tibetan Plateau on Asian Climate

Abstract Professor Yanai is remembered in our hearts as an esteemed friend. Based on his accomplishments in tropical meteorology and with his flashes of insight he led his group at the University of California, Los Angeles, in the 1980s and 1990s to explore the thermal features of the Tibetan Plateau (TP) and its relation to the Asian monsoon, and he brought forward the TP meteorology established by Ye Duzheng et al. in 1957 to a new stage. In cherishing the memory of Professor Yanai and his great contribution to the TP meteorology, the authors review their recent study on the impacts of the TP and contribute this chapter as an extension of their chapter titled “Effects of the Tibetan Plateau” published by Yanai and Wu in 2006 in the book The Asian Monsoon. The influence of a large-scale orography on climate depends not only on the mechanical and thermal forcing it exerts on the atmosphere, but also on the background atmospheric circulation. In winter the TP possesses two leading heating modes resulting from the relevant dominant atmospheric circulations, in particular the North Atlantic Oscillation and the North Pacific Oscillation. The prevailing effect of the mechanical forcing of the TP in wintertime generates a dipole type of circulation, in which the anticyclonic gyre in the middle and high latitudes contributes to the warm inland area to the west, and the cold seashore area to the east, of northeast Asia, whereas the cyclonic gyre in low latitudes contributes to the formation of a prolonged dry season over central and southern Asia and moist climate over southeastern Asia. Such a dipole circulation also generates a unique persistent rainfall in early spring (PRES) over southern China. In 1980s, Yanai and his colleagues analyzed the in situ observation and found that the constant potential temperature boundary layer over the TP can reach about 300 hPa before the summer monsoon onset. This study supports these findings, and demonstrates that such a boundary layer structure is a consequence of the atmospheric thermal adaptation to the surface sensible heating, which vanishes quickly with increasing height. The overshooting of rising air, which is induced by surface sensible heating, then can form a layer of constant potential temperature with a thickness of several kilometers. The thermal forcing of the TP on the lower tropospheric circulation looks like a sensible heat–driven air pump (SHAP). It is the surface sensible heating on the sloping sides of the plateau that the SHAP can effectively influence the Asian monsoon circulation. In spring the SHAP contributes to the seasonal abrupt change of the Asian circulation and anchors the earliest Asian summer monsoon onset over the eastern Bay of Bengal. In summer, this pumping, together with the thermal forcing over the Iranian Plateau, produces bimodality in the South Asian high activity in the upper troposphere, which is closely related to the climate anomaly patterns over South and East Asia. Because the isentropic surfaces in the middle and lower troposphere intersect with the TP, in summertime the plateau becomes a strong negative vorticity source of the atmosphere and affects the surrounding climate and even the Northern Hemispheric circulation via Rossby wave energy dispersion. Future prospects in related TP studies are also addressed.

Tibetan Plateau climate dynamics: recent research progress and outlook

Abstract This paper reviews progress in the study of Tibetan Plateau (TP) climate dynamics over the past decade. Several theoretical frameworks, including thermal adaptation and the TP sensible heat (SH) driving air-pump, have been developed to identify the mechanisms responsible for the circulation anomaly produced by thermal forcing of the TP. Numerical simulations demonstrate that the thermal effects of large-scale orography, including the Tibetan and Iranian Plateaus (TIP), are crucial for the formation of the East Asian and South Asian summer monsoons (SASM) because the surface SH of the TIP is the major driver of the water vapor transport required for the genesis of the north branch of the SASM. The large-scale orography of the TP affects the Asian climate through thermal forcing in spring and summer, and mechanical forcing in winter. The TP forcing can also influence the Asian summer monsoon (ASM) onset over the Bay of Bengal (BOB) by enhancing the BOB warm pool at the surface and by modulating the South Asian High (SAH) in the upper troposphere. On intra-seasonal timescales, the TP thermal forcing significantly modulates spring rainfall in southern China and generates the biweekly oscillation of the SAH in summer. Despite climate warming, the atmospheric heat source over the TP, particularly the spring SH, exhibits a clear weakening trend from the 1980s to 2000s. This weakening of the spring SH contributed to the anomalous ‘dry in the north’ and ‘wet in the south’ rainfall pattern observed over East China. Also discussed are challenges to further understanding the mechanism of TP forcing on the multi-scale variability of the ASM.

