Windward Mountain Slopes Research Articles

Термическое воздействие на приземный слой тропосферы как способ управления рисками природных чрезвычайных ситуаций

Purpose. Natural emergencies that occur in mountainous regions annually damage ecosystems, the country's economy and lead to the death of people. On leeward mountain slopes, where precipitation is insufficient, there is an increased fire hazard of territories, complicating water supply of the population and ecosystems. On windward slopes excessive precipitation leads to the risk of avalanches, floods, and mudflows. The search for ways to redistribute the volume of precipitation between the windward and leeward slopes to control the risks of natural emergencies is relevant. The intensity of precipitation is significantly affected by the dew point height (DPH), which determines the position of the lower cloud boundary (the lower the dew point height, the greater the probability of precipitation). The article discusses the possibility of local thermal impact on the ground layer of the troposphere by heating a section of the mountain windward slope in order to increase the dew point height for the subsequent natural transfer of the precipitation zone to its leeward slope. The purpose of the study is to determine the values intervals of the difference in average temperatures of the tropospheric ground layer at its opposite boundaries and values intervals of average wind speed over the relief, where the thermal interaction of the tropospheric ground layer with a local area of the earth's surface can be used to effectively control the dew point height over the entire windward slope of the mountain. Methods. Three-dimensional field modeling is applied using the Fire Dinamic Simulator software product, which provides a numerical solution of the complete nonlinear Navier-Stokes thermohydrodynamic equations for low-speed temperature-dependent flows. Findings. It has been established that the thermal effect on the ground layer of the troposphere above the foot of the mountain windward slope under forced convection conditions can be used to control the distribution of the dew point height over this entire slope. Research application field. The conducted numerical study is the initial stage of a new scientific direction related to natural emergencies risks control caused by excess or lack of mountain slopes moisture (floods, droughts, forest and steppe fires). The obtained results indicate the fundamental possibility of such control by means of local temperature impact from the earth's surface to the ground layer of the troposphere. The practical significance of the results, including financial costs, required for the specified impact on the ground layer of the troposphere, can be revealed in subsequent publications after conducting full-scale experiments. Conclusions. The possibility of reducing the natural emergencies risks caused by excess or lack of moisture in mountain slopes territories by controlling the dew point height value over windward mountain slopes is shown. A solution to this problem is proposed through thermal impact on the ground layer of the troposphere from the earth's surface.

Potential impact of aerosols on convective clouds revealed by Himawari-8 observations over different terrain types in eastern China

Abstract. Convective clouds are common and play a major role in Earth's water cycle and energy balance; they may even develop into storms and cause severe rainfall events. To understand the convective cloud development process, this study investigates the impact of aerosols on convective clouds by considering the influence of both topography and diurnal variation in radiation. By combining texture analysis, clustering, and thresholding methods, we identify all convective clouds in two warm seasons (May–September, 2016/17) in eastern China based on Himawari-8 Level 1 data. Having large diurnally resolved cloud data together with surface meteorological and environmental measurements, we investigate convective cloud properties and their variation, stratified by elevation and diurnal change. We then analyze the potential impact of aerosol on convective clouds under different meteorological conditions and topographies. In general, convective clouds tend to occur preferentially under polluted conditions in the morning, which reverses in the afternoon. Convective cloud fraction first increases then decreases with aerosol loading, which may contribute to this phenomenon. Topography and diurnal meteorological variations may affect the strength of aerosol microphysical and radiative effects. Updraft is always stronger along the windward slopes of mountains and plateaus, especially in northern China. The prevailing southerly wind near the foothills of mountains and plateaus is likely to contribute to this windward strengthening of updraft and to bring more pollutant into the mountains, thereby strengthening the microphysical effect, invigorating convective clouds. By comparison, over plain, aerosols decrease surface heating and suppress convection by blocking solar radiation reaching the surface.

