Response of Orographic Precipitation to Subsaturated Low-Level Layers

2020 ◽  
Author(s):  
Shizuo Fu ◽  
Richard Rotunno ◽  
Huiwen Xue
Keyword(s):  
Critical Value ◽  
Vapor Transport ◽  
Orographic Precipitation ◽  
Surface Precipitation ◽  
Low Level ◽  
Nondimensional Parameter ◽  
Transport Effect ◽  
Width Effect ◽  
Upper Level ◽  
The Impact

<p>Orographic precipitation is, on the one hand, an important source of fresh water, and on the other hand, a potential cause of floods and other disasters. Previous studies have focused on the situation where the whole atmosphere is saturated and nearly moist-neutral. However, there are times when subsaturated low-level layers are observed to be below saturated, nearly moist-neutral, upper-level layers.</p><p>A series of idealized two-dimensional simulations are performed here to investigate the impact of this subsaturated low-level layer on orographic precipitation. It is found that the impact is mainly controlled by a nondimensional parameter and two competing effects. The nondimensional parameter is N<sub>2</sub>z<sub>t</sub>/U, where N<sub>2</sub> and z<sub>t</sub> are, respectively, the dry Brunt–Väisälä frequency and depth of the subsaturated low-level layer, and U the cross-mountain wind speed. When the nondimensional parameter exceeds a critical value, the decelerated region on the upwind side of the mountain moves upwind, resulting in weak surface precipitation near the mountain peak. When it is smaller than the critical value, surface precipitation occurs near the mountain peak.</p><p>The two competing effects are: 1) the vapor-transport effect, meaning that increasing z<sub>t</sub> decreases the amount of vapor transported to the mountain, and hence tends to decrease surface precipitation; and 2) the updraft width effect, meaning that increasing z<sub>t</sub> enhances flow blocking, producing a wider updraft over the upwind slope, and hence tends to increase surface precipitation. When the vapor-transport effect dominates, surface precipitation decreases with z<sub>t</sub>. When the updraft-width effect dominates, surface precipitation increases with z<sub>t</sub>.</p>

scholarly journals Response of Orographic Precipitation to Subsaturated Low-Level Layers

2019 ◽  
Vol 76 (12) ◽  
pp. 3753-3771
Author(s):  
Shizuo Fu ◽  
Richard Rotunno ◽  
Huiwen Xue
Keyword(s):  
Liquid Phase ◽  
Critical Value ◽  
Vapor Transport ◽  
Orographic Precipitation ◽  
Surface Precipitation ◽  
Low Level ◽  
Transport Effect ◽  
Width Effect ◽  
Upper Level ◽  
Weak Surface

Abstract In orographic precipitation events, there are times when subsaturated low-level layers are observed to be below saturated, nearly moist-neutral, upper-level layers. By performing a series of idealized two-dimensional simulations, this study investigates the response of orographic precipitation to subsaturated low-level layers. When the nondimensional parameter N2zt/U, where N2 and zt are, respectively, the dry Brunt–Väisälä frequency and depth of the subsaturated low-level layer, and U the cross-mountain wind speed, exceeds a critical value, the decelerated region on the upwind side of the mountain moves upwind, resulting in weak surface precipitation near the mountain peak. The critical value determined from the simulations is close to that derived from linear theory. When N2zt/U is less than the critical value, increasing zt has two competing effects: 1) the vapor-transport effect, meaning that increasing zt decreases the amount of vapor transported to the mountain, and hence tends to decrease surface precipitation; and 2) the updraft-width effect, meaning that increasing zt enhances flow blocking, producing a wider updraft over the upwind slope, and hence tends to increase surface precipitation. When the vapor-transport effect dominates, surface precipitation decreases with zt. When the updraft-width effect dominates, surface precipitation increases with zt. Increasing the maximum mountain height hm or U generally increases surface precipitation. However, for certain combinations of hm and U, the simulations produce lee waves, which substantially reduce surface precipitation. Finally, the response of orographic precipitation in the simulations with both liquid-phase and ice-phase microphysics is similar to that in the simulations with only liquid-phase microphysics.

scholarly journals The Impact of the Sierra Nevada on Low-Level Winds and Water Vapor Transport

2007 ◽  
Vol 8 (4) ◽  
pp. 790-804 ◽  
Author(s):  
Jinwon Kim ◽  
Hyun-Suk Kang
Keyword(s):  
Water Vapor ◽  
Sierra Nevada ◽  
Climate Model ◽  
Northern Region ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Western Slope ◽  
Mountain Range ◽  
Low Level ◽  
The Impact