Evolution of the Asian monsoon and the coupled atmosphere-ocean system in the tropics associated with the uplift of the Tibetan Plateau – A simulation with the MRI coupled atmosphere-ocean GCM

Large-scale orographies, such as the Tibetan Plateau and the Rocky mountains, play an important role to form the present global climate system (Kutzbach et al. 1993). The large-scale orography has dynamical and thermodynamical effects to the atmosphere, which in turn influences the ocean circulation through the atmosphere-ocean interaction. Many previous studies investigated the effect of mountains on the Asian summer monsoon using atmospheric general circulation models (AGCMs), by comparing the equilibrium climate states with and without mountains (Tibetan Plateau) and that without mountains (e.g., Hahn and Manabe 1974). However, these studies could not reveal to what degree the change of mountain height affects a climate and monsoon. It can also be postulated that the effect of mountains does not necessarily change linearly with increasing mountain heights. In addition, the Asian summer monsoon has been noted to have an active role of the interannual variability of the coupled atmosphereocean system in the tropical Pacific through the east-west circulation (e.g., Yasunari 1990, Yasunari and Seki 1992). The coupling of the Asian summer monsoon with the tropical Pacific Ocean system may, therefore, be crucial for the formation and variability of the tropical climate. An essential issue may be to investigate to what extent and how the tropical climate system, which is a coupled system of the Pacific/Indian Oceans and the Asian monsoon, is regulated by the uplift of the Tibetan plateau and other large-scale orographies. Thus, to understand changes of the atmosphere/ocean coupled system associated with gradual uplift of the orography, a numerical experiment was conducted using the Meteorological Research Institute (MRI) coupled atmosphere–ocean general circulation model I (CGCM-I) (Abe et al. 2003). The MRI CGCMI is the global grid model. Horizontal resolution of atmospheric part of the CGCM is 5° in longitude and 4° in latitude, and the vertical is 15 layers. The oceanic part has nonuniform meridional resolution ranging from 0.5° to 2.0°, with a finer grid in the tropics, fixed zonal resolution of 2.5°, and 21 layers vertically.

Relative Roles of Large-Scale Orography and Land Surface Processes in the Global Hydroclimate. Part II: Impacts on Hydroclimate over Eurasia

Abstract A series of simplistic simulations from an AGCM coupled to a simple land surface scheme and water vapor tracers was performed to explore the relative roles of basic factors in land surface conditions, with regard to the seasonal evolution of the hydroclimate over Eurasia. Large-scale orography in Asia and vegetation (further decomposed to soil and vegetation skin) were evaluated, with orography represented in the model by surface altitude, soil represented by water-holding capacity, and vegetation skin represented by surface albedo and roughness. The percentage of global annual precipitation over land (occupying 25.6% of the total surface) was 14.8%, 15.0%, and 21.7% for the mountainless “bare rock” (i.e., vegetationless) surface, and the bare-rock and vegetated surface, respectively. The result for evaporation was 8.9%, 9.0%, and 16.2%, respectively, showing higher sensitivity to the land surface changes than precipitation. The orography and vegetation (i.e., soil and vegetation skin) showed different impacts on Eurasian hydroclimate on the seasonal and regional scales. Thermodynamical forcings to the atmosphere increased over the continent with the inclusion of both. Large-scale orography in Asia exerted east–west contrast in the surface energy exchange in summer in eastern Eurasia. An increase in extratropical winter precipitation with mountains was also noticed because of the atmospheric vapor transport changes. Impact of soil and vegetation skin was clearly found in the warm season in the extratropics; soil impacts extratropical summer precipitation due to enhanced recycling of water and the resultant increased water supply.

