scholarly journals Dropsonde Observations in Low-Level Jets over the Northeastern Pacific Ocean from CALJET-1998 and PACJET-2001: Mean Vertical-Profile and Atmospheric-River Characteristics

2005 ◽  
Vol 133 (4) ◽  
pp. 889-910 ◽  
Author(s):  
F. Martin Ralph ◽  
Paul J. Neiman ◽  
Richard Rotunno
Keyword(s):  
Water Vapor ◽  
Pacific Ocean ◽  
Static Stability ◽  
Vapor Transport ◽  
Vertical Shear ◽  
Water Vapor Transport ◽  
Low Level ◽  
Stable Conditions ◽  
Thermodynamic Conditions ◽  
The Mean

Dropsonde observations are used to document the mean vertical profiles of kinematic and thermodynamic conditions in the pre-cold-frontal low-level-jet (LLJ) region of extratropical cyclones over the eastern Pacific Ocean. This is the region within storms that is responsible not only for the majority of heavy rainfall induced by orography when such storms strike the coast, but also for almost all meridional water vapor transport at midlatitudes. The data were collected from NOAA’s P-3 aircraft in 10 storms during the California Land-falling Jets Experiment (CALJET) of 1998 and in 7 storms during the Pacific Land-falling Jets Experiment (PACJET) of 2001. The mean position of the dropsondes was 500 km offshore, well upstream of orographic influences. The availability of data from two winters that were characterized by very different synoptic regimes and by differing phases of ENSO—that is, El Niño in 1998 and La Niña in 2001—allowed examination of interannual variability. The composite pre-cold-frontal profiles reveal a well-defined LLJ at 1.0-km altitude with a wind speed of 23.4 m s−1 and a wind direction of 216.7°, as well as vertical shear characteristic of warm advection. The composite thermodynamic conditions were also documented, with special attention given to moist static stability due to the nearly saturated conditions that were prevalent. Although the dry static stability indicated very stable conditions (4.5 K km−1), the moist static stability was approximately zero up to 2.8-km altitude. Although the composite winds, temperatures, and water vapor mixing ratios in 2001 differed markedly from 1998, the moist static stability remained near zero from the surface up to 2.8–3.0-km altitude for both seasons. Hence, orographic precipitation enhancement is favored in this sector of the storm, regardless of the phase of ENSO. The dropsonde data were also used to characterize the depth and strength of atmospheric rivers, which are responsible for most of the meridional water vapor transport at midlatitudes. The vertically integrated along-river water vapor fluxes averaged 525 × 105 kg s−1 (assuming a 100-km-wide swath), while the meridional and zonal components were 387 × 105 kg s−1 and 302 × 105 kg s−1, respectively. Although the composite meridional transport in 2001 was less than half that in 1998 (230 × 105 kg s−1 versus 497 × 105 kg s−1), the characteristic scale height of the meridional water vapor transport remained constant; that is, 75% of the transport occurred below 2.25-km altitude.

Extreme Surface Winds During Landfalling Atmospheric Rivers: The Modulating Role of Near-Surface Stability

2021 ◽  
Author(s):  
Terence J. Pagano ◽  
Duane E. Waliser ◽  
Bin Guan ◽  
Hengchun Ye ◽  
F. Martin Ralph ◽  
...  
Keyword(s):  
Water Vapor ◽  
Static Stability ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Surface Winds ◽  
Near Surface ◽  
Extreme Surface ◽  
Atmospheric Rivers ◽  
Explained Variance

AbstractAtmospheric rivers (ARs) are long and narrow regions of strong horizontal water vapor transport. Upon landfall, ARs are typically associated with heavy precipitation and strong surface winds. A quantitative understanding of the atmospheric conditions that favor extreme surface winds during ARs has implications for anticipating and managing various impacts associated with these potentially hazardous events. Here, a global AR database (1999–2014) with relevant information from MERRA-2 reanalysis, QuikSCAT and AIRS satellite observations are used to better understand and quantify the role of near-surface static stability in modulating surface winds during landfalling ARs. The temperature difference between the surface and 1 km MSL (ΔT; used here as a proxy for near-surface static stability), and integrated water vapor transport (IVT) are analyzed to quantify their relationships to surface winds using bivariate linear regression. In four regions where AR landfalls are common, the MERRA-2-based results indicate that IVT accounts for 22-38% of the variance in surface wind speed. Combining ΔT with IVT increases the explained variance to 36-52%. Substitution of QuikSCAT surface winds and AIRS ΔT in place of the MERRA-2 data largely preserves this relationship (e.g., 44% compared to 52% explained variance for USA West Coast). Use of an alternate static stability measure–the bulk Richardson number–yields a similar explained variance (47%). Lastly, AR cases within the top and bottom 25% of near-surface static stability indicate that extreme surface winds (gale or higher) are more likely to occur in unstable conditions (5.3%/14.7% during weak/strong IVT) than in stable conditions (0.58%/6.15%).

scholarly journals Momentum and scalar transport within a vegetation canopy following atmospheric stability and seasonal canopy changes: the CHATS experiment