Abstract To understand the influence of the Sierra Nevada on the water cycle in California the authors have analyzed low-level winds and water vapor fluxes upstream of the mountain range in regional climate model simulations. In a low Froude number (Fr) regime, the upstream low-level wind disturbances are characterized by the rapid weakening of the crosswinds and the appearance of a stagnation point over the southwestern foothills. The weakening of the low-level inflow is accompanied by the development of along-ridge winds that take the form of a barrier jet over the western slope of the mountain range. Such upstream wind disturbances are either weak or nonexistent in a high-Fr case. A critical Fr (Frc) of 0.35 inferred in this study is within the range of those suggested in previous observational and numerical studies. The depth of the blocked layer estimated from the along-ridge wind profile upstream of the northern Sierra Nevada corresponds to Frc between 0.3 and 0.45 as well. Associated with these low-level wind disturbances are significant low-level southerly moisture fluxes over the western slope and foothills of the Sierra Nevada in the low-Fr case, which result in significant exports of moisture from the southern Sierra Nevada to the northern region. This along-ridge low-level water vapor transport by blocking-induced barrier jets in a low-Fr condition may result in a strong north–south precipitation gradient over the Sierra Nevada.

scholarly journals The Impact of the Asian Summer Monsoon Circulation on the Tropopause

2016 ◽  
Vol 29 (24) ◽  
pp. 8689-8701 ◽  
Author(s):  
Yutian Wu ◽  
Tiffany A. Shaw
Keyword(s):  
Asian Summer Monsoon ◽  
Potential Temperature ◽  
Surface Heating ◽  
Monsoon Circulation ◽  
Low Level ◽  
Sverdrup Balance ◽  
Upper Level ◽  
Extratropical Tropopause ◽  
Asian Summer Monsoon Circulation ◽  
The Impact

Abstract Previous studies have identified two important features of summertime thermodynamics: 1) a significant correlation between the low-level distribution of equivalent potential temperature and the potential temperature θ of the extratropical tropopause and 2) a northwestward shift of the maximum tropopause θ relative to the maximum low-level . Here, the authors hypothesize these two features occur because of the Asian monsoon circulation. The hypothesis is examined using a set of idealized prescribed sea surface temperature (SST) aquaplanet simulations. Simulations with a zonally symmetric background climate exhibit a weak moisture–tropopause correlation. A significant correlation and northwestward shift occurs when a zonal wave-1 SST perturbation is introduced in the Northern Hemisphere subtropics. The equivalent zonal-mean subtropical warming does not produce a significant correlation. A mechanism is proposed to explain the moisture–tropopause connection that involves the circulation response to zonally asymmetric surface heating and its impact on the tropopause defined by the 2-potential-vorticity-unit (PVU; 1 PVU = 10−6 K kg−1 m2 s−1) surface. While the circulation response to diabatic heating is well known, here the focus is on the implications for the tropopause. Consistent with previous research, surface heating increases the low-level and produces low-level convergence and a cyclonic circulation. The low-level convergence is coupled with upper-level divergence via convection and produces an upper-level anticyclonic circulation consistent with Sverdrup balance. The anticyclonic vorticity lowers the PV northwest of the surface heating via Rossby wave dynamics. The decreased PV leads to a northwestward shift of the 2-PVU surface on fixed pressure levels. The θ value to the northwest of the surface heating is higher, and consequently the maximum tropopause θ increases.

scholarly journals Impacts of Ice Particle Shape and Density Evolution on the Distribution of Orographic Precipitation

2018 ◽  
Vol 75 (9) ◽  
pp. 3095-3114 ◽  
Author(s):  
Anders A. Jensen ◽  
Jerry Y. Harrington ◽  
Hugh Morrison
Keyword(s):  
Particle Shape ◽  
Traditional Approach ◽  
Orographic Precipitation ◽  
Coast Range ◽  
Surface Precipitation ◽  
Particle Properties ◽  
Cloud Properties ◽  
Orographic Cloud ◽  
Spatial Locations ◽  
The Impact

Abstract An IMPROVE-2 orographic precipitation case is simulated using the Ice-Spheroids Habit Model with Aspect-Ratio Evolution (ISHMAEL) microphysics. In ISHMAEL, the evolution of ice particle properties such as mass, shape, size, density, and fall speed are predicted. These ice particle properties along with the ice size distributions from ISHMAEL and model-derived spatial distribution of accumulated precipitation are compared to observations. ISHMAEL predicts planar and columnar particles at spatial locations that agree with observations. Sensitivity simulations are used to explore the impact of predicting ice particle shape evolution on orographic cloud properties and precipitation compared to the traditional approach of representing snow and graupel using separate categories with conversion from snow to graupel during riming. High biases in both IWCs aloft and surface precipitation accumulation occur in the Umpqua River valley using separate snow and graupel categories because snow that does not convert to graupel is advected over the Coast Range and precipitates out in the valley. Improvements in IWCs aloft and surface precipitation using ISHMAEL occur from both predicting various vapor-grown habits and predicting the impact of partial riming on ice particle properties. Compared to traditional microphysics schemes, ISHMAEL also produces less spatial variability in accumulated precipitation.

