Cloud-resolving-model simulations of nocturnal precipitation over the Himalayan slopes and foothills

A numerical experiment with a 2-km resolution was conducted using the Weather Research and Forecasting (WRF) model to investigate physical processes driving nocturnal precipitation over the Himalayas during the mature monsoon seasons between 2003 and 2010. The WRF model simulations of increases in precipitation twice a day, one in the afternoon and another around midnight, over the Himalayan slopes, and of the single nocturnal peak over the Himalayan foothills were reasonably accurate. To understand the synoptic-scale moisture transport and its local-scale convergence generating the nocturnal precipitation, composite analyses were conducted using the reanalysis dataset and model outputs. In the synoptic scale, moisture transport associated with the westward propagation of low pressure systems was found when nocturnal precipitation dominated over the Himalayan slopes. In contrast, moisture was directly provided from the synoptic-scale monsoon westerlies for nocturnal precipitation over the foothills. The model outputs suggested that precipitation occurred on the mountain ridges in the Himalayas during the afternoon, and expanded horizontally towards lower-elevation areas through the night. During the nighttime, the downslope wind was caused by radiative cooling at the surface and was intensified by evaporative cooling by hydrometeors in the near-surface layer. As a result, convergence between the downslope wind and the synoptic-scale flow promoted nocturnal precipitation over the Himalayas and to the south, as well as the moisture convergence by orography and/or synoptic-scale circulation patterns. The nocturnal precipitation over the Himalayas was not simulated well when we used the coarse topographic resolution and the smaller number of vertical layers.

A Modeling Study on the Downslope Wind of “Katevatos” in Greece and Implications for the Battle of Arachova in 1826

Downslope winds and lee gravity waves are common features of mountainous environments. A similar weather type at Mt. Parnassos in Arachova, Greece is known as “Katevatos” and has devastating results for the population and visitors at the local touristic resorts. In this study, we analyze three incidents of this atmospheric pattern at local scale resolution (1 × 1 km) with WRF model. This is the first study of this local weather hazard, and the following key factors are identified. (I) The main synoptic forcing is the propagation of an upper-level trough from central Europe towards the Balkans. (II) The associated generation of a surface low-pressure system over the Aegean Sea results in a northeast flow in the lower troposphere that is perpendicular to the main topographic ridge of Mt. Parnassos. (III) Generation of gravity waves and downward reflection of wave energy at the critical level between the upper level flow and the undercutting northeast current result in the formation of “Katevatos” downslope wind at the lee side of the mountain. This hurricane-scale wind is accompanied with horizontal transport of frozen rain and snow from the mountain tops towards the village of Arachova. This wind pattern appeared also during the battle of Arachova in November 1826 between the Greek and Ottoman forces resulting in enormous casualties due to the adverse weather conditions.

Urban-Induced Changes on Local Circulation in Complex Terrain: Central Mexico Basin

Land use land cover (LULC) significantly impacts local circulation in the Mexico Basin, particularly wind field circulations such as gap winds, convergence lines, and thermally induced upslope/downslope wind. A case study with a high-pressure system over the Mexico Basin isolates the influence of large-scale synoptic forcing. Numerical simulations reveal a wind system composed of meridional circulation and a zonal component. Thermal pressure gradients between the Mexico basin and its colder surroundings create near-surface convergence lines as part of the meridional circulation. Experiments show that the intensity and organization of meridional circulations and downslope winds increase when LULC changes from natural and cultivated land to urban. Zonal circulation exhibits a typical circulation pattern with the upslope flow and descending motion in the middle of the basin. Large values of moist static energy are near the surface where air parcels pick up energy from the surface either as fluxes of enthalpy or latent heat. Surface heat fluxes and stored energy in the ground are drivers of local circulation, which is more evident in zonal circulation patterns.