2012 ◽  
Vol 12 (13) ◽  
pp. 5913-5935 ◽  
Author(s):  
S. Dupont ◽  
E. G. Patton
Keyword(s):  
Water Vapor ◽  
Forced Convection ◽  
Atmospheric Stability ◽  
Vapor Transport ◽  
Scalar Transport ◽  
Water Vapor Transport ◽  
Heat Sources ◽  
Thermal Plumes ◽  
Vapor Transfer ◽  
Stable Conditions

Abstract. Momentum and scalar (heat and water vapor) transfer between a walnut canopy and the overlying atmosphere are investigated for two seasonal periods (before and after leaf-out), and for five thermal stability regimes (free and forced convection, near-neutral condition, transition to stable, and stable). Quadrant and octant analyses of momentum and scalar fluxes followed by space-time autocorrelations of observations from the Canopy Horizontal Array Turbulence Study's (CHATS) thirty meter tower help characterize the motions exchanging momentum, heat, and moisture between the canopy layers and aloft. During sufficiently windy conditions, i.e. in forced convection, near-neutral and transition to stable regimes, momentum and scalars are generally transported by sweep and ejection motions associated with the well-known canopy-top "shear-driven" coherent eddy structures. During extreme stability conditions (both unstable and stable), the role of these "shear-driven" structures in transporting scalars decreases, inducing notable dissimilarity between momentum and scalar transport. In unstable conditions, "shear-driven" coherent structures are progressively replaced by "buo-yantly-driven" structures, known as thermal plumes; which appear very efficient at transporting scalars, especially upward thermal plumes above the canopy. Within the canopy, downward thermal plumes become more efficient at transporting scalars than upward thermal plumes if scalar sources are located in the upper canopy. We explain these features by suggesting that: (i) downward plumes within the canopy correspond to large downward plumes coming from above, and (ii) upward plumes within the canopy are local small plumes induced by canopy heat sources where passive scalars are first injected if there sources are at the same location as heat sources. Above the canopy, these small upward thermal plumes aggregate to form larger scale upward thermal plumes. Furthermore, scalar quantities carried by downward plumes are not modified when penetrating the canopy and crossing upper scalar sources. Consequently, scalars appear to be preferentially injected into upward thermal plumes as opposed to in downward thermal plumes. In stable conditions, intermittent downward and upward motions probably related to elevated shear layers are responsible for canopy-top heat and water vapor transport through the initiation of turbulent instabilities, but this transport remains small. During the foliated period, lower-canopy heat and water vapor transport occurs through thermal plumes associated with a subcanopy unstable layer.

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 Momentum and scalar transport within a vegetation canopy following atmospheric stability and seasonal canopy changes: the CHATS experiment

2012 ◽  
Vol 12 (2) ◽  
pp. 6363-6418 ◽  
Author(s):  
S. Dupont ◽  
E. G. Patton
Keyword(s):  
Water Vapor ◽  
Forced Convection ◽  
Atmospheric Stability ◽  
Vapor Transport ◽  
Scalar Transport ◽  
Water Vapor Transport ◽  
Heat Sources ◽  
Thermal Plumes ◽  
Vapor Transfer ◽  
Stable Conditions

Abstract. Momentum and scalar (heat and water vapor) transfer between a walnut canopy and the overlying atmosphere are investigated for two seasonal periods (before and after leaf-out), and for five thermal stability regimes (free and forced convection, near-neutral condition, transition to stable, and stable). Quadrant and octant analyses of momentum and scalar fluxes followed by space-time autocorrelations of observations from the Canopy Horizontal Array Turbulence Study's (CHATS) thirty meter tower help characterize the motions exchanging momentum, heat, and moisture between the canopy layers and aloft. During sufficiently windy conditions, i.e. in forced convection, near-neutral and transition to stable regimes, momentum and scalars are generally transported by sweep and ejection motions associated with the well-known canopy-top "shear-driven" coherent eddy structures. During extreme stability conditions (both unstable and stable), the role of these "shear-driven" structures in transporting scalars decreases, inducing notable dissimilarity between momentum and scalar transport. In unstable conditions, "shear-driven" coherent structures are progressively replaced by "buoyantly-driven" structures, known as thermal plumes; which appear very efficient at transporting scalars, especially upward thermal plumes above the canopy. Within the canopy, downward thermal plumes become more efficient at transporting scalars than upward thermal plumes if scalar sources are located in the upper canopy as the heat source. We explain these features by suggesting that: (i) downward plumes within the canopy correspond to large downward plumes coming from above, and (ii) upward plumes within the canopy are local small plumes induced by canopy heat sources where passive scalars are first injected if there sources are at the same location than heat sources. Above the canopy, these small upward thermal plumes aggregate to form larger scale upward thermal plumes. Furthermore, scalar quantities carried by downward plumes are not modified when penetrating the canopy and crossing upper scalar sources. Consequently, scalars appear to be preferentially injected into upward thermal plumes as opposed to in downward thermal plumes. In stable conditions, intermittent downward and upward motions probably related to elevated shear layers are responsible for canopy-top heat and water vapor transport through the initiation of turbulent instabilities, but this transport remains small. During the foliated period, lower-canopy heat and water vapor transport occurs through thermal plumes associated with a subcanopy unstable layer.

