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Protat A.,Center for Australian Weather and Climate Research | Protat A.,Laboratoire Atmosphere Milieux Observations Spatiales | Williams C.R.,Cooperative Institute for Research in Environmental Science
Journal of Applied Meteorology and Climatology | Year: 2011

Doppler radar measurements at different frequencies (50 and 2835 MHz) are used to characterize the terminal fall speed of hydrometeors and the vertical air motion in tropical ice clouds and to evaluate statistical methods for retrieving these two parameters using a single vertically pointing cloud radar. For the observed vertical air motions, it is found that the mean vertical air velocity in ice clouds is small on average, as is assumed in terminal fall speed retrieval methods. The mean vertical air motions are slightly negative (downdraft) between the melting layer (5-km height) and 6.3-km height, and positive (updraft) above this altitude, with two peaks of 6 and 7 cm s-1 at 7.7- and 9.7-km height. For the retrieved hydrometeor terminal fall speeds, it is found that the variability of terminal fall speeds within narrow reflectivity ranges is typically within the acceptable uncertainties for using terminal fall speeds in ice cloud microphysical retrievals. This study also evaluates the performance of previously published statistical methods of separating terminal fall speed and vertical air velocity from vertically pointing Doppler radar measurements using the 50-/2835-MHz radar retrievals as a reference. It is found that the variability of the terminal fall speed-radar reflectivity relationship (Vt-Ze) is large in ice clouds and cannot be parameterized accurately with a single relationship.A well-defined linear relationship is found between the two coefficients of a power-law Vt-Ze relationship, but a more accurate microphysical retrieval is obtained using Doppler velocity measurements to better constrain the Vt-Ze relationship for each cloud. When comparing the different statistical methods to the reference, the distribution of terminal fall speed residual is wide, with most residuals being in the ±30-40 cm s-1 range about the mean. The typical mean residual ranged from 15 to 20 cm s-1, with different methods having mean residuals of ,10 cm s-1 at some heights, but not at the same heights for all methods. The so-called Vt-Ze technique was the most accurate above 9-km height, and the running-mean technique outperformed the other techniques below 9-km height. Sensitivity tests of the running-mean technique indicate that the 20-min average is the best trade-off for the type of ice clouds considered in this analysis. A new technique is proposed that incorporates simple averages of Doppler velocity for each (Ze,H) couple in a given cloud. This technique, referred to as DOP-Ze-H, was found to outperform the three other methods at most heights, with a mean terminal fall residual of < 10 cm s-1 at all heights. This error magnitude is compatible with the use of such retrieved terminal fall speeds for the retrieval of microphysical properties. © 2011 American Meteorological Society.

Roberts J.B.,Florida State University | Clayson C.A.,Florida State University | Robertson F.R.,NASA | Jackson D.L.,Cooperative Institute for Research in Environmental science
Journal of Geophysical Research: Atmospheres | Year: 2010

The turbulent heat fluxes play a pivotal role in the exchange of energy between the atmosphere and ocean. The calculation of these fluxes over the global oceans requires the use of bulk aerodynamic or flux-gradient methods that rely on estimates of the sea surface temperature (SST), near-surface wind speed, air temperature, and specific humidity. Errors in current methodologies of satellite retrievals of near-surface properties have been shown to be the main sources of error for calculation of the fluxes. A new neural network technique is presented here that significantly improves the error characteristics of the air temperature and specific humidity compared to previous methods. Improvements in predicting near-surface wind speed and SST are also seen. Additional improvements are also made by accounting for the effects of high cloud liquid water contents, the effects of which can be mitigated through the use of regime-specific linear and nonlinear retrieval methods. The use of a first-guess SST is shown to result in significant improvement in retrieval accuracy. Copyright © 2010 by the American Geophysical Union.