The Impact of Mount Washington on the Height of the Boundary Layer and the Vertical Structure of Temperature and Moisture

Discrimination of the type of air mass along mountain slopes can be a challenge and is not commonly performed, but is critical for identifying factors responsible for influencing montane weather, climate, and air quality. A field campaign to measure air mass type and transitions on the summit of Mount Washington, New Hampshire, USA was performed on 19 August 2016. Meteorological observations were taken at the summit and at several sites along the east and west slopes. Ozone concentrations were measured at the summit and on the valley floor. Additionally, water vapor stable isotopes were measured from a truck that drove up and down the Mount Washington Auto Road concurrent with radiosonde launches that profiled the free atmosphere. This multivariate perspective revealed thermal, moisture, and air mass height differences among the free atmosphere, leeward, and windward mountain slopes. Both thermally and mechanically forced upslope flows helped shape these differences by altering the height of the boundary layer with respect to the mountain surface. Recommendations for measurement strategies hoping to develop accurate observational climatologies of air mass exposure in complex terrain are discussed and will be important for evaluating elevation-dependent warming and improving forecasting for weather and air quality.

Statistical Characteristics of Convective Clouds over the Western Ghats Derived from Weather Radar Observations

AbstractX‐band radar observations at Mandhardev (18.04°N, 73.85°E) are used to investigate statistics of convective clouds over the Western Ghats during monsoon season (June–September 2014). Convective storms (cells) are identified using an objective‐tracking method to examine their spatiotemporal variability, thus quantifying the time‐continuous aspects of convective cloud population over the region for the first time. An increased frequency of storm location and initiation along the windward mountains compared to coastal and lee side highlights orographic response to southwesterly flow, with superimposed diurnal cycle. An eastward progression of convective activity from upstream the barrier through windward slopes of mountains over to the lee side is observed. Storm area, height, and duration follow lognormal distributions; wherein, small‐sized storms contribute more to total population and unimodal distribution of 35 dBZ top heights (peaking at 5.5 km) depicts the dominance of shallow convection. Storms exhibit a pronounced diurnal cycle with a peak in afternoon hours, while the convective area maximum is delayed by several hours to that of precipitation flux. Cell lifetime and propagation show that cells move with slow speeds and have mean duration of 46 min. They align east‐west nearly parallel to mountain ridges, and their direction of movement is steered mostly by large‐scale winds at lower levels. Based on top heights, convective cells are further classified into cumulus, congestus, and deep clouds. In general, congestus (deep) cells are most abundant in the windward (leeward) side. A lead‐lag relationship between congestus and deep cells indicates midtroposphere moistening by congestus cells prior to deep convection.

Radar‐observed diurnal cycle and propagation of convection over the Pearl River Delta during Mei‐Yu season

AbstractUsing operational Doppler radar and regional reanalysis data from 2007–2009, the climatology and physical mechanisms of the diurnal cycle and propagation of convection over the Pearl River Delta (PRD) region of China during the Mei‐Yu seasons are investigated. Analyses reveal two hot spots for convection: one along the south coastline of PRD and the other on the windward slope of mountains in the northeastern part of PRD. Overall, convection occurs most frequently during the afternoon over PRD due to solar heating. On the windward slope of the mountains, convection occurrence frequency exhibits two daily peaks, with the primary peak in the afternoon and the secondary peak from midnight to early morning. The nighttime peak is shown to be closely related to the nocturnal acceleration and enhanced lifting on the windward slope of southwesterly boundary layer flow, in the form of boundary layer low‐level jet. Along the coastline, nighttime convection is induced by the convergence between the prevailing onshore wind and the thermally induced land breeze in the early morning. Convection on the windward slope of the mountainous area is more or less stationary. Convection initiated near the coastline along the land breeze front tends to propagate inland from early morning to early afternoon when land breeze cedes to sea breeze and the prevailing onshore flow.

Rainfall asymmetries of tropical cyclones prior to, during, and after making landfall in South China and Southeast United States