scholarly journals Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers

2017 ◽  
Vol 18 (9) ◽  
pp. 2577-2596 ◽  
Author(s):  
F. M. Ralph ◽  
S. F. Iacobellis ◽  
P. J. Neiman ◽  
J. M. Cordeira ◽  
J. R. Spackman ◽  
...  
Keyword(s):  
Water Vapor ◽  
Liquid Water ◽  
North Pacific ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Amazon River ◽  
Vertical Profiles ◽  
Total Water ◽  
Atmospheric Rivers ◽  
The Mean

Abstract Aircraft dropsonde observations provide the most comprehensive measurements to date of horizontal water vapor transport in atmospheric rivers (ARs). The CalWater experiment recently more than tripled the number of ARs probed with the required measurements. This study uses vertical profiles of water vapor, wind, and pressure obtained from 304 dropsondes across 21 ARs. On average, total water vapor transport (TIVT) in an AR was 4.7 × 108 ± 2 × 108 kg s−1. This magnitude is 2.6 times larger than the average discharge of liquid water from the Amazon River. The mean AR width was 890 ± 270 km. Subtropical ARs contained larger integrated water vapor (IWV) but weaker winds than midlatitude ARs, although average TIVTs were nearly the same. Mean TIVTs calculated by defining the lateral “edges” of ARs using an IVT threshold versus an IWV threshold produced results that differed by less than 10% across all cases, but did vary between the midlatitudes and subtropical regions.

scholarly journals Evaluation of Atmospheric River Predictions by the WRF Model Using Aircraft and Regional Mesonet Observations of Orographic Precipitation and Its Forcing

2018 ◽  
Vol 19 (7) ◽  
pp. 1097-1113 ◽  
Author(s):  
Andrew Martin ◽  
F. Martin Ralph ◽  
Reuben Demirdjian ◽  
Laurel DeHaan ◽  
Rachel Weihs ◽  
...  
Keyword(s):  
Water Vapor ◽  
Forecast Error ◽  
Wrf Model ◽  
Static Stability ◽  
Regional Model ◽  
Global Model ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Atmospheric Rivers ◽  
Vapor Flux

Abstract Accurate forecasts of precipitation during landfalling atmospheric rivers (ARs) are critical because ARs play a large role in water supply and flooding for many regions. In this study, we have used hundreds of observations to verify global and regional model forecasts of atmospheric rivers making landfall in Northern California and offshore in the midlatitude northeast Pacific Ocean. We have characterized forecast error and the predictability limit in AR water vapor transport, static stability, onshore precipitation, and standard atmospheric fields. Analysis is also presented that apportions the role of orographic forcing and precipitation response in driving errors in forecast precipitation after AR landfall. It is found that the global model and the higher-resolution regional model reach their predictability limit in forecasting the atmospheric state during ARs at similar lead times, and both present similar and important errors in low-level water vapor flux, moist-static stability, and precipitation. However, the relative contribution of forcing and response to the incurred precipitation error is very different in the two models. It can be demonstrated using the analysis presented herein that improving water vapor transport accuracy can significantly reduce regional model precipitation errors during ARs, while the same cannot be demonstrated for the global model.

scholarly journals Atmospheric Rivers over the Northwestern Pacific: Climatology and Interannual Variability

2017 ◽  
Vol 30 (15) ◽  
pp. 5605-5619 ◽  
Author(s):  
Youichi Kamae ◽  
Wei Mei ◽  
Shang-Ping Xie ◽  
Moeka Naoi ◽  
Hiroaki Ueda
Keyword(s):  
Water Vapor ◽  
Interannual Variability ◽  
El Niño ◽  
El Nino ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Boreal Summer ◽  
Northwestern Pacific ◽  
Atmospheric Rivers ◽  
Low Level

Atmospheric rivers (ARs), conduits of intense water vapor transport in the midlatitudes, are critically important for water resources and heavy rainfall events over the west coast of North America, Europe, and Africa. ARs are also frequently observed over the northwestern Pacific (NWP) during boreal summer but have not been studied comprehensively. Here the climatology, seasonal variation, interannual variability, and predictability of NWP ARs (NWPARs) are examined by using a large ensemble, high-resolution atmospheric general circulation model (AGCM) simulation and a global atmospheric reanalysis. The AGCM captures general characteristics of climatology and variability compared to the reanalysis, suggesting a strong sea surface temperature (SST) effect on NWPARs. The summertime NWPAR occurrences are tightly related to El Niño–Southern Oscillation (ENSO) in the preceding winter through Indo–western Pacific Ocean capacitor (IPOC) effects. An enhanced East Asian summer monsoon and a low-level anticyclonic anomaly over the tropical western North Pacific in the post–El Niño summer reinforce low-level water vapor transport from the tropics with increased occurrence of NWPARs. The strong coupling with ENSO and IPOC indicates a high predictability of anomalous summertime NWPAR activity.

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