Cesana G.,University Pierre and Marie Curie | Kay J.E.,NCAR | Chepfer H.,University Pierre and Marie Curie | English J.M.,NCAR | And 3 more authors.
Geophysical Research Letters | Year: 2012

Ground-based observations show that persistent liquid-containing Arctic clouds occur frequently and have a dominant influence on Arctic surface radiative fluxes. Yet, without a hemispheric multi-year perspective, the climate relevance of these intriguing Arctic cloud observations was previously unknown. In this study, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations are used to document cloud phase over the Arctic basin (60-82N) during a five-year period (2006-2011). Over Arctic ocean-covered areas, low-level liquid-containing clouds are prevalent in all seasons, especially in Fall. These new CALIPSO observations provide a unique and climate-relevant constraint on Arctic cloud processes. Evaluation of one climate model using a lidar simulator suggests a lack of liquid-containing Arctic clouds contributes to a lack of "radiatively opaque" states. The surface radiation biases found in this one model are found in multiple models, highlighting the need for improved modeling of Arctic cloud phase. © 2012. American Geophysical Union. All Rights Reserved.

Pegion P.J.,National Oceanic and Atmospheric Administration | Pegion P.J.,Cooperative Institute for Research in Environmental science | Kumar A.,National Oceanic and Atmospheric Administration
Journal of Climate | Year: 2010

A set of idealized global model experiments was performed by several modeling centers as part of the Drought Working Group of the U.S. Climate Variability and Predictability component of the World Climate Research Programme (CLIVAR). The purpose of the experiments was to assess the role of the leading modes of sea surface temperature (SST) variability on the climate over the continents, with particular emphasis on the influence of SSTs on surface climate variability and droughts over the United States. An analysis based on several models gives more creditability to the results since it relies on the assessment of impacts that are robust across different models. Coordinated atmospheric general circulation model (AGCM) simulations forced with three modes of SST variability were analyzed. The results showthat the SST-forced precipitation variability over the centralUnited States is dominated by the SST mode with maximum loading in the central Pacific Ocean. The SST mode with loading in the Atlantic Ocean, and a mode that is dominated by trends in SSTs, lead to a smaller response. Based on the response to the idealized SSTs, the precipitation response for the twentieth century was also reconstructed. A comparison with the Atmospheric Model Intercomparison Project (AMIP) simulations forced with the observed SSTs illustrates that the reconstructed precipitation variability was similar to the one in the AMIP simulations, further supporting the conclusion that the SST modes identified in the present analysis play a dominant role in the precipitation variability over the United States. One notable exception is the Dust Bowl of the 1930s, and further analysis regarding this major climate extreme is discussed. © 2010 American Meteorological Society.

Cucurull L.,National Oceanic and Atmospheric Administration | Cucurull L.,Cooperative Institute for Research in Environmental science
Atmospheric Measurement Techniques | Year: 2015

In preparation for the launch of the first six satellites of the COSMIC-2 mission in equatorial orbit, and the larger number of observations that such a mission will provide in the lower tropical troposphere, work is underway at the National Oceanic and Atmospheric Administration (NOAA) to improve the assimilation of radio occultation (RO) observations, particularly in the lower tropical troposphere. As part of the improvement of the bending angle forward operator at the National Centers for Environmental Prediction (NCEP), additional quality controls aimed to detect and reject observations that might have been affected by super-refraction conditions have been implemented and tested. The updated quality control procedures also address the situation where the model detects atmospheric super-refraction conditions. This paper describes the limitations of the current standard quality controls and discusses the implementation of additional quality control procedures to address the limitations of assimilating observations likely affected by the super-refraction conditions, either in the model simulation or in the retrieval process. © 2015 Author(s).

