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Shortwave Radiative Fluxes on Slopes

JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY 2016年第7期55卷 1513-1532页

作者： Ma, Yingtao Pinker, Rachel T. Wonsick, Margaret M. Li, Chuan Hinkelman, Laura M. Univ Maryland Dept Atmospher & Ocean Sci Comp & Space Sci Bldg College Pk MD 20742 USA Univ Washington Joint Inst Study Atmosphere & Ocean Seattle WA 98195 USA

Snow-covered mountain ranges are a major source of water supply for runoff and groundwater recharge. Snowmelt supplies as much as 75% of the surface water in basins of the western United States. Net radiative fluxes make up about 80% of the energy balance over snow-covered surfaces. Because of the large extent of snow cover and the scarcity of ground observations, use of remotely sensed data is an attractive option for estimating radiative fluxes. Most of the available methods have been applied to low-spatial-resolution satellite observations that do not capture the spatial variability of snow cover, clouds, or aerosols, all of which need to be accounted for to achieve accurate estimates of surface radiative fluxes. The objective of this study is to use high-spatial-resolution observations that are available from the Moderate Resolution Imaging Spectroradiometer (MODIS) to derive surface shortwave (0.2-4.0 mm) downward radiative fluxes in complex terrain, with attention on the effect of topography (e.g., shadowing or limited sky view) on the amount of radiation received. The developed method has been applied to several typical melt seasons (January-July during 2003, 2004, 2005, and 2009) over the western part of the United States, and the available information was used to derive metrics on spatial and temporal variability of shortwave fluxes. Issues of scale in both the satellite and ground observations are also addressed to illuminate difficulties in the validation process of satellite-derived quantities. It is planned to apply the findings from this study to test improvements in estimation of snow water equivalent.

关键词： Physical Meteorology and Climatology Budgets Radiation budgets Radiative fluxes Shortwave radiation observational techniques and algorithms algorithms Remote sensing

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Error Characteristics of Ceilometer-Based Observations of Cloud Amount

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第7期33卷 1557-1567页

作者： Wagner, Timothy J. Kleiss, Jessica M. Univ Wisconsin Space Sci & Engn Ctr Cooperat Inst Meteorol Satellite Studies Madison WI USA Lewis & Clark Coll Environm Studies Program Portland OR 97219 USA

Ceilometer observations of cloud cover are an important component of the automated weather observation network. However, the accuracy of its measurements of cloud amount is impacted by the limited vertical range and areal extent of its observations. A multiyear collocated dataset of observations from a laser ceilometer, a total sky imager (TSI), and a micropulse lidar (MPL) at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility is used to simulate the observations of operational ceilometers and to analyze the magnitude of the errors associated with ceilometer-based observations of cloud amount. The limited areal coverage of ceilometers results in error when skies are heterogeneous, but these errors are small compared to those caused by the limited vertical range: observations of clear sky or few clouds are often in error as the instrument cannot detect the presence of upper-level clouds. The varying quantities of upper-level clouds mean that errors are diurnally and seasonally dependent, with the greatest error at the SGP site happening in the morning and summer, respectively. Overall, the spatial homogeneity and low base of stratus clouds means that ceilometer-based observations of overcast skies are the most accurate, with a root-mean-square error of cloud fraction in overcast conditions an order of magnitude lower than for the dataset as a whole.

关键词： Atm/Ocean Structure/ Phenomena Clouds Instrumentation/sensors Lidars/Lidar observations observational techniques and algorithms Surface observations

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Impact of Scatterometer Surface Wind Data in the ECMWF Coupled Assimilation System

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MONTHLY WEATHER REVIEW 2016年第3期144卷 1203-1217页

作者： Laloyaux, Patrick Thepaut, Jean-Noel Dee, Dick European Ctr Medium Range Weather Forecasts Shinfield Pk Reading RG2 9AX Berks England

The European Centre for Medium-Range Weather Forecasts (ECMWF) has developed a coupled assimilation system that ingests simultaneously ocean and atmospheric observations in a coupled ocean-atmosphere model. Employing the coupled model constraint in the analysis implies that assimilation of an ocean observation has immediate impact on the atmospheric state estimate, and, conversely, assimilation of an atmospheric observation affects the ocean state. In this context, observing system experiments have been carried out withholding scatterometer surface wind data over the period September-November 2013. Impacts in the coupled assimilation system have been compared to the uncoupled approach used in ECMWF operations where atmospheric and ocean analyses are computed sequentially. The assimilation of scatterometer data has reduced the background surface wind root-mean-square error in the coupled and uncoupled assimilation systems by 3.7% and 2.5%, respectively. It has been found that the ocean temperature in the mixed layer is improved in the coupled system, while the impact is neutral in the uncoupled system. Further investigations have been conducted over a case of a tropical cyclone when strong interactions between atmospheric wind and ocean temperature occur. Cyclone Phailin in the Bay of Bengal has been selected since the conventional observing system has measured surface wind speed and ocean temperature at a high frequency. In this case study, the coupled assimilation system outperforms the uncoupled approach, being able to better use the scatterometer measurements to estimate the cold wake after the cyclone.