This study examines rainfall asymmetries of tropical cyclones (TCs) prior to, during, and after making landfall in South China and the Southeast United States from 1998 to 2011 based on TRMM rainfall products. The climatological TC rainfall asymmetries are then linked to various potential environmental factors. These factors include TC intensity and motion, vertical wind shear, topography, sea surface temperature (SST), and total precipitable water (TPW). All these factors are examined in combination, as it is in the real world. Results show that effects of the deep vertical wind shear (200–850hPa) dominate the rainfall asymmetries producing maxima on the downshear and downshear-left sides. This wind shear effect is still dominant even during TCs making landfall, or after landfall, or interacting with mountains. The magnitude of the rainfall asymmetry increases with shear magnitude, but decreases with increasing TC intensity. Furthermore, the shear effect is less efficient for TCs over relatively cool SST and low TPW oceans when TCs produce less precipitation. On the other hand, effects of storm motion (producing rainfall maxima at front quadrants) are not evident in this study, mainly due to opposing vertical wind shear effects. However, landfalling TCs experience a significant increase in the rainfall percentage toward the right quadrants relative to the coast, likely due to surface frictional gradient between land and sea. There is no evident signature of precipitation enhancement over the windward slope of mountains (400–600m) in SCHN and SECHN.

Rebuilding after collapse: evidence for long‐term cohort dynamics in the native Hawaiian rain forest

AbstractQuestionsDo long‐term observations in permanent plots confirm the conceptual model of Metrosideros polymorpha cohort dynamics as postulated in 1987? Do regeneration patterns occur independently of substrate age, i.e. of direct volcanic disturbance impact?LocationThe windward mountain slopes of the younger Mauna Loa and the older Mauna Kea volcanoes (island of Hawaii, USA).MethodsAfter widespread forest decline (dieback), permanent plots were established in 1976 in 13 dieback and 13 non‐dieback patches to monitor the population structure of M. polymorpha at ca. 5‐yr intervals. Within each plot of 20 × 20 m, all trees with DBH >2.5 cm were individually tagged, measured and tree vigour assessed; regeneration was quantified in 16 systematically placed subplots of 3 × 5 m. Data collected in the subplots included the total number of M. polymorpha seedlings and saplings (five stem height classes). Here we analyse monitoring data from six time steps from 1976 to 2003 using repeated measures ANOVA to test specific predictions derived from the 1987 conceptual model.ResultsRegeneration was significantly different between dieback and non‐dieback plots. In dieback plots, the collapse in the 1970s was followed by a ‘sapling wave’ that by 2003 led to new cohort stands of M. polymorpha. In non‐dieback stands, seedling emergence did not result in sapling waves over the same period. Instead, a ‘sapling gap’ (i.e. very few or no M. polymorpha saplings) prevailed as typical for mature stands. Canopy dieback in 1976, degree of recovery by 2003 and the number of living trees in 2003 were unrelated to substrate age.ConclusionsPopulation development of M. polymorpha supports the cohort dynamics model, which predicts rebuilding of the forest with the same canopy species after dieback. The lack of association with substrate age suggests that the long‐term maintenance of cohort structure in M. polymorpha does not depend on volcanic disturbance but may be related to other environmental mechanisms, such as climate anomalies.

Development and Evaluation of Mei-Yu Season Quantitative Precipitation Forecasts in Taiwan River Basins Based on a Conceptual Climatology Model

Abstract Using rain gauge data during 134 mei-yu frontal cases in May–June 1991–2006, a rainfall climatology in relation to the positions of fronts every 0.5° in Taiwan is obtained, showing widespread precipitation with maxima over windward mountain slopes associated with frontal passages. For six major river basins, rainfall characteristics and synoptic factors are further analyzed to build a conceptual climatology model for short-term quantitative precipitation forecasts (QPFs). The four basins in central–southern Taiwan exhibit increased heavy-rainfall frequencies (>25%) roughly from 2° south to 1° north of the front, while the two basins in northern Taiwan have lower frequencies with different characteristics mainly due to the differences in their topography. The synoptic factors in the checklist employed by the Central Weather Bureau and important to heavy rainfall are identified for each of the six basins through statistical tests and their threat score (TS). These factors include those related to mei-yu fronts, low-level jets and moisture, upper-level divergence–diffluence, and short-wave troughs. A conceptual climatology model that uses both synoptic and probability forecasting guidance is developed, and in practice the average rainfall climatology is replaced by one obtained for heavy-rainfall periods if either of the two guidance schemes indicates heavy-rainfall possibility. This model for 0–6- and 0–12-h QPFs is also evaluated for its usefulness using cases during the 2007–08 seasons. With typical TSs of 0.2–0.3 (for heavy rainfall), this approach outperforms simple climatology in all six basins especially toward higher thresholds (about 20–50 mm) and for 12-h events, where it also shows advantages over model QPFs in southern Taiwan. Thus, the model can provide useful information for operational use.