scholarly journals Organization of convective ascents in a warm conveyor belt

2020 ◽  
Author(s):  
Nicolas Blanchard ◽  
Florian Pantillon ◽  
Jean-Pierre Chaboureau ◽  
Julien Delanoë
Keyword(s):  
Large Scale ◽  
Doppler Radar ◽  
Heavy Precipitation ◽  
Conveyor Belt ◽  
Isolated Cells ◽  
Low Level ◽  
The North ◽  
Low Level Jet ◽  
Upper Level ◽  
The Impact

Abstract. Warm conveyor belts (WCBs) are warm, moist airstreams of extratropical cyclones leading to widespread clouds and heavy precipitation, where associated diabatic processes can influence midlatitude dynamics. Although WCBs are traditionally seen as continuous slantwise ascents, recent studies have emphasized the presence of embedded convection and the production of mesoscale bands of negative potential vorticity (PV), the impact of which on large-scale dynamics is still debated. Here, detailed cloud and wind measurements obtained with airborne Doppler radar provide unique information on the WCB of the Stalactite cyclone on 2 October 2016 during the North Atlantic Waveguide and Downstream Impact Experiment. The measurements are complemented by a convection-permitting simulation, enabling online Lagrangian trajectories and 3-D objects clustering. The simulation reproduces well the mesoscale structure of the cyclone shown by satellite infrared observations, while the location of trajectories rising by 150 hPa during a relatively short 12 h window matches the WCB region expected from high clouds. One third of those trajectories, categorized as fast ascents, further reach a 100 hPa (2h)−1 threshold during their ascent and follow the cyclonic flow mainly at lower levels. In agreement with radar observations, convective updrafts are found in the WCB and are characterized by moderate reflectivity values up to 20 dBz and vertical velocities above 0.3 m s−1. Updraft objects and fast ascents consistently show three main types of convection in the WCB: (i) frontal convection along the surface cold front and the western edge of the low-level jet; (ii) banded convection at about 2 km altitude along the eastern edge of the low-level jet; (iii) mid-level convection below the upper-level jet. Mesoscale PV dipoles with strong positive and negative values are located in the vicinity of convective ascents and appear to accelerate both low-level and upper-level jets. Both convective ascents and negative PV organize into structures with coherent shape, location and evolution, thus suggesting a dynamical linkage. The results show that convection embedded in WCBs occurs in a coherent and organized manner rather than as isolated cells.

scholarly journals Surface processes in the 7 November 2014 medicane from air–sea coupled high-resolution numerical modelling

2019 ◽  
Author(s):  
Marie-Noëlle Bouin ◽  
Cindy Lebeaupin Brossier
Keyword(s):  
Tropical Cyclones ◽  
Surface Processes ◽  
Convective Precipitation ◽  
Surface Cooling ◽  
Air Masses ◽  
Low Level ◽  
Cold Pools ◽  
Ocean Atmosphere ◽  
Upper Level ◽  
The Impact

Abstract. A medicane, or Mediterranean cyclone with characteristics similar to tropical cyclones, is simulated using a kilometre-scale ocean–atmosphere coupled modelling platform. A first baroclinic phase of the cyclone leads to strong convective precipitation, with high potential vorticity anomalies aloft due to an upper-level trough. The deepening and tropicalization of the cyclone is due first to the crossing of the upper-level jet, then to low-level convergence and uplift of conditionally unstable air masses by cold pools, resulting either from rain evaporation or from advection of continental air masses from North Africa. Backtrajectories show that air–sea heat exchanges warm and moisten the low-level inflow feeding the latent heat release during the mature phase of the medicane. However, the impact of ocean–atmosphere coupling on the cyclone track, intensity and lifecycle is very weak, due to a surface cooling one order of magnitude weaker than for tropical cyclones, even on the area of strong enthalpy fluxes. Isolating the influence of the surface parameters on the surface fluxes at sea during the different phases of the cyclone confirms the impact of the cold pools on the surface processes. The evaporation is controlled mainly by the sea surface temperature and wind, with a significant additional impact of the humidity and temperature at first level during the development phase. The sensible heat flux is influenced mainly by the temperature at first level throughout the whole medicane lifetime. This study shows that the tropical transition, in this case, is dependent on processes widespread in the Mediterranean Basin, like advection of continental air, rain evaporation, and dry air intrusion.