关键词： Variational analysis Atm/Ocean Structure/ Phenomena Mathematical and statistical techniques observational techniques and algorithms Tropical cyclones Satellite observations Coupled models Models and modeling In situ oceanic observations

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Toward an Algorithm for Estimating Latent Heat Release in Warm Rain Systems

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第6期33卷 1309-1329页

作者： Nelson, Ethan L. L'Ecuyer, Tristan S. Saleeby, Stephen M. Berg, Wesley Herbener, Stephen R. van den Heever, Susan C. Univ Wisconsin Dept Atmospher & Ocean Sci 1225 W Dayton St Madison WI 53706 USA Colorado State Univ Ft Collins CO 80523 USA

This paper outlines an approach for estimating latent heating, surface rainfall rate, and liquid water path in warm rain from downward-viewing W-band radar observations using a Bayesian Monte Carlo algorithm. The algorithm utilizes observed vertical and path-integrated characteristics of precipitating liquid clouds to identify the most appropriate hydrometeor and latent heating structures in a large database of profiles generated using a cloud-resolving model. These characteristics are selected by applying multiple performance metrics to synthetic retrievals. Analysis of the retrievals suggests that a combination of cloud-top, rain-top, and maximum reflectivity heights;vertically integrated reflectivity and attenuation;and a measure of near-surface intensity is sufficient to constrain bulk properties and the vertical structure of warm rain systems. When applied to observations at CloudSat resolution, biases in retrieved liquid water path and surface rainfall rate are small (less than 10%). The algorithm also captures the vertical structure of latent heating, although the magnitudes of integrated heating and cooling exhibit nearly compensating low biases. Random errors are larger owing to the limitations of single-frequency radar observations in constraining drop size distributions. Uncertainties in the altitudes of peak heating and cooling at the pixel scale are typically less than one vertical level, while uncertainties in vertically resolved estimates of heating and cooling rates are on the order of a factor of 2. The utility of the technique is illustrated through application to case studies from airborne radar data from the VAMOS Ocean-Cloud-Atmosphere-Land Study field campaign and satellite observations from CloudSat.

关键词： Latent heating/cooling observational techniques and algorithms Physical Meteorology and Climatology Radars/Radar observations Satellite observations

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Estimating Confidence Intervals around Relative Humidity Profiles from Satellite Observations: Application to the SAPHIR Sounder

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第5期33卷 1005-1022页

作者： Brogniez, Helene Fallourd, Renaud Mallet, Cecile Sivira, Ramses Dufour, Christophe Univ Paris 06 CNRS UVSQ Univ Paris Saclay LATMOS IPSL Guyancourt France Meteo Protect SAS Paris France

A novel scheme for the estimation of layer-averaged relative humidity (RH) profiles from spaceborne observations in the 183.31-GHz line is presented. Named atmospheric relative humidity profiles including analysis of confidence intervals (ARPIA), it provides for each vector of observations the parameters of the distribution of the RH instead of its expectation, as is usually done by the current methods. The profiles are composed of six layers distributed between 100 and 950 hPa. The approach combines the six channels of the Sondeur Atmospherique du Profil d'Humidite Intertropical par Radiometrie (SAPHIR) instrument on board the Megha-Tropiques satellite and the generalized additive model for location, scale and shape (GAMLSS) method to infer the parametric distributions, assuming that they follow a Gaussian law. The knowledge of the conditional uncertainty is an asset in the evaluation using radiosounding profiles of RH with a dedicated Bayesian method. Taking the uncertainties into account in both the ARPIA estimates and the in situ measurements yields biases, root-mean-square, and correlation coefficients in the range of -0.56% to 9.79%, 1.58% to 13.32%, and 0.55 to 0.98, respectively, with the largest biases being obtained over the continent, in the midtropospheric layers.

