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Role of a Cross-Barrier Jet and Turbulence on Winter Orographic Snowfall

MONTHLY WEATHER REVIEW 2016年第9期144卷 3277-3300页

作者： Aikins, Joshua Friedrich, Katja Geerts, Bart Pokharel, Binod Univ Colorado Boulder Dept Atmospher & Ocean Sci UCB 311 Boulder CO 80309 USA Univ Wyoming Dept Atmospher Sci Laramie WY 82071 USA

Natural small-scale microphysical and dynamical mechanisms are identified in a winter orographic snowstorm over the Sierra Madre Range of Wyoming during an intensive observational period (TOP) from the AgI Seeding Cloud Impact Investigation (ASCII;January-March 2012). A suite of high-resolution radars, including a ground-based scanning X-band dual-polarization Doppler on Wheels radar, vertically pointing Ka-band Micro Rain Radar (MRR), and airborne W-band Wyoming Cloud Radar (WCR), and additional in situ and remote sensing instruments are used in the analysis. The analysis focuses on a deep postfrontal period on 16 January 2012 (I0P2) when clouds extended throughout the troposphere and cloud liquid water was absent following the passage of a baroclinic front. A turbulent shear layer was observed in this postfrontal environment that was created by a midlevel cross-barrier jet riding over a decoupled Arctic air mass that extended above mountaintop. MRR and WCR observations indicate additional regions of turbulence aloft that favor rapid particle growth at upper levels of the cloud. Plunging flow in the lee of the Sierra Madre was also observed during this case, which caused sublimation of snow up to 20 km downwind. The multi instrument analysis in this paper suggests that 1) shear-induced turbulent overturning cells do exist over cold continental mountain ranges like the Sierra Madre, 2) the presence of cross-barrier jets favors these turbulent shear zones, 3) this turbulence is a key mechanism in enhancing snow growth, and 4) snow growth enhanced by turbulence primarily occurs through deposition and aggregation in these cold (<-15 degrees C) postfrontal continental environments.

关键词： Geographic location/entity Complex terrain Circulation/ Dynamics Mesoscale processes Orographic effects Turbulence Wind shear observational techniques and algorithms Radars/Radar observations

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Two Types of Strong Local Wind Captured by Simultaneous Multiple-Site Radiosonde Soundings across a Mountain Range

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MONTHLY WEATHER REVIEW 2016年第10期144卷 3915-3936页

作者： Komatsu, Kensuke K. Tachibana, Yoshihiro Mie Univ Weather & Climate Dynam Div 1577 Kurimamachiya Cho Tsu Mie 5148507 Japan

A radiosonde observation method is presented, consisting of simultaneous radiosonde observations at closely spaced multiple sites using balloons with varied buoyancies. This method was employed during a strong wind event (Suzuka-oroshi) on the lee side of the Suzuka mountain range, Japan, to derive the detailed structure of the wind as it crossed the mountains. Batches of six radiosondes were launched simultaneously from a line of four sites, using balloons with three different degrees of buoyancy. The four sites were 13 km apart along a 35-km-long transect roughly aligned with the prevailing wind. The observations documented two flow regimes: a downslope flow perpendicular to the mountain range, similar to a windstorm, and an unexpectedly strong low-level jet flowing parallel to the mountain range. The method was more successful at delineating the first regime than the second. The first regime was well simulated by a numerical experiment, but the second regime was not. The vertical wind associated with the downslope windstorm was inferred from the changing slopes of potential temperature isentropes. Comparison of the balloon ascent rates with these isentropes meanders and the simulated vertical wind showed that fluctuations in balloon ascent rate provide reliable information on the vertical direction of the wind. An analysis of the second regime using a long-term meteorological dataset shows that the onset of the low-level jet is related to the synoptic-scale shift in vorticity from positive to negative in the observation area. This vorticity shift appears to be a useful indicator for the low-level jet regime.

关键词： Circulation/ Dynamics Downslope winds Mountain waves Orographic effects observational techniques and algorithms Radiosonde observations

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Improving Radar Refractivity Retrieval by Considering the Change in the Refractivity Profile and the Varying Altitudes of Ground Targets

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第5期33卷 989-1004页

作者： Feng, Ya-Chien Fabry, Frederic Weckwerth, Tammy M. McGill Univ Dept Atmospher & Ocean Sci 805 Sherbrooke St W Montreal PQ H3A 0B9 Canada Natl Ctr Atmospher Res Earth Observing Lab POB 3000 Boulder CO 80307 USA

