Windward Ridge Research Articles

Microphysical Enhancement Processes within Stratiform Precipitation on the Barrier and Sub-Barrier Scale of the Olympic Mountains

AbstractHigh-resolution numerical model simulations of six different cases during the 2015/16 Olympic Mountains Experiment (OLYMPEX) are used to examine dynamic and microphysical precipitation processes on both the full barrier-scale and smaller sub-barrier-scale ridges and valleys. The degree to which stratiform precipitation within midlatitude cyclones is modified over the coastal Olympic Mountains range was found to be strongly dependent on the synoptic environment within a cyclone’s prefrontal and warm sectors. In prefrontal sectors, barrier-scale ascent over stably stratified flow resulted in enhanced ice production aloft at the coast and generally upstream of higher terrain. At low levels, stable flow orientated transverse to sub-barrier-scale windward ridges generated small-scale mountain waves, which failed to produce enough cloud water to appreciably enhance precipitation on the scale of the windward ridges. In moist-neutral warm sectors, the upstream side of the barrier exhibited broad ascent oriented along the windward ridges with lesser regions of adjacent downward motion. Significant quantities of cloud water were produced over coastal foothills with further production of cloud water on the lower-windward slopes. Ice production above the melting layer occurred directly over the barrier where the ice particles were further advected downstream by cross-barrier winds and spilled over into the lee. The coastal foothills were found to be essential for the production and maintenance of cloud water upstream of the primary topographic barrier, allowing additional time for hydrometeors to grow to precipitation size by autoconversion and collection before falling out on the lower-windward slopes.

Investigation of Orographic Precipitation over an Isolated, Three-Dimensional Complex Topography with a Dense Gauge Network, Radar Observations, and Upslope Model

Abstract This study uses a dense rain gauge network, radar observations, and an upslope model to document the detailed aspects of precipitation over Da-Tun Mountain (DT) of northern Taiwan under prevailing northeasterly monsoonal flow. DT is one of the most concentrated areas of heavy rainfall in Taiwan, with a size of ~15 km and a terrain peak of ~1 km (MSL), and it constitutes a concave ridge with two windward ridge arms that encompass a funnel-shaped valley. Twenty-one rainfall events identified over DT from January 2011 to February 2015 are chosen for analysis. More than half of the studied cases exhibit two local maxima of rainfall over the ridge arms, and asymmetric characteristics between these two maxima are evident. The other frequent rainfall pattern, observed as upstream winds are more from the east-northeast, is characterized by a local maximum of precipitation inside the valley. Analyses from the upslope model confirm that the occurrence of the windward maxima of rainfall is primarily caused by upslope lifting, and their asymmetric characteristics are closely related to the difference in the azimuthal variations of slope steepness between the two ridge arms. The observed characteristics of rainfall intensities inside the valley, however, cannot be well described by the upslope model. It is found that the lateral flow confluence induced by the deflected flows over the ridge arms may play an essential role in intensifying upslope-forced precipitation within the valley. This effect emerges as upstream winds are roughly parallel to the central axis of funneling regions located between the ridge arms.

Effectiveness of ridge vent to wind-driven natural ventilation in monoslope multi-span greenhouses

A large eddy simulation (LES) model and wind tunnel experiments were used to investigate the wind-driven ventilation of monoslope greenhouses with ridge vents. The accuracy of the numerical simulation was verified by the results of wind tunnel experiments. The numerical model was then employed to systematically examine the influences of wind direction, opening configuration and greenhouse length on the cross ventilation. In addition, an analytic model, based on inviscid flow assumption and mass conservation, was used to compute the ventilation rate. The results showed that the total ventilation rates of the multi-span greenhouses with open ridge vents were much larger than that of with closed ridge vents. The windward ridge vent (directly facing the wind direction when θ = 180°) can substantially increase the ventilation rate of multi-span monoslope greenhouses, as compared to the leeward ridge vent when wind direction θ = 0°. In other words, to take full advantage of the wind-driven ventilation the ridge vents of the greenhouses should directly face the prevailing wind direction. The ventilation rates can be reduced by adjusting the degree of opening and the number of the operable ridge vents.