关键词： Inverse methods Mathematical and statistical techniques Microwave observations observational techniques and algorithms Physical Meteorology and Climatology Satellite observations Water vapor

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Observable Surface Anomalies Preceding Simulated Isolated Convective Initiation

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MONTHLY WEATHER REVIEW 2016年第6期144卷 2265-2284页

作者： Madaus, Luke E. Hakim, Gregory J. Univ Washington Seattle WA 98195 USA

Idealized ensemble simulations of isolated convective initiation (CI) are analyzed to identify storm-scale features in surface weather fields that precede initiation in a variety of background environments and the observations that would be needed to resolve these features. Precipitating storms are identified with an object-based method and composites of surface anomalies are generated for the variables of interest surrounding times and locations of initiation. Correlation length scales and anomaly magnitudes throughout the CI process are examined in detail with the latter comparing favorably to anomaly estimates obtained from previous observational and modeling studies. Negative temperature anomalies due to cloud shadowing are found to be the most prominent storm-scale feature prior to initiation. Significant spatial correlations are shown to extend from the surface throughout the boundary layer and even into the cloud-bearing layer once deep convective clouds become established. The findings are discussed in the context of data assimilation, particularly with respect to current assumptions about surface observation error. It is shown that, to resolve the storm-scale anomalies in these simulations, the minimum necessary temperature and wind observation densities would likely be limited by spatial correlation length scale while moisture and pressure observations are more limited by observation error.

关键词： Circulation Dynamics Convective storms Convective-scale processes Physical Meteorology and Climatology Surface fluxes observational techniques and algorithms Surface observations Models and modeling Data assimilation Numerical weather prediction forecasting

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Toward Improving Ice Water Content and Snow-Rate Retrievals from Radars. Part I: X and W Bands, Emphasizing CloudSat

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JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY 2016年第9期55卷 2063-2090页

作者： Heymsfield, Andrew J. Matrosov, Sergey Y. Wood, Norman B. Natl Ctr Atmospher Res 3450 Mitchell Lane Boulder CO 80301 USA Univ Colorado Cooperat Inst Res Environm Sci Boulder CO 80309 USA NOAA Earth Syst Res Lab Boulder CO USA Univ Wisconsin Madison Cooperat Inst Meteorol Satellite Studies Madison WI USA

Microphysical data and radar reflectivities (Z(e), -15 10 dB, this Z(e)-S relationship conforms well to earlier relationships, but for Z(e) < 10 dB it yields higher IWC and S. Because not all retrieval algorithms ... 详细信息

Microphysical data and radar reflectivities (Z(e), -15 < Z(e) < 10 dB) measured from flights during the NASA Tropical Clouds, Convection, Chemistry and Climate field program are used to relate Z(e) at X and W band to measured ice water content (IWC). Because nearly collocated Z(e) and IWC were each directly measured, Z(e)-IWC relationships could be developed directly. Using the particle size distributions and ice particle masses evaluated based on the direct IWC measurements, reflectivity-snowfall rate (Z(e)-S) relationships were also derived. For -15 < Z(e) < 10 dB, the relationships herein yield larger IWC and S than given by the retrievals and earlier relationships. The sensitivity of radar reflectivity to particle size distribution and size-dependent mass, shape, and orientation introduces significant uncertainties in retrieved quantities since these factors vary substantially globally. To partially circumvent these uncertainties, a W-band Z(e)-S relationship is developed by relating four years of global CloudSat reflectivity observations measured immediately above the melting layer to retrieved rain rates at the base of the melting layer. The supporting assumptions are that the water mass flux is constant through the melting layer, that the air temperature is nearly 0 degrees C, and that the retrieved rain rates are well constrained. Where Z(e) > 10 dB, this Z(e)-S relationship conforms well to earlier relationships, but for Z(e) < 10 dB it yields higher IWC and S. Because not all retrieval algorithms estimate either or both IWC and S, the authors use a large aircraft-derived dataset to relate IWC and S. The IWC can then be estimated from S and vice versa.

关键词： observational techniques and algorithms Aircraft observations algorithms Cloud retrieval Radars/Radar observations Satellite observations

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On the Impacts of Different Definitions of Maximum Dimension for Nonspherical Particles Recorded by 2D Imaging Probes

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第5期33卷 1056-1072页

作者： Wu, Wei McFarquhar, Greg M. Univ Illinois Dept Atmospher Sci 105 S Gregory St Urbana IL 61801 USA

Knowledge of ice crystal particle size distributions (PSDs) is critical for parameterization schemes for atmospheric models and remote sensing retrieval schemes. Two-dimensional in situ images captured by cloud imaging probes are widely used to derive PSDs in term of maximum particle dimension (D-max). In this study, different definitions of D-max for nonspherical particles recorded by 2D probes are compared. It is shown that the derived PSDs can differ by up to a factor of 6 for D-max < 200 mu m and D-max > 2 mm. The large differences for D-max < 200 mu m are caused by the strong dependence of sample volume on particle size, whereas differences for D-max < 2 mm are caused by the small number of particles detected. Derived bulk properties can also vary depending on the definitions of D-max because of discrepancies in the definition of D-max used to characterize the PSDs and that used to describe the properties of individual ice crystals. For example, the mass-weighted mean diameter can vary by 2 times, the ice water content (IWC) by 3 times, and the mass-weighted terminal velocity by 6 times. Therefore, a consistent definition of D-max should be used for all data and single-particle properties. As an invariant measure with respect to the orientation of particles in the imaging plane for 2D probes, the diameter of the smallest circle enclosing the particle (DS) is recommended as the optimal definition of D-max. If the 3D structure of a particle is observed, then the technique can be extended to determine the minimum enclosing sphere.