Accurate radar refractivity retrievals are critical for quantitative applications, such as assimilating refractivity into numerical models or studying boundary layer and convection processes. However, the technique as originally developed makes some simplistic assumptions about the heights of ground targets (HT) and the vertical gradient of refractivity (dN/dh). In reality, the field of target phases used for refractivity retrieval is noisy because of varying terrain and introduces estimation biases. To obtain a refractivity map at a constant height above terrain, a 2D horizontal refractivity field at the radar height must be computed and corrected for altitude using an average dN/dh. This is achieved by theoretically clarifying the interpretation of the measured phase considering the varying HT and the temporal change of dN/dh. Evolving dN/dh causes systematic refractivity biases, as it affects the beam trajectory, the associated target range, and the refractivity field sampled between selected targets of different heights. To determine HT and dN/dh changes, a twofold approach is proposed: first, HT can be reasonably inferred based on terrain height;then, a new method of dN/dh estimation is devised by using the property of the returned powers of a pointlike target at successive antenna elevations. The dN/dh obtained shows skill based on in situ tower observation. As a result, the data quality of the retrieved refractivity may be improved with the newly added information of dN/dh and HT.

关键词： Atm/Ocean Structure/ Phenomena Boundary layer Data quality control Diurnal effects observational techniques and algorithms Radars/Radar observations Variability

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The Mississippi Valley Convection Minimum on Summer Afternoons: Observations and Numerical Simulations

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MONTHLY WEATHER REVIEW 2016年第1期144卷 263-272页

作者： Kirshbaum, Daniel J. Fabry, Frederic Cazenave, Quitterie McGill Univ Dept Atmospher & Ocean Sci Montreal PQ H3A 0B9 Canada Ecole Natl Meteorol Toulouse France

Analysis of 15 years of composite radar images over the continental United States reveals a distinct minimum of deep-convection occurrence over the interior lower Mississippi Valley on summer afternoons, relative to surrounding areas. To understand the mechanisms behind this convection signature, quasi-idealized numerical simulations with the Weather Research and Forecasting (WRF) Model are performed. The simulations, which broadly reproduce the valley convection minimum, suggest that convective inhibition is maximized, and low-level ascent minimized, over the flat valley terrain. By contrast, weaker inhibition and stronger mechanically forced ascent over the hills flanking the valley combine to initiate convection more readily. Although the orography of the region is unremarkable, it has a stronger influence on the regional convection pattern than do variations in land use.

关键词： Circulation Dynamics Convective storms Orographic effects Atm Ocean Structure Phenomena Diurnal effects Physical Meteorology and Climatology Atmosphere-land interaction observational techniques and algorithms Radars Radar observations Models and modeling Mesoscale models

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A Procedure to Compensate for the Response Drift of a Large Set of Thermistors

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第7期33卷 1495-1508页

作者： Cimatoribus, Andrea A. van Haren, Hans Gostiaux, Louis Royal Netherlands Inst Sea Res NIOZ Het Horntje North Holland Netherlands Ecole Polytech Fed Lausanne Sch Architecture Civil & Environm Engn Inst Environm Engn Ecol Engn Lab ECOL Lausanne Switzerland Univ Lyon Ecole Cent Lyon CNRS LMFAUMR 5509 Lyon France

The drift of temperature measurements by semiconductor negative temperature coefficient thermistors is a well-known problem. This study analyzes the drift characteristics of the thermistors designed and used at the Royal Netherlands Institute for Sea Research for measuring high-frequency O(1Hz) temperature fluctuations in the ocean. These thermistors can be calibrated to high precision and accuracy (better than 1mK) and have very low noise levels. The thermistors can measure independently for long periods of time (more than one year), and the identification and compensation of the drift are thus essential processing steps. A laboratory analysis showing that the drift is similar, in its functional form, to the drift of commercial thermistors described in the literature is presented. An effective procedure to estimate this drift from ocean observations is described and tested using three datasets from the deep Atlantic Ocean. Since the functional form of the drift rate is, with good approximation, universal among different sensors, the procedure could easily be adapted to other datasets and, the authors argue, to measurements from thermistors by other manufacturers too.

关键词： In situ oceanic observations Instrumentation/sensors Mathematical and statistical techniques observational techniques and algorithms Quality assurance/control

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Dynamics of the Changing Near-Inertial Internal Wave Field in the Arctic Ocean

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第2期46卷 395-415页

作者： Dosser, Hayley V. Rainville, Luc Univ Washington Appl Phys Lab 1013 NE 40th St Seattle WA 98105 USA

The dynamics of the wind-generated near-inertial internal wave field in the Canada Basin of the Arctic Ocean are investigated using the drifting Ice-Tethered Profiler dataset for the years 2005 to 2014, during a decade when sea ice extent and thickness decreased dramatically. This time series, with nearly 10 years of measurements and broad spatial coverage, is used to quantify a seasonal cycle and interannual trend for internal waves in the Arctic, using estimates of the amplitude of near-inertial waves derived from isopycnal displacements. The internal wave field is found to be most energetic in summer when sea ice is at a minimum, with a second maximum in early winter during the period of maximum wind speed. Amplitude distributions for the near-inertial waves are quantifiably different during summer and winter, due primarily to seasonal changes in sea ice properties that affect how the ice responds to the wind, which can be expressed through the "wind factor"-the ratio of sea ice drift speed to wind speed. A small positive interannual trend in near-inertial wave energy is linked to pronounced sea ice decline during the last decade. Overall variability in the internal wave field increases significantly over the second half of the record, with an increased probability of larger-than-average waves in both summer and winter. This change is linked to an overall increase in variability in the wind factor and sea ice drift speeds, and reflects a shift in year-round sea ice characteristics in the Arctic, with potential implications for dissipation and mixing associated with internal waves.