Relative Roles of Large-Scale Orography and Land Surface Processes in the Global Hydroclimate. Part I: Impacts on Monsoon Systems and the Tropics

Abstract A series of numerical simulations by an atmospheric general circulation model (AGCM) was conducted to evaluate the relative roles of Tibetan Plateau (TP) and continental-scale land surface processes on the Asian monsoon and hydroclimates in other regions in the Tropics. Four boundary conditions were used to define experiments: a flat and bare-rock (nonvegetated) surface (NMR), a realistic TP with bare-rock surface (MR), MR conditions but with a water-holding soil layer (with 20 cm of field capacity) (MS), and TP with a vegetated surface with specified albedo and roughness of the current vegetation and the soil layer (MVS). Systematic increases of precipitation (P) over the Asian monsoon region and other land areas in the Tropics were noticed from NMR, MR, MS, to MVS runs. Precipitation over the warm tropical western Pacific, in contrast, decreased from NMR to MVS runs although the same SST climatology was prescribed in all the experiments. The effect of TP orography produces wet (dry) climate regions to the west (east) of TP as shown in other GCM studies. In the Asian monsoon region both land surface effect (LSE) and TP effect (TE) contribute nearly equally to increase P, but in the West African monsoon region, TE contributes to decrease P while LSE plays a major role to increase P. LSE also enhances the subtropical anticyclone over the North Pacific through enhanced land–ocean thermal contrast. In most of the monsoon regions and Tropics, the albedo (and roughness) effect of vegetation substantially increases available radiation, which in turn increases both evaporation (E) and atmospheric moisture convergence (C), and, as a result, significantly increases P. This positive feedback between E and C is likely to be a characteristic nature when and where vegetation albedo effect is significant under sufficient radiative energy, suggesting a prerequisite for Charney’s original hypothesis.

Effects of Large-scale Orography on the Coupled Atmosphere-Ocean System in the Tropical Indian and Pacific Oceans in Boreal Summer

We investigated the effects of large-scale orography on the tropical coupled atmosphere-ocean system over the Indian and Pacific Oceans in northern summer, using the Meteorological Research Institute coupled atmosphere-ocean General Circulation Model (GCM). Six different experiments were conducted with mountain heights of 100%, 80%, 60%, 40%, 20%, and 0% of the standard mountain height. The results show that a pool of warm sea surface temperatures (SSTs) appears in the western Pacific as orography increases, although SST in the tropical Pacific decreases as a whole. In addition, easterly winds at low levels over the equatorial Pacific strengthen as mountains rise. The enhanced easterlies alter surface heat flux and ocean dynamics, changing the water temperature field in the upper Pacific Ocean. Water temperatures between the surface and 300 m in the western Pacific increase as upwelling is suppressed and the thermocline deepens. Water temperatures in the eastern Pacific decrease and the thermocline rises. Therefore, the east-west gradient of water temperature in the Pacific is enhanced for cases with mountain heights of 80% and 100% of the standard mountain height. In the equatorial Indian Ocean, the east-west gradient of ocean heat content weakens as mountain heights increase, in connection with the evolution of the Asian summer monsoon. An increase in diabatic heating over South Asia as mountain heights increase causes sea level pressure (SLP) to decline over the Indian Ocean, and enhances upper atmospheric divergence over the eastern hemisphere. Consequently, the east-west circulation over the Indian and Pacific Oceans strengthens as mountains become taller. The east-west circulation may also be enhanced by changes in convective activity associated with SST changes. The coupled general circu- lation model (GCM) results show that uplift of large-scale orography, particularly the Tibetan Plateau, significantly affects the tropical atmospheric and oceanic climate, by changing the east-west circulation and altering the evolution of the Asian summer monsoon.

Stationary Eddy Response to Surface Boundary Forcing: Idealized GCM Experiments*

Abstract A set of atmospheric general circulation model (AGCM) experiments under idealized conditions is performed to investigate atmospheric response to surface boundary forcing by extratropical land–sea contrast, large-scale orography, and tropical sea surface temperature (SST) distribution. Stationary eddies forced by the extratropical land–sea distribution are strongest in high latitudes, but their amplitudes are modest and comparable to internal chaotic variability. By contrast, the stationary eddy response to zonal variations in tropical SST is strong and robust in both the subtropics and midlatitudes. While these SST-forced stationary waves are trapped within the troposphere, those induced by orography show a strong vertical propagation into the stratosphere. Analysis of transient eddies indicates that orography is effective in generating a zonally localized storm track while extratropical land–sea contrast has little effect on the zonal variation of upper-level storm activity. A vorticity budget a...