Observational case study of a persistent cold pool and gap flow in the Columbia River Basin

Persistent cold pools form as layers of cold stagnant air within topographical depressions mainly during wintertime when the near-surface air cools and/or the air aloft warms and daytime surface heating is insufficient to mix out the stable layer. An area often affected by persistent cold pools is the Columbia River Basin in the Pacific Northwest, when a high-pressure system east of the Cascade Range promotes radiative cooling and easterly flow. The only major outflow for the easterly flow is through the narrow Columbia River Gorge which cuts through the north-south oriented Cascade Range and often experiences very strong gap flows. Observations collected during the Second Wind Forecast Improvement Project (WFIP2) are used to study a persistent cold pool in the Columbia River Basin between 10-19 Jan 2017 which was associated with a strong gap flow. We used data from various remote sensing and in situ instruments and a optimal estimation physical retrieval to obtain thermodynamic profiles to address the temporal and spatial characteristics of the cold pool and gap flow and to investigate the physical processes involved during formation, maintenance and decay. While large-scale temperature advection occurred during all phases, we found that the cold pool vertical structure was modulated by the existence of low-level clouds and that turbulent shear-induced mixing and downslope wind storms likely played a role during its decay.

The spatiotemporal characteristics of near-surface water vapor in a coastal region revealed from radar-derived refractivity

The radar-retrieved refractivity fields provide detailed depictions of the near-surface moisture distribution at the meso-gamma scale. This study represents a novel application of the refractivity fields by examining the spatiotemporal characteristics of moisture variability in a summertime coastal region in Taiwan over four weeks. The physiography in Taiwan lends itself to a variety of flow features and corresponding moisture behavior, which has not been well-studied. High-resolution of refractivity analyses demonstrate how a highly variable moisture field is related to the complex interaction between the synoptic-scale winds, diurnal local circulations, terrain, storms, and heterogeneous land-use. On average, higher refractivity (water vapor) is observed along the coastline and refractivity decreases inland toward the foothills. Under weak synoptic forcing conditions, the daytime refractivity field develops differently under local surface wind directions determined by the synoptic-scale prevailing wind and the sea breeze fronts. High moisture penetrates inland toward the foothills with southwesterly winds, but it stalls along the coastline with southerly and the northwesterly winds. The moisture distribution may further affect the occurrence of the inland afternoon storms. During the nighttime, the dry downslope wind decreases the moisture from the foothills toward the coast and forms a refractivity gradient perpendicular to the meridionally-oriented mountains. Furthermore, the refractivity fields illustrate higher resolution moisture distribution over surface station point measurements by showing the lagged daytime sea-breeze front between the urban and rural areas and the detailed nighttime heterogeneous moisture distribution related to land-use and rivers.

Evaluation of the near-surface evolution of Foehn events in COSMO-1

<p>Foehn is a downslope wind with a large impact on society due to its gusty nature and the associated high-temperature extremes. It is influenced by and interacts with near-surface processes, such as the cold air pool (CAP), an important phenomenon that is often present in the foehn valleys during the cold season. Therefore, it is challenging to accurately forecast the foehn characteristics, in terms of the onset, strength, and decay as the near-surface evolution is the result of multi-scale interactions between the larger atmosphere and the mountain/local valley topography. From a meso-/micro-scale perspective, this study investigates the skill of the COSMO model (v5.7) at 1.1 km grid spacing in simulating the near-surface foehn evolution for a set of south foehn events, representative of different foehn types around the Alps. The evaluation is based on a comparison to station data from the automatic monitoring network of MeteoSwiss, with a focus on the Rhine Valley. Significant cold and moist biases are found in the model during foehn hours in all the chosen cases. Biases in Foehn duration and spatial extent are also studied. The sensitivity of these biases to several land-surface parameterization choices, e.g., skin layer, bare soil evaporation, and resistance for heat fluxes, are investigated and presented. Simulations for the same foehn events with COSMO at 500 m grid spacing are also evaluated for a better understanding of the model biases. Further studies from the perspective of climatological statistics are needed to establish the relationships between model biases and foehn types.</p>

Episode with strong downslope wind in the Southern and Curvature Carpathians in the period 5-6 february 2020

The phenomenon that occurred during the blizzard from February 5-6 in the mountains and especially on the southern slopes of the Southern Carpathians, is known in the literature as "strong downslope winds". This phenomenon occurred in a typical blizzard configuration, in which the differentiated advection of temperature led to the formation of a very stable air layer, with thermal inversion approximately between the levels of 850 and 700 hPa; and it also contributed in this layer to the change of wind direction to vertical. Thus, the existence in the same air layer of two factors favorable to the formation of a critical level, created the ideal conditions for generating strong downslope winds.