关键词： Interannual variability Variability Seasonal cycle Profilers, oceanic Arctic Circulation/ Dynamics Geographic location/entity Sea ice Internal waves observational techniques and algorithms

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Moisture and Precipitation Evolution during Tropical Cyclone Formation as Revealed by the SSM/I-SSMIS Retrievals

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第7期73卷 2773-2781页

作者： Wang, Zhuo Hankes, Isaac Univ Illinois Dept Atmospher Sci Urbana IL 61801 USA Northern Illinois Univ Dept Geog De Kalb IL USA

The simultaneous precipitation and column water vapor retrievals from the SSM/I and SSMIS passive microwave instruments were used to examine the convective and moisture evolution during tropical cyclone formation. Using a wave-pouch-track dataset, composites of precipitation and column water vapor were constructed with more than 2000 satellite overpasses for a 3-day time period prior to genesis. It was found that high column water vapor occurs near the pouch center and starts to increase about 42 h prior to genesis while a substantial increase in precipitation occurs within 24 h prior to genesis. These features are consistent with a recently proposed two-stage conceptual model for tropical cyclone formation, in which gradual moisture preconditioning precedes an abrupt transition to sustained deep convection leading up to genesis. The relationship between precipitation and saturation fraction (SF) is examined for the developing waves and compared with the general tropical North Atlantic. Precipitation rate is found to increase at the same exponential rate above the same critical point of SF in the two groups, but convection in the developing waves has a higher probability of occurrence near and above criticality. This can be attributed to the positive feedback between convection and the low-level moisture convergence, which counteracts the negative feedback of convection on water vapor and makes convection in a developing tropical cyclone more sustainable.

关键词： Atm/Ocean Structure/ Phenomena Convective clouds Precipitation Tropical cyclones observational techniques and algorithms Remote sensing

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OSSEs for an Ensemble 3DVAR Data Assimilation System with Radar Observations of Convective Storms

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第6期73卷 2403-2426页

作者： Gao, Jidong Fu, Chenghao Stensrud, David J. Kain, John S. NOAA Natl Severe Storms Lab Norman OK 73069 USA Hunan Meteorol Bur Changsha Hunan Peoples R China Univ Oklahoma Cooperate Inst Mesoscale Meteorol Studies Norman OK 73019 USA Penn State Univ Dept Meteorol 503 Walker Bldg University Pk PA 16802 USA

An ensemble of the three-dimensional variational data assimilation (En3DA) method for convective-scale weather has been developed. It consists of an ensemble of three-dimensional variational data assimilations and forecasts in which member differences are introduced by perturbing initial conditions and/or observations, and it uses flow-dependent error covariances generated by the ensemble forecasts. The method is applied to the assimilation of simulated radar data for a supercell storm. Results indicate that the flow-dependent ensemble covariances are effective in enabling convective-scale analyses, as the most important features of the simulated storm, including the low-level cold pool and midlevel mesocyclone, are well analyzed. Several groups of sensitivity experiments are conducted to test the robustness of the method. The first group demonstrates that incorporating a mass continuity equation as a weak constraint into the En3DA algorithm can improve the quality of the analyses when radial velocity observations contain large errors. In the second group of experiments, the sensitivity of analyses to the microphysical parameterization scheme is explored. Results indicate that the En3DA analyses are quite sensitive to differences in the microphysics scheme, suggesting that ensemble forecasts with multiple microphysics schemes could reduce uncertainty related to model physics errors. Experiments also show that assimilating reflectivity observations can reduce spinup time and that it has a small positive impact on the quality of the wind field analysis. Of the threshold values tested for assimilating reflectivity observations, 15 dBZ provides the best analysis. The final group of experiments demonstrates that it is not necessary to perturb radial velocity observations for every ensemble number in order to improve the quality of the analysis.