Петрофитные сообщества высокогорных известняковых массивов Западного Кавказа

The paper presents description of new syntaxa of pertrophyte associations from West Caucasus lime­stone massifs. The work is based on the author’s long-term observations between years 1998 and 2003, made in the basins of the rivers Belaya, Bolshaja Laba and Malaya Laba within the National Nature Park «Bol­shoi Tkhach» and limestone massifs of Caucasian State Natural Biosphere Reserve. Original geobotanical rele­veйs have been used for the classification. Collec­tion and material processing involved Braun-Blanqet me­thod. The size of sampling plots varied from 4 to 25 m2. The paper is based upon 120 releveés. New syntaxa names have been given in accordance with the Code of Phytosociological Nomenclature (Weber et al., 2000). Ass. Veronica minutae—Chaerophylletum humilis Onipchenko 2002 is common for upper alpine belt of the Reserve and distributed over mobile calcareous screes at 2400—2800 m alt. occupying small areas of Oshten, Fisht, Pshekha-Su and Yatygvarta mountains. Ass. Helianthemetum cani ass. nov. includes open chionophobic groupings of alpine belt, developing on windward mountain slopes and crests at 2000—2800 m alt., occurs on rocky outcrops and dry sedentary screes, and covers significant areas of Lagonaki Pla­teau, mountains Bolshoi Tkhach, Yatygvarta and Snegovalka Ridge. These open associations preferen­tially occupy slopes of «warm» (southern, southern-west, southern-east) expositions of different abruptness from almost horizontal up to 70° (in average 25°). Ass. Saxifragetum sibiricae typicum Onipchenko et Lubez­nova 2002 includes chionophytic groupings of seden­tary screes, characterized by significant snow accu­mulation in winter, forms patches of vegetation around snowbeds and glaciers of Lagonaki Plateau in the Reserve, occurs sporadically on the studied territory. Ass. Saxifragetum sibiricae arabidetosum caucasicae subass. nov. combines open communities of alpine belt at unstable damp rocky substrates and predominantly occupies abrupt slopes (20—35°) of northern—nor­thern-east exposition of damp, slightly mobile screes close to glaciers at 2000—2700 м alt. on Lagonaki Plateau in the Reserve. Ass. Saxifrago cartilagineae—Asplenietum rutae-murariae ass. nov. occurs on rocks in subalpine belt predominantly on abrupt slopes (20—90°, average 69°) of different expositions at 1400—1700 m alt. on Lagonaki Plateau in the Reserve. Ass. Valeriana saxicolae—Helianthemetum cani ass. nov. includes plant communities of limestone rocks occupying abrupt slopes (20—70°, in average 53°) of predominantly southern expositions at 2000—2700 m alt. on Lagonaki Plateau, Triu-Yatygvarta massif, mountain Bolshoi Tkhach.

Cross-Barrier Flow during Orographic Precipitation Events: Results from MAP and IMPROVE

Abstract Ground-based and airborne Doppler radar data collected during passage of frontal rainstorms over the European Alps and Cascade Mountains of Oregon are found to exhibit characteristic cross-barrier flow and precipitation patterns. A stably stratified layer of blocked (or partially blocked) low-momentum flow below mountain-crest level is separated from strong cross-barrier flow aloft by a concentrated layer of vertical wind shear. This shear layer slopes upward toward the mountain crest and persists for several hours as the storm passes over the mountain range. This pattern is remarkably similar from case to case and from one mountain range to the other. The orographically enhanced precipitation in these cases exhibits stratiform structure, marked by a radar bright band, which often drops to a lower height (indicating locally cooler conditions) immediately adjacent to the windward mountain slopes. A terrain-induced gravity wave produces strong downslope flow and spillover of precipitation onto the lee side of the barrier. An elevated reflectivity maximum appears above the mountain crest and extends a short distance (generally 20–40 km) upstream, apparently as a result of gravity wave lifting. While baroclinically induced shear may contribute to the observed pattern, two-dimensional idealized simulations indicate that, in the presence of sufficient upstream static stability, orographic effects alone are sufficient to support development of the upward-sloping shear layer on the windward side of the barrier. Furthermore, idealized simulations show that if there is preexisting vertical shear in the incident upstream flow, this shear is strengthened by the orography and that surface friction may also enhance the strength of the shear layer. The repeatable sheared velocity pattern identified here is apparently an essential feature of cases in which stably stratified low-level flow associated with a baroclinic system impinges upon a sufficiently tall mountain range. Since the layer of orographically generated shear promotes turbulence, which has been shown to aid precipitation growth over the windward slopes, these stable flow dynamics represent a potentially important mechanism for orographic precipitation enhancement in association with the passage of frontal systems over mountainous terrain.