Box J.E.,Ohio State University | Colgan W.,Geological Survey of Denmark | Colgan W.,Cooperative Institute for Research in Environmental science
Journal of Climate | Year: 2013

Greenland ice sheet mass loss to the marine environment occurs by some combination of iceberg calving and underwater melting (referred to here as marine ice loss, LM). This study quantifies the relation between LM and meltwater runoff(R) at the ice sheet scale. A theoretical basis is presented explaining how variability in R can be expected to govern much of the LM variability over annual to decadal time scales. It is found that R enhances LM through three processes: 1) increased glacier discharge by ice warming-softening and basal lubrication-sliding; 2) increased calving susceptibility through undercutting glacier front geometry and reducing ice integrity; and 3) increased underwater melting from forcing marine convection. Applying a semiempirical LM f(R) parameterization to a surfacemass balance reconstruction enables total ice sheet mass budget closure over the 1840-2010 period. The estimated cumulative 171-yr net ice sheet sea level contribution is 25±10mm, the rise punctuated by periods of ice sheet netmass gain (sea level drawdown) (1893-1900, 1938-47, and 1972-98). The sea level contribution accelerated at 27.6mmyr-1 century-1 over the entire reconstruction, reaching a peak sea level rise contribution of 6.1mmdecade-1 during 2002-10. © 2013 American Meteorological Society.

DuVivier A.K.,Cooperative Institute for Research in Environmental science | DuVivier A.K.,University of Colorado at Boulder | Cassano J.J.,Cooperative Institute for Research in Environmental science | Cassano J.J.,University of Colorado at Boulder
Climate Dynamics | Year: 2015

Strong, mesoscale tip jets and barrier winds that occur over the ocean near southern Greenland have the potential for impacting deep convection in the ocean. The self-organizing map (SOM) training algorithm was used to identify and classify the range of 10 m wind patterns present during ten winters (1997–2007, NDJFM) in the ECMWF interim reanalysis (ERA-I) and from a regional simulation using the weather research and forecasting (WRF) model at 50 km into a SOM. The SOM is used to identify differences in the manifestation of westerly tip jets, easterly tip jets, and barrier flow. The North Atlantic Oscillation (NAO) index is well correlated with the type of tip jet present at Cape Farewell, but the NAO was not well correlated with the absence or presence of barrier flow. WRF simulated patterns with strong barrier-parallel flow more frequently than ERA-I, and WRF also had faster coastal winds than ERA-I during all types of strong wind events. The difference in coastal winds is likely related to model resolution and the resulting ability of each model to simulate strong mesoscale winds that are driven by Greenland’s steep terrain. © 2015 Springer-Verlag Berlin Heidelberg

Macknick J.,National Renewable Energy Laboratory | Sattler S.,Union of Concerned Scientists | Averyt K.,Cooperative Institute for Research in Environmental science | Clemmer S.,Union of Concerned Scientists | Rogers J.,Union of Concerned Scientists
Environmental Research Letters | Year: 2012

The power sector withdraws more freshwater annually than any other sector in the US. The current portfolio of electricity generating technologies in the US has highly regionalized and technology-specific requirements for water. Water availability differs widely throughout the nation. As a result, assessments of water impacts from the power sector must have a high geographic resolution and consider regional, basin-level differences. The US electricity portfolio is expected to evolve in coming years, shaped by various policy and economic drivers on the international, national and regional level; that evolution will impact power sector water demands. Analysis of future electricity scenarios that incorporate technology options and constraints can provide useful insights about water impacts related to changes to the technology mix. Utilizing outputs from the regional energy deployment system (ReEDS) model, a national electricity sector capacity expansion model with high geographical resolution, we explore potential changes in water use by the US electric sector over the next four decades under various low carbon energy scenarios, nationally and regionally. © 2012 IOP Publishing Ltd.

Joughin I.,University of Washington | Smith B.E.,University of Washington | Abdalati W.,Cooperative Institute for Research in Environmental science | Abdalati W.,University of Colorado at Boulder
Journal of Glaciology | Year: 2011

Spaceborne interferometric synthetic aperture radar (InSAR) techniques for measuring ice flow velocity and topography have developed rapidly over the last decade and a half, revolutionizing the study of ice dynamics. Spaceborne interferometry has contributed to major progress in many areas of glaciological study by: providing the first comprehensive measurements of ice-stream flow velocity over the major outlets of Greenland and Antarctica; revealing that ice-stream and outlet-glacier flow can change rapidly (months to years); improving understanding of several ice-sheet and ice-shelf processes; providing velocity for flux-gate based