关键词： Circulation Dynamics Convective storms observational techniques and algorithms Radars Radar observations Models and modeling Data assimilation Ensembles

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Thirty Years of Atmospheric Extinction from Telescopes of the North Atlantic Canary Archipelago

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JOURNAL OF CLIMATE 2016年第1期29卷 227-240页

作者： Laken, Benjamin A. Parviainen, Hannu Garcia-Gil, Alejandro Munoz-Tunon, Casiana Varela, Antonia M. Fernandez-Acosta, Sergio Palle, Pere Univ Oslo Dept Geosci N-0315 Oslo Norway Univ Oxford Dept Phys Oxford England Agencia Estatal Meteorol Izana Atmospher Res Ctr Santa Cruz De Tenerife Spain Inst Astrofis Canarias Tenerife Spain Univ La Laguna Inst Astrofis Canarias Tenerife Spain Univ La Laguna Dept Astrophys Tenerife Spain

This study examines 30 years of atmospheric extinction, , obtained from both stellar and solar telescope measurements, at similar to 2.4 km MSL, from the North Atlantic Canary Archipelagoan island chain located at approximately 28 degrees N, around 100 km from the west coast of Africa. Data from three AERONET monitors, located at varying heights on one of the main islands, were also used, although these are only available over a shorter (<10 yr) period. The Canary Archipelago is regularly affected by dust intrusions into the local atmosphere as they intersect one of the primary export pathways of mineral dust from the Sahara. The of baseline and dust influenced conditions were statistically distinguished by fitting normal-gamma mixture distributions to the observations using Markov chain Monte Carlo methods, and then the seasonal and long-term characteristics of these data were examined. The telescope data show that baseline conditions are usually stable at < 0.1 (except during periods influenced by volcanic aerosols) and indicate the existence of a low-amplitude () seasonal variation. During dust-influenced conditions, regularly reaches values of a factor of 2-6 times higher than normal. The majority of dust intrusions take place during the months of July and August, when they may occur 44 +/- 15% of the time, predominantly at high altitudes (with similar to 94.3 +/- 1.6% of intrusions occurring 2.4 km), whereas during the months of November-May, dust intrusions occur far less frequently (similar to 19 +/- 7%) and are more common at lower altitudeswith intrusions at <2.4 km comprising similar to 79.5 +/- 3.2% of all outbreaks. Year-to-year variations in the frequency of dust-influenced conditions (of similar to 9%) were found but no long-term trend over the observed 30-yr period.

关键词： Physical Meteorology and Climatology Aerosols observational techniques and algorithms In situ atmospheric observations Mathematical and statistical techniques Statistics Time series Variability Annual variations Seasonal variability

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Microwave Sounder Cloud Detection Using a Collocated High-Resolution Imager and Its Impact on Radiance Assimilation in Tropical Cyclone Forecasts

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MONTHLY WEATHER REVIEW 2016年第10期144卷 3937-3959页

作者： Han, Hyojin Li, Jun Goldberg, Mitch Wang, Pei Li, Jinlong Li, Zhenglong Sohn, B. -J. Li, Juan Univ Wisconsin Cooperat Inst Mesoscale Meteorol Studies 1225 West Dayton St Madison WI 53706 USA Korea Meteorol Adm Natl Inst Meteorol Sci Jeju Do South Korea NOAA NESDIS Joint Polar Satellite Syst Lanham MD USA Univ Wisconsin Dept Atmospher & Ocean Sci Madison WI USA Seoul Natl Univ Sch Earth & Environm Sci Seoul South Korea China Meteorol Adm Numer Weather Predict Ctr Beijing Peoples R China

Accurate cloud detection is one of the most important factors in satellite data assimilation due to the uncertainties associated with cloud properties and their impacts on satellite-simulated radiances. To enhance the accuracy of cloud detection and improve radiance assimilation for tropical cyclone (TC) forecasts, measurements from the Advanced Microwave Sounding Unit-A (AMSU-A) on board the Aqua satellite and the Advanced Technology Microwave Sounder (ATMS) are collocated with high spatial resolution cloud products from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board Aqua and the Visible Infrared Imager Radiometer Suite (VIIRS) on board the Suomi-National Polar-Orbiting Partnership (Suomi-NPP) satellite. The cloud-screened microwave radiance measurements are assimilated for Hurricane Sandy (2012) and Typhoon Haiyan (2013) forecasts using the Weather Research and Forecasting (WRF) Model and the three-dimensional variational (3DVAR)-based Gridpoint Statistical Interpolation (GSI) data assimilation system. Experiments are carried out to determine the optimal thresholds of cloud fraction (CF) for minimizing track and intensity forecast errors. The results indicate that the use of high spatial resolution cloud products can improve the accuracy of TC forecasts by better eliminating cloud-contaminated microwave sounder field-of-views (FOVs). In conclusion, the combination of advanced microwave sounders and collocated high spatial resolution imagers is able to improve the radiance assimilation and TC forecasts. The methodology used in this study can be applied to process data from other pairs of microwave sounders and imagers on board the same platform.

关键词： Physical Meteorology and Climatology Clouds observational techniques and algorithms Satellite observations Models and modeling Data assimilation

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