Dynamics of coarse riparian detritus in a Hawaiian stream ecosystem: a comparison of drought and post-drought conditions

AbstractInput, standing crop, and export of coarse (>4 mm2) particulate organic matter (CPOM) was measured in Kaiwiki Stream, a forested stream on the Island of Hawaii. The study was conducted during and after a severe drought associated with the 1997–1998 El Nino/Southern Oscillation event. During the drought, stream discharge was reduced by ∼70% and no spates occurred. Hawaiian streams draining windward mountain slopes normally experience >20 spates per year. During the drought, the mean rate of CPOM export was 8% of the mean input rate; following the drought, the export rate increased to 40% of input. Rates of marked leaf transport within the study area increased 30- to 50-fold following the drought, and rates of stem transport increased 10- to 30-fold. Lower rates of CPOM export and transport during the drought suggested that retention and potential availability to detritivores were increased compared to the post-drought period. Differences in CPOM standing crops during and after the drought were not ...

Vegetation Dynamics and Slope Management on the Mountains of the Hawaiian Islands

Lyon's conclusion (cf. 1919) with regard to the stand reduction dieback on the lower wet slope of Haleakala Mountain led to a forest management policy that saw importation of alien tree species as necessary for protecting Hawaiian watersheds. Unfortunately, this also led to a further demise of native vegetation.It is possible that the low-canopy species diversity in Hawaii may increase the rate of paludification and swamp (or bog) formation, under wet climatic conditions, over that in a similar environment with a broader spectrum of successional species. However, a boggy landscape can also be a good watershed. Historic evidence comes from the sugar-cane planters on Maui—the supposed beneficiaries of the exotic tree-planting programme—who had objected to it on empirical grounds (Holt, 1983). A more recent study of a canopy dieback in the Hilo watershed showed that extensive tree mortality had no effect on the rate and quality of the water yield from this area. The forest hydrologist, R.D. Doty (1983), who investigated this relationship, attribut ed the outcome to the vigorous undergrowth which remained intact during the dieback event.Further landscape ageing in Hawaii's shield-shaped mountains involves a process of fluviomorphic dissecting of the gentle slopes lying inland. This results first in amphitheatre-headed valleys and eventually, on the windward sides, in steep residual escarpments with numerous secondary footslopes. Wirthmann & Hüser (1987) derived details of this process from the side-by-side comparison of younger with older Hawaiian mountains. When the waterlogged surfaces of the plateau-like lower wet slopes of shield volcanoes become laterally drained, closedMetrosiderosforests can become re-established. That this natural re-establishment process works in the Hawaiian mountains can be seen by going from the shield-shaped slopes of the younger, to the more dissected slopes of the older, high islands. VigorousMetrosidewsforests occur also on the older islands, but they are typically of lower stature than those on the younger mountains.Unfortunately, most of the secondary slopes formed at the base of the steeply-cut windward mountain slopes of the older high islands have been deforested to make way for crop agriculture. Many of these lower slopes have subsequently been abandoned for economic reasons; but instead of reverting to forests, they have become invaded in many areas by alien grasses and shrubs. Particularly damaging has been the introduced pyrophytic Broomsedge (Andropogon virginicus). This grass has retained a temperate-zone phenology, whereby it undergoes partial dormancy during the winter season when rainfall increases (Sorenson, 1980). As such it has contributed to accelerated erosion in two ways: (1) by preventing recirculation of soil water in areas where excessive runoff is a problem, and (2) by attracting repeated fires and preventing succession to a forest, which would provide much-improved protection against the accelerated soil erosion (Mueller-Dombois, 1973) that is such a common feature on the lower windward slopes of the older islands.