Sensitivity of a Simulated Midlatitude Squall Line to Parameterization of Raindrop Breakup

2012 ◽  
Vol 140 (8) ◽  
pp. 2437-2460 ◽  
Author(s):  
Hugh Morrison ◽  
Sarah A. Tessendorf ◽  
Kyoko Ikeda ◽  
Gregory Thompson
Keyword(s):  
Wind Shear ◽  
Squall Line ◽  
Cold Pool ◽  
Density Currents ◽  
Drop Breakup ◽  
Surface Precipitation ◽  
Low Level ◽  
Fall Speed ◽  
Storm Characteristics ◽  
The Impact

Abstract This paper describes idealized simulations of a squall line observed on 20 June 2007, in central Oklahoma. Results are compared with measurements from dual-polarization radar and surface disdrometer. The baseline model configuration qualitatively reproduces key storm features, but underpredicts precipitation rates and generally overpredicts median volume raindrop diameter. The sensitivity of model simulations to parameterization of raindrop breakup is tested under different low-level (0–2.5 km) environmental vertical wind shears. Storm characteristics exhibit considerable sensitivity to the parameterization of breakup, especially for moderate (0.0048 s−1) shear. Simulations with more efficient breakup tend to have higher domain-mean precipitation rates under both moderate and higher (0.0064 s−1) shear, despite the smaller mean drop size and hence lower mass-weighted fall speed and higher evaporation rate for a given rainwater content. In these runs, higher evaporation leads to stronger cold pools, faster propagation, larger storm size, greater updraft mass flux (but weaker convective updrafts at mid- and upper levels), and greater total condensation that compensates for the increased evaporation to give more surface precipitation. The impact of drop breakup on mass-weighted fall speed is also important and leads to a nonmonotonic response of storm characteristics (surface precipitation, cold pool strength, etc.) to changes in breakup efficiency under moderate wind shear. In contrast, the response is generally monotonic at higher wind shear. Interactions between drop breakup, convective dynamics, cold pool intensity, and low-level environmental wind shear are also described in the context of “Rotunno–Klemp–Weisman (RKW) theory,” which addresses how density currents evolve in sheared environments.

scholarly journals Life Cycle Study of a Diabatic Rossby Wave as a Precursor to Rapid Cyclogenesis in the North Atlantic—Dynamics and Forecast Performance

2011 ◽  
Vol 139 (6) ◽  
pp. 1861-1878 ◽  
Author(s):  
Maxi Boettcher ◽  
Heini Wernli
Keyword(s):  
Life Cycle ◽  
Rossby Wave ◽  
North Atlantic ◽  
Vertical Motion ◽  
Environmental Parameters ◽  
Forecast Performance ◽  
Low Level ◽  
The North ◽  
Upper Level ◽  
The Impact

Abstract The life cycle of a North Atlantic cyclone in December 2005 that included a rapid propagation phase as a diabatic Rossby wave (DRW) is investigated by means of operational analyses and deterministic forecasts from the ECMWF. A quasigeostrophic omega diagnostic has been applied to assess the impact of upper-level forcing during the genesis, propagation, and intensification phase, respectively. The system was generated in the Gulf of Mexico as a mesoscale convective vortex (MCV) influenced by vertical motion forcing from a nearby upper-level trough. The DRW propagation phase was characterized by a shallow, low-level, diabatically produced potential vorticity (PV) anomaly that rapidly propagated along the southern border of an intense baroclinic zone. No significant upper-level forcing could be identified during this phase of the development. Eventually, explosive intensification occurred as the region of vertical motion forced by an approaching upper-level trough reached the position of the DRW. The rapid intensification of 34 hPa in 24 h led to a mature extratropical cyclone in the central North Atlantic with marked frontal structures associated with a pronounced PV tower. The performance of four operational deterministic ECMWF forecasts has been investigated for the DRW propagation and cyclone intensification. The forecasts showed a highly variable skill. Despite the fact that the DRW was initially well represented in all forecasts, two of them failed to capture the explosive intensification. By applying a DRW tracking tool, the low-level baroclinicity downstream of the DRW and the moisture supply to the south of the DRW could be identified as the key environmental parameters during DRW propagation. The subsequent cyclone intensification went wrong in two of the forecasts because of the missing interaction of the DRW and the upper-level trough. It is shown that this interaction can fail if the intensity of the DRW and/or the approaching upper-level wave are too weak, or in case of an erroneous structure of the upper-level trough leading to a phasing problem of the vertical interaction with the DRW. Therefore, the DRW intensification bears similar characteristics and forecast challenges as the extratropical reintensification of tropical cyclones.