Mechanistic Model Simulations of the East African Climate Using NCAR Regional Climate Model: Influence of Large-Scale Orography on the Turkana Low-Level Jet

Abstract The National Center for Atmospheric Research regional climate model (RegCM) is employed to study the dynamics of the Turkana low-level jet that lies between the Ethiopian and the East African highlands, and also investigate the mechanisms responsible for the observed dry conditions over the Lake Turkana basin that lies in the wider section of the Turkana channel. The role of the large-scale orography and two other forcing factors namely the large-scale monsoonal flow and the Turkana channel depth are investigated in order to understand the kinematics of the jet. The simulated patterns of the Turkana easterly low-level jet compares well with its observed characteristics. Strong winds are indicated in the channel throughout the study period of October to December, with the wind speed decreasing in the middle and wider region of the channel. A split in the jet core is also shown in the middle of the channel. The level of maximum winds (∼11 m s−1) occurs in the layers 930-hPa and 650-hPa levels. The ...

Formation of subtropical westerly jet core in an idealized GCM without mountains

In boreal winter, subtropical westerlies in the upper troposphere reach maxima in speed over the eastern edges of the Asian and North American continents. The zonal variations in the westerlies are generally attributed to large‐scale orography and thermal forcing, but the latter mechanism remains largely unsubstantiated. Here we conduct general circulation model (GCM) experiments without orography to identify the most important thermal forcing for generating zonal asymmetries in subtropical westerlies. By changing sea surface temperature (SST) distribution in the GCM, we find that the tropical SST distribution plays a decisive role in producing a subtropical jet core to the north of the tropical warm water pool, while the effects of extratropical continent‐ocean heating contrast on upper‐level zonal wind speed distribution are secondary. The results from Aqua Planet runs further support this conclusion.

The solitary waves of the barotropic quasi-geostrophic model with the large-scale orography

Starting from a modified barotropic quasi-geostrophic model equation, considering the actual situation of the large-orography of the Tibetan Plateau, neglecting its slope in x direction, and using the reductive perturbation method, then the solitary waves are obtained. The results show that the orography is essential factor exciting solitary Rossby waves in a flow without shear.

Factors Controlling Nonlinearity in Mechanically Forced Stationary Waves over Orography

Abstract The forcing of stationary waves by the earth’s large-scale orography is studied using a nonlinear stationary wave model based on the quasigeostrophic equations. The manner in which wind speed, meridional temperature gradient, Ekman pumping parameter, linear damping, orographic shape, and meridional wind structure affect the validity of the linearized equations is examined and the nonlinear response is investigated. A critical mountain height that separates the linear from the nonlinear regime is defined based on the linear quasigeostrophic potential temperature equation applied at the surface. The largest critical heights (those responses in which nonlinearity is least important) are obtained when the surface damping is weak or nonexistent. Also, relative maximums in mountain critical heights are obtained when the ratio of surface wind to surface wind shear does not vary in the meridional direction. These critical height results are validated using the fully nonlinear stationary wave model. The n...

The influence of weakly–nonlinear vertical advection on the wind field of PBL with large–scale orography

The analytical solutions of the PBL wind distribution under the equilibrium of four forces including both horizontal and vertical advections are obtained in this paper using small parameter method. Utilizing this simple PBL model, we also compute the wind spirals of a certain circular vortex over level ground and over a west-east ranged mountain, respectively. The results show that when there exsits large-scale orographic forcing,vertical advection may exert considerable influence on the wind distribution of PBL, the angle between geostrophic flow and surface wind, and the height of the top of PBL.

Orographic control of storm zones on Mars

Low-pressure cyclones and their accompanying frontal systems in the middle latitudes of the Earth's troposphere develop, travel eastwards and decay preferentially within latitudinally and longitudinally confined geographical regions known as storm zones1. These zones can be readily identified in the circulation statistics of terrestrial weather systems2. Mars, like the Earth, is a rapidly rotating solid planet and has a seasonally varying shallow atmosphere, large-scale orography, and (in a broadly defined context) continental structures3. Although there are also important differences between the two planets, travelling weather systems do exist on Mars3. This raises the question of whether storm zones also occur there, and if so, by what mechanisms. Here we report the results of numerical simulations of global atmospheric circulation patterns on Mars. We find that storm zones can exist during the northern winter, and that continental-scale orography (rather than surface thermal contrasts) is the main factor determining the development of these zones. Storm zones on Mars should play an important role in the martian climate cycle4,5, by influencing the transport of (for example) heat, momentum, water vapour and atmospheric dust3,6–8 towards the poles.