Two-Dimensional Idealized Simulations of the Impact of Multiple Windward Ridges on Orographic Precipitation

Abstract This paper presents two-dimensional (2D) idealized simulations at 1-km grid spacing using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) in order to illustrate how a series of ridges along a broad windward slope can impact the precipitation distribution and simulated microphysics. The number of windward ridges for a 2000-m mountain of 50-km half-width is varied from 0 to 16 over a 150-km distance using different stratifications, freezing levels, uniform ambient flows, and ridge amplitudes. A few (200–400 m) windward ridges can enhance the precipitation locally over each ridge crest by a factor of 2–3. Meanwhile, a series of 8–16 ridges that are 200–400 m in height can increase the net precipitation averaged over the windward slope by 10%–35%. This average precipitation enhancement is maximized when the ridge spacing is relatively small (<20 km), since there is less time for subsidence drying within the valleys and the mountain waves become more evanescent, which favors a simple upward and downward motion couplet over each ridge. In addition, small ridge spacing is shown to have a synergistic effect on precipitation over the lower windward slope, in which an upstream ridge helps increase the precipitation over the adjacent downwind ridge. There is little net precipitation enhancement by the ridges for small moist Froude numbers (Fr < 0.8), since flow blocking limits the flow up and over each ridge. For a series of narrow ridges (∼10 km wide), the largest precipitation enhancement for a 500-mb freezing level occurs over lower windward slope of the barrier through warm-rain processes. In contrast, a 1000-mb freezing level has the largest precipitation enhancement over the middle and upper portions of a barrier for a series of narrow (∼10 km wide) ridges given the horizontal advection of snow aloft.

Topographic Effects on a Wintertime Cold Front in Taiwan

Abstract This paper describes an observational and numerical study of an intense wintertime cold front that occurred in Taiwan on 8 January 1996. The front was associated with rope clouds at the leading edge, and a broad area of stratiform clouds behind. The front was blocked by the Central Mountain Range of Taiwan and divided into two sections on each side of the mountain range. As the cold air moved southward along the east coast, the increasing westward Coriolis force induced a landward acceleration. After the cold air piled up against the mountains, a coastal pressure ridge developed. The cold air damming yielded a geostrophic balance between the westward Coriolis force and the eastward component of the pressure gradient force in the x direction, and a southward acceleration in the y direction mainly caused by the southward pressure gradient force component. Over the Taiwan Strait, southward pressure gradient forces increased when the low-level stable cold air was confined over the Taiwan Strait, leading to a southward acceleration of the cold air. The formation of a windward ridge off the northwest coast of Taiwan contributed to a large southward acceleration, resulting in the development of a coastal jet. Over the Taiwan Strait, the cold air moved southward the fastest due to the channeling effect. The air parcels along the east coast of Taiwan experienced a downgradient acceleration from the cold air damming and advanced at a slower speed. Those traveling over the western plains and the nearshore coast advanced at the slowest speed. Two sensitivity runs, one without Taiwan’s topography (flat land only) and the other without Taiwan’s landmass, demonstrated the influences of Taiwan’s terrain and water–land contrast on the airflow. The run with no surface fluxes showed that the ocean modified the low-level cold air by supplying surface heat and moisture fluxes. This weakened the front, reduced low-level stability, and increased forced shallow convection (formation of rope clouds) at the leading edge.

The modeling and observation of a lee trough event over eastern Tasmania

Due to its complex orography, a lee trough can occur along leeward coasts of Tasmania. Of particular interest to meteorologists and others, is the lee trough that can form along eastern parts of Tasmania. This effect is often observed under broad westerly flow due mainly to a topographic barrier, which lies to the west of the area of interest. The lee trough seems to be most pronounced when the troposphere is stable with an inversion just above the highest parts of the western and central highlands, known as the Western Mountains and Central Plateau, respectively. Blocking of the low-level airflow, coupled with the formation of a windward ridge of high pressure may also contribute to the formation of the lee trough. From time to time, mesoscale thermal lows and pronounced standing wave activity has also been observed to form over parts of the state co-incident with the lee trough. These can further complicate the overall wind pattern over parts of the Tasmania. This study attempts to investigate and verify some aspects of this lee trough by the utilization of a very high resolution numerical weather prediction model, known as HIRES (HIghRESolution).