关键词： Aircraft observations algorithms Cloud microphysics Data processing In situ atmospheric observations observational techniques and algorithms Physical Meteorology and Climatology

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Interannual Variability of the Mindanao Current/Undercurrent in Direct Observations and Numerical Simulations

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第2期46卷 483-499页

作者： Hu, Shijian Hu, Dunxin Guan, Cong Wang, Fan Zhang, Linlin Wang, Fujun Wang, Qingye Chinese Acad Sci Inst Oceanol 7 Nanhai Rd Qingdao 266071 Peoples R China Chinese Acad Sci Key Lab Ocean Circulat & Wave Qingdao 266071 Peoples R China Qingdao Natl Lab Marine Sci & Technol Lab Ocean & Climate Dynam Qingdao Peoples R China Univ Chinese Acad Sci Beijing Peoples R China

The interannual variability of the boundary currents east of the Mindanao Island, including the Mindanao Current/Undercurrent (MC/MUC), is investigated using moored acoustic Doppler current profiler (ADCP) measurements combined with a series of numerical experiments. The ADCP mooring system was deployed east of the Mindanao Island at 7 degrees 59 ' N, 127 degrees 3 ' E during December 2010-August 2014. Depth-dependent interannual variability is detected in the two western boundary currents: strong and lower-frequency variability dominates the upper-layer MC, while weaker and higher-frequency fluctuation controls the subsurface MUC. Throughout the duration of mooring measurements, the weakest MC was observed in June 2012, in contrast to the maximum peaks in December 2010 and June 2014, while in the deeper layer the MUC shows speed peaks circa December 2010, January 2011, April 2013, and July 2014 and valleys circa June 2011, August 2012, and November 2013. Diagnostic analysis and numerical sensitivity experiments using a 2.5-layer reduced-gravity model indicate that wind forcing in the western Pacific Ocean is a driving agent in conditioning the interannual variability of MC and MUC. Results suggest that westward-propagating Rossby waves that generate in the western Pacific Ocean (roughly 150 degrees-180 degrees E) are of much significance in the interannual variability of the two boundary currents. Fluctuation of Ekman pumping due to local wind stress curl anomaly in the far western Pacific Ocean (roughly 120 degrees-150 degrees E) also plays a role in the interannual variability of the MC. The relationship between the MC/MUC and El Nino is discussed.

关键词： In situ oceanic observations Geographic location/entity Pacific Ocean observational techniques and algorithms Boundary currents Circulation/ Dynamics

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On the Use of X-Band Weather Radar for Wind Field Retrieval in Coastal Zone

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第5期33卷 899-917页

作者： Forget, P. Saillard, M. Guerin, C. -A. Testud, J. Le Bouar, E. Univ Aix Marseille Univ Toulon Mediterranean Inst Oceanog CNRS INSUIRDUM110 CS60584 Toulon France NOVIMET Guyancourt France

This paper documents the study of the radar signature of the sea surface on the images collected by an X-band weather radar and its application to remote sensing of the coastal zone. The main radar parameters considered here are the reflectivity factor [which was converted into a normalized radar cross section (NRCS)], the differential reflectivity, and the mean radial velocity. Measurements of the NRCS in the vicinity of an instrumented offshore buoy allowed for identifying its variations with wind speed and relative direction, which are found to be consistent with the Georgia Institute of Technology (GIT) model and other measurements. A more accurate empirical model for the NRCS is developed. For moderate to strong winds, the mean radial velocity is linearly related to the radial component of the wind speed, showing the potentiality of weather radars to map the radial surface wind speed at the scale of a basin. The influence of surface currents is weak here but should be likely taken into account in other places with stronger currents. Measurements of the differential reflectivity are also consistent with other measurements made elsewhere for the polarization ratio. The analysis of radar data over the entire coverage could also be performed using surface wind estimates from a meteorological model. Similar but less accurate results are obtained for the NRCS and the mean radial velocity. The extra spatial variability not due to wind speed variations is identified. An inversion procedure is proposed to recover the wind vector field from radar measurements after calibration of the NRCS values. Results are promising and pave the way to the concept of weather radar for ocean remote sensing.

关键词： Microwave observations observational techniques and algorithms Radars/Radar observations

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