Episode with strong downslope wind in the Southern and Curvature Carpathians in the period 5-6 February 2020

The phenomenon that occurred during the blizzard from February 5-6 in the mountains and especially on the southern slopes of the Southern Carpathians, is known in the literature as "strong downslope winds". This phenomenon occurred in a typical blizzard configuration, in which the differentiated advection of temperature led to the formation of a very stable air layer, with thermal inversion approximately between the levels of 850 and 700 hPa; and it also contributed in this layer to the change of wind direction to vertical. Thus, the existence in the same air layer of two factors favorable to the formation of a critical level, created the ideal conditions for generating strong downslope winds.

Observational study for strong downslope wind event under fine weather condition during ICE-POP 2018

A strong downslope wind event under fine weather condition on 13–15 February 2018 was examined by various observational and high resolution reanalysis datasets during the 2018 Winter Olympic and Paralympic games in Pyeongchang, Korea. High spatio-temporal resolution of wind information was obtained by Doppler lidars, automatic weather stations (AWS), wind profiler, and sounding observations under the International Collaborative Experiments for Pyeongchang 2018 Olympic and Paralympic winter games (ICE-POP 2018). This study aimed to understand the possible generation mechanisms of localized strong wind event across high mountainous areas and in the lee side of mountains associated with the underlying large-scale pattern of a low-pressure system (LPS). The spatial distribution of linear trends for surface wind shows different patterns, exhibiting increased trend in the lee side and a persistent one in mountainous areas with the approaching LPS. Surface wind speed was intensified dramatically from ~3 to ~12 m s−1 (gust was stronger than 20 m s−1 above ground) at a surface station in the lee side (named as GWW). However, the mountainous station at DGW site appeared to have a persistently strong wind (~10 m s−1) during the research period. Budget analysis of horizontal momentum equation and local reanalysis data suggests that the pressure gradient force (PGF) derived by adiabatic warming along the downslope and subsequent hydraulic jump in the lee side of mountains was a main factor in the acceleration of the surface wind at the GWW site. Detailed analysis of the retrieved 3D winds reveals that the PGF also dominate at the DGW site, which causes the persistent strong wind that is related to the channeling effect across the valley areas in the mountain range. The observational evidence presented here shows that the different mechanisms in local areas under the same synoptic condition with LPS are important references in determining the strength and persistence of the orographic-induced strong winds under fine weather condition.

The Strong, Dry Winds of Central and Northern California: Climatology and Synoptic Evolution

Strong, dry downslope winds over Northern and central California have played a critical role in regional wildfires. These events, sometimes called Diablo or North winds, are more frequent over the Bay Area and nearby coastal terrain than along the western slopes of the Sierra Nevada, where the highest frequency occurs over the midslopes of the barrier. For the Bay Area, there is a frequency minimum during midsummer, a maximum in October, and a declining trend from November to June. The Sierra Nevada locations have their minimum frequency from February to August, and a maximum from October to January. There is little trend in event frequency during the past two decades over either region. For the Bay Area sites, there is a maximum frequency during the early morning hours and a large decline midday, while the Sierra Nevada locations have a maximum frequency approximately three hours earlier. Before the onset of these downslope wind events, there is substantial amplification of upper-level ridging over the eastern Pacific, with sea level pressure increasing first over the Pacific Northwest and then over the Intermountain West. The coincident development of a coastal sea level pressure trough leads to a large pressure gradient over the Sierra Nevada and Northern California. Diablo–North wind events are associated with below-normal temperatures east of the Sierra Nevada, with rapid warming of the air as it subsides into coastal California. The large horizontal variability in the frequency and magnitude of these events suggests the importance of exposure, elevation, and mountain-wave-related downslope acceleration.