The Role of Orography in Determining the Sahelian Climate

We have examined numerical modelling evidence based on the NASA GEOS-1 general circulation model in order to investigate the role of orography in determining the West African summer climate anomalies. Orographic climatic control and the influence of sea-surface temperature (SST) anomaly forcing were investigated by systematic suppression or inclusion of these effects in the numerical experiments. Our main finding is that the orography of Africa plays a major role in determining the climate of the Sahel and the coastal region of West Africa during the northern hemispheric summer season. These orographic effects are comprised of: (i) a quasi-stationary orographic ridge–trough dipole generated by the passage of the low-level prevailing easterlies over the Atlas–Ahaggar mountain complex over northern Africa, and (ii) an elongated zonal windward orographic ridge generated as the cross-equatorial summer monsoons from the South Atlantic Ocean basin ascends over the elevated landmass of West Africa. Over the Sahel, the moist south-westerly flow emanating from the Atlas–Ahaggar orographic trough over the Atlantic Ocean and the dry hot north-easterlies originating from the corresponding orographic windward ridge tend to converge along a zonal axis thus reinforcing the Inter-Tropical Convergence Zone. This results in wetter conditions over the Sahel than would have been the case in the absence of orography. The model simulations show that further to the south along the coastal region of West Africa, the zonal orographic ridge results in drier conditions than in the model runs when orography is suppressed. The net result is a permanent orographic-induced rainfall dipole pattern over West Africa. Further model simulations were conducted in which the 1973 SST anomaly conditions were described with or without the orography of Africa. The 1973 SST anomaly pattern is characterized by positive anomalies in the Southern Hemisphere and negative anomalies in the Northern Hemisphere. The model results show a weaker orographic-induced rainfall dipole pattern across West Africa in response to the 1973 SST anomaly pattern. The net result is wetter conditions along the coastal region and rainfall deficits over the Sahelian zonal strip. © 1997 by the Royal Meteorological Society.

Analysis of a Relatively Dry Front during the Taiwan Area Mesoscale Experiment

Abstract The evolution of a relatively dry front during the early-summer rainy Season of Taiwan is analyzed. Because of the synoptic subsidence associated with a subtropical high pressure cell over the northern South China Sea, prefrontal soundings over the Taiwan area exhibited a shallow, warm, moist layer in the lowest levels, capped by an inversion with extremely dry air aloft. Over the Taiwan area, the southwest flow ahead of the surface front was more than 10 m s−1 at the 850-mb level. It interacted with the central mountain range, resulting in the windward ridge, leeside trough. Downstream of the blocked region, strong southwesterly winds (∼1 5 m s−1) developed in the lowest levels along the northwest coast, where the flow deflected by the mountain barrier merged with the undetected southwest monsoon flow. The hilly terrain along the southeastern China coast retarded the cold air behind the surface front. The cold air was then ducted around the southeastern China coast. At the 850-mb level, a weak s...

Comparison of snow drifting measurements at an alpine ridge crest

Experiments at Weissfluhjoch, Davos, Switzerland, compared mass flux measured by rocket-shaped snowdrift gages (modified Mellor traps), with gages made of porous filter fabric, and with several electronic systems. A new method of electronic analysis gave results similar to the mass flux measured by mechanical traps. The experiments also reexamined the vertical profile of drift flux in the vicinity of the ridge crest, confirming the flux maximum at about 1 m height above the ridge crest, and adding details near the surface. Profiles measured at a secondary change in the windward ridge face, within a few meters of the crest, showed only a slight maximum, supporting the view that the drift flux maximum develops near the crest.