scholarly journals A Numerical Study of the Interaction between the Large-Scale Monsoon Circulation and Orographic Precipitation over South and Southeast Asia

2012 ◽  
Vol 25 (7) ◽  
pp. 2440-2455 ◽  
Author(s):  
Zhuo Wang ◽  
Chih-Pei Chang
Keyword(s):  
Southeast Asia ◽  
Large Scale ◽  
Monsoon Onset ◽  
Orographic Precipitation ◽  
Monsoon Circulation ◽  
Indochina Peninsula ◽  
Low Level ◽  
South And Southeast Asia ◽  
Sensitivity Tests ◽  
The Impact

Abstract A regional climate model is used to simulate the summer monsoon onset in South and Southeast Asia during the year 2000 to explore the interaction between orographic precipitation and the large-scale monsoon circulation. In the control run, the model uses the U. S. Geological Survey topography data and simulates the observed monsoon onset reasonably well. In the sensitivity tests, mountains are removed within different regions south of the Tibetan Plateau. It is found that the Indochina Peninsula monsoon onset is closely related to the local wind–terrain–precipitation interaction, while the Indian monsoon onset is more controlled by the large-scale land–sea thermal contrast. The sensitivity tests suggest two opposite effects of high terrain on the monsoon circulation and precipitation. When the terrain height is below the lifted condensation level (LCL), the low-level westerlies and the orographic precipitation weaken with increasing terrain height due to the surface drag effect. When the terrain height is above the LCL, the positive feedback associated with the diabatic forcing of orographic precipitation is dominant, and a large mountain height leads to heavier orographic precipitation and stronger low-level westerlies. The sensitivity tests also show that the impact of orographic precipitation in the Indochina Peninsula extends up to 30° longitude upstream and affects monsoon precipitation along the western coast of India.

scholarly journals Observed and WRF-Simulated Low-Level Winds in a High-Ozone Episode during the Central California Ozone Study

2008 ◽  
Vol 47 (9) ◽  
pp. 2372-2394 ◽  
Author(s):  
J-W. Bao ◽  
S. A. Michelson ◽  
P. O. G. Persson ◽  
I. V. Djalalova ◽  
J. M. Wilczak
Keyword(s):  
Large Scale ◽  
Central Valley ◽  
Transport Processes ◽  
Central California ◽  
Low Level ◽  
Lateral Boundary Conditions ◽  
Downslope Flows ◽  
Upper Level ◽  
Physical Reasons ◽  
The Impact

Abstract A case study is carried out for the 29 July–3 August 2000 episode of the Central California Ozone Study (CCOS), a typical summertime high-ozone event in the Central Valley of California. The focus of the study is on the low-level winds that control the transport and dispersion of pollutants in the Central Valley. An analysis of surface and wind profiler observations from the CCOS field experiment indicates a number of important low-level flows in the Central Valley: 1) the incoming low-level marine airflow through the Carquinez Strait into the Sacramento River delta, 2) the diurnal cycle of upslope–downslope flows, 3) the up- and down-valley flow in the Sacramento Valley, 4) the nocturnal low-level jet in the San Joaquin Valley, and 5) the orographically induced mesoscale eddies (the Fresno and Schultz eddies). A numerical simulation using the advanced research version of the Weather Research and Forecasting Model (WRF) reproduces the overall pattern of the observed low-level flows. The physical reasons behind the quantitative differences between the observed and simulated low-level winds are also analyzed and discussed, although not enough observations are available to diagnose thoroughly the model-error sources. In particular, hodograph analysis is applied to provide physical insight into the impact of the large-scale, upper-level winds on the locally forced low-level winds. It is found that the diurnal rotation of the observed and simulated hodographs of the local winds varies spatially in the Central Valley, resulting from the combining effect of topographically induced local forcing and the interaction between the upper-level winds and the aforementioned low-level flows. The trajectory analysis not only further confirms that WRF reproduces the observed low-level transport processes reasonably well but also shows that the simulated upper-level winds have noticeable errors. The results from this study strongly suggest that the errors in the WRF-simulated low-level winds are related not only to the errors in the model’s surface conditions and atmospheric boundary layer physics but also to the errors in the upper-level forcing mostly prescribed in the model’s lateral boundary conditions.

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