Introduction of irrigated agriculture changes the partitioning of the surface energy flux between sensible and latent heat (H vs LE) and alters the albedo alpha and emissivity epsilon. In the absence of changes in the...
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Introduction of irrigated agriculture changes the partitioning of the surface energy flux between sensible and latent heat (H vs LE) and alters the albedo alpha and emissivity epsilon. In the absence of changes in the radiation components of the surface energy balance, the change in the Bowen ratio due to irrigation typically suppresses the local air temperature T but increases the total near-surface atmospheric heat content (as measured using equivalent potential temperature theta(e)). While the effect of irrigation on surface energy partitioning due to enhanced surface and subsurface water availability has long been acknowledged, the roles of associated changes in epsilon and alpha have received less attention, and the scales and magnitudes of these effects remain uncertain. A new methodology designed for application to in situ and remote sensing data is presented and used to demonstrate that the net impact of irrigation on T and theta(e) is strongly dependent on the regional climate, land cover in surrounding areas, and the amount of irrigation in the upwind fetch. The results suggest that the impact of the radiative forcing terms on net available energy is not negligible and may amplify or offset the impact from changed energy partitioning on T and theta(e) depending on the specific regional climate and land cover.
Three methods for estimation of the weather radar antenna azimuth and elevation pointing offsets are compared. Two of the methods reviewed use the known location of the sun as a reference. The first of these methods i...
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Three methods for estimation of the weather radar antenna azimuth and elevation pointing offsets are compared. Two of the methods reviewed use the known location of the sun as a reference. The first of these methods is based on an offline scan of the sun disk. The second method detects and characterizes solar interferences in operative scans. The third method consists of correlating measured ground clutter echoes with echoes simulated using a high-resolution digital elevation model. The main objectives are to review the characteristics in each case, studying their performance in actual operative conditions, and to examine the reasons for the discrepancies between the reported pointing bias estimates, with the aim of laying the groundwork for an optimized individual or combined application and interpretation of the methods. Daily pointing biases estimated through the sun-scanning procedure in a dedicated one-month, short-term campaign are the base for the intercomparison. When applied to the three weather radars operated by the Meteorological Service of Catalonia, the short-term study reveals the advantages and limitations of the methods. A one-year, long-term analysis serves to confirm and clarify the discrepancies inferred from the short-term study and highlights how the antenna position at the time of the measurement may influence the pointing bias estimates. Based on the long-term results, a combination of the two sun-based methods for detection and simultaneous quantification of the pointing bias and the system leveling error is discussed.
Estimation of the exchange of seawater of various properties between the Arctic and North Atlantic Oceans presents a challenging observational problem. The strong current systems within Fram Strait induce recirculatio...
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Estimation of the exchange of seawater of various properties between the Arctic and North Atlantic Oceans presents a challenging observational problem. The strong current systems within Fram Strait induce recirculations and a turbulent ocean environment dominated by mesoscale variations of 4-10-km scale. By employing a simple parameterized model for mesoscale variability within Fram Strait, the authors examine the ability of a line array of closely spaced moorings and an acoustic tomography line to measure the average sound speed, a proxy variable for ocean temperature or heat content. Objective maps are employed to quantify the uncertainties resulting from the different measurement approaches. While measurements by a mooring line and tomography result in similar uncertainties in estimations of range and depth-averaged sound speed, the combination of the two approaches gives uncertainties 3 times smaller. The two measurements are sufficiently different as to be complementary;one measurement provides resolution for the aspects of the temperature section that the other misses. The parameterized model and its assumptions as to the magnitudes and scales of variability were tested by application to a hydrographic section across Fram Strait measured in 2011. This study supports the deployment of the 2013-16 Arctic Ocean under Melting Ice ( UNDER-ICE) network of tomographic transceivers spanning the ongoing moored array line across Fram Strait. Optimal estimation for this ocean environment may require combining disparate data types as constraints on a numerical ocean model using data assimilation.
This paper furthers previous investigations into the zenith angular effect of cloud contamination within infrared (IR) window radiance observations commonly used in the retrieval of environmental data records (EDRs). ...
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This paper furthers previous investigations into the zenith angular effect of cloud contamination within infrared (IR) window radiance observations commonly used in the retrieval of environmental data records (EDRs). Here analyses were performed of clear-sky forward radiance calculations versus observations obtained under clear to partly cloudy conditions over ocean. The authors utilized high-resolution IR spectra observed by the aircraft-based National Polar-Orbiting Partnership (NPP) Aircraft Sounder Test Bed-Interferometer (NAST-I) during the Joint Airborne Infrared Atmospheric Sounding Interferometer (IASI) Validation Experiment (JAIVEx) and performed forward calculations using collocated dropsondes. An aerosol optical depth EDR product derived from Geostationary Operational Environmental Satellite (GOES) was then applied to detect clouds within NAST-I fields of view (FOVs). To calculate the angular variation of clouds, expressions were derived for estimating cloud aspect ratios from visible imagery where cloud shadow lengths can be estimated relative to cloud horizontal diameters. In agreement with sensitivity calculations, it was found that a small cloud fraction within window radiance observations can have a measurable impact on the angular agreement with clear-sky calculations on the order of 0.1-0.4K in brightness temperature. It was also found that systematic sun-glint contamination can likewise have an impact on the order of 0.1 K. These results are germane to IR sensor data record (SDR) calibration/validation and EDR retrieval schemes depending upon clear-sky SDRs, as well as radiative transfer modeling involving randomly distributed broken cloud fields.
The role of the ocean in Earth's climate is fundamentally influenced by the locally dominant stratifying property (heat or salt), which in turn can be used to categorize the ocean into three classes: alpha, beta, ...
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The role of the ocean in Earth's climate is fundamentally influenced by the locally dominant stratifying property (heat or salt), which in turn can be used to categorize the ocean into three classes: alpha, beta, and transition zone oceans. Alpha and beta oceans are regions where the stratification is permanently set by heat and salt, respectively. Transition zone oceans exist between alpha and beta oceans and are regions where the stratification is seasonally or intermittently set by heat or salt. Despite their large ranges of temperature and salinity, transition zone oceans are the most weakly stratified regions of the upper oceans, making them ideal locations for thermobaric effects arising from the nonlinear equation of state of seawater. Here a novel definition and quantification of alpha, beta, and transition zone oceans is presented and used to analyze 4 years (2010-13) of hydrographic data developed from the Argo profiling float array. Two types of thermobaric instabilities are defined and identified in the hydrographic data. The first type arises from the vertical relocation of individual water parcels. The second type is novel and relates to the effect of pressure on the stratification through the pressure dependence of the thermal expansion coefficient;water that is stably stratified for one pressure is not necessarily stable for other pressures. The upper 1500 m of the global ocean is composed of 67% alpha, 15% beta, and 17% transition zone oceans, with 5.7% identified as thermobarically unstable. Over 63% of these thermobarically unstable waters exist in transition zone oceans, suggesting that these are important locations for efficient vertical transport of water-mass properties.
Interest in validating the eddy covariance (EC) technique under wave-induced flows led to a series of experiments in a 104-m-long large wave flume (LWF) using an acoustic Doppler velocimeter (ADV) and two oxygen micro...
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Interest in validating the eddy covariance (EC) technique under wave-induced flows led to a series of experiments in a 104-m-long large wave flume (LWF) using an acoustic Doppler velocimeter (ADV) and two oxygen microelectrodes (tips ~2 mm apart) mounted on a sturdy tripod. Four additional ADVs positioned within the flume provided comparative near-bed velocity measurements during experiments with irregular waves over a sand bed. These measurements revealed that modifications of local turbulence by the tripod frame were insignificant. However, errors in velocity measurements were at times observed for setups where the microelectrode tips protruded into the ADV's measurement volume. Disparate oxygen microelectrode velocity effects (stirring sensitivities) combined with response time offsets were also identified as problems, adding biases to EC flux derivations. Microelectrode velocity effects were further investigated through modeling designed to mimic the LWF data, and through examination of a 12-h dataset from the Oregon shelf. The modeling showed that under progressive waves, an artificial EC flux, or bias, arises most severely when the velocity sensitivity of the microelectrode is unequal in opposing flow directions or augmented by horizontal currents, and the velocity and oxygen data are not perfectly aligned in time. Sensitivities to wave motions were seen in the oxygen measurements from the Oregon shelf, contributing to an average flux of +2.7 +/- 0.6 mmol m(-2) day(-1) (SE, n = 22) at wave frequencies. Since overall EC fluxes equaled only -4.1 +/- 1.8 mmol m(-2) day(-1) (SE, n = 22), sources of EC biasing coupled to waves cannot be ruled out as potential problems for estimating exact benthic oxygen fluxes under common continental shelf field conditions.
To improve the prediction of heavy rainfall in southern China during the prerainy season, horizontal wind data from wind profiling radars (WPRs) were assimilated in the partial-cycle data assimilation (DA) system base...
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To improve the prediction of heavy rainfall in southern China during the prerainy season, horizontal wind data from wind profiling radars (WPRs) were assimilated in the partial-cycle data assimilation (DA) system based on a three-dimensional variational method. The analyses from the DA system are used as initial conditions for the convection-permitting Global/Regional Assimilation and Prediction System (GRAPES) model. The impact of assimilating WPR data on the quantitative precipitation forecast (QPF) in southern China was evaluated over the period of the Southern China Monsoon Rainfall Experiment (SCMREX) in May 2014, by comparing the results of a control experiment with WPR data assimilated and a denial experiment without WPR data. The positive impact of WPR DA was significant on the forecasts of atmospheric variables in the vertical and diagnostic fields at the surface, especially those of surface wind fields in the 0-6-h range. The inclusion of WPR data also improved the QPF skill of light and heavy rainfall throughout the 12-h forecast period by reducing the predicted spurious precipitation (thereby alleviating overestimations and false alarms), with the largest improvement in 6-h heavy rainfall forecasts. WPR DA considerably alleviated the spinup problem, remarkably improving the QPF of heavy rainfall (especially extreme rainfall). The improved representation of wind and moisture at lower levels in the analyses due to WPR DA was the physical cause of the QPF improvement, as is illustrated using a case study.
With the development of convection-permitting numerical weather prediction the efficient use of high-resolution observations in data assimilation is becoming increasingly important. The operational assimilation of the...
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With the development of convection-permitting numerical weather prediction the efficient use of high-resolution observations in data assimilation is becoming increasingly important. The operational assimilation of these observations, such as Doppler radar radial winds (DRWs), is now common, although to avoid violating the assumption of uncorrelated observation errors the observation density is severely reduced. To improve the quantity of observations used and the impact that they have on the forecast requires the introduction of the full, potentially correlated, error statistics. In this work, observation error statistics are calculated for the DRWs that are assimilated into the Met Office high-resolution U.K. model (UKV) using a diagnostic thatmakes use of statistical averages of observation-minus-background and observation-minus-analysis residuals. This is the first in-depth study using the diagnostic to estimate both horizontal and along-beam observation error statistics. The new results obtained show that the DRW error standard deviations are similar to those used operationally and increase as the observation height increases. Surprisingly, the estimated observation error correlation length scales are longer than the operational thinning distance. They are dependent both on the height of the observation and on the distance of the observation away from the radar. Further tests show that the long correlations cannot be attributed to the background error covariance matrix used in the assimilation, although they are, in part, a result of using superobservations and a simplified observation operator. The inclusion of correlated error statistics in the assimilation allows less thinning of the data and hence better use of the high-resolution observations.
Physical uncertainties in global-warming projections are dominated by uncertainties about how the fraction of incoming shortwave radiation that clouds reflect will change as greenhouse gas concentrations rise. Differe...
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Physical uncertainties in global-warming projections are dominated by uncertainties about how the fraction of incoming shortwave radiation that clouds reflect will change as greenhouse gas concentrations rise. Differences in the shortwave reflection by low clouds over tropical oceans alone account for more than half of the variance of the equilibrium climate sensitivity (ECS) among climate models, which ranges from 2.1 to 4.7 K. Space-based measurements now provide an opportunity to assess how well models reproduce temporal variations of this shortwave reflection on seasonal to interannual time scales. Here such space-based measurements are used to show that shortwave reflection by low clouds over tropical oceans decreases robustly when the underlying surface warms, for example, by -(0.96 +/- 0.22)% K-1 (90% confidence level) for de-seasonalized variations. Additionally, the temporal covariance of low-cloud reflection with temperature in historical simulations with current climate models correlates strongly (r = -0.67) with the models' ECS. Therefore, measurements of temporal low-cloud variations can be used to constrain ECS estimates based on climate models. An information-theoretic weighting of climate models by how well they reproduce the measured deseasonalized covariance of shortwave cloud reflection with temperature yields a most likely ECS estimate around 4.0 K;an ECS below 2.3K becomes very unlikely (90% confidence).
Use of an airborne platform to determine the dynamics of atmospheric motion has been ongoing for over three decades. Much of the effort has been centered on the determination of the horizontal pressure gradient force ...
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Use of an airborne platform to determine the dynamics of atmospheric motion has been ongoing for over three decades. Much of the effort has been centered on the determination of the horizontal pressure gradient force along an isobaric surface, and with wind measurements the nongeostrophic components of motion can be obtained. Recent advances using differential GPS-based altitude measurements allow accurate assessment of the geostrophic wind. Porpoise or sawtooth maneuvers are used to determine the vertical cross section of the horizontal pressure gradient force. D-values, the difference of the height of a given pressure level from that in a reference atmosphere, are used to isolate the vertical structure of the horizontal component of the pressure gradient force from the vastly larger hydrostatic pressure gradient. Comparison of measured D-value cross sections with airborne measurements of the horizontal pressure gradient is shown. Comparison of D-values with output from the WRF Model demonstrates that the airborne measurements are consistent with finescale numerical simulations. This technique provides a means of inferring the thermal wind, thereby enabling a detailed examination of the vertical structure of the forcing of mesoscale and synoptic-scale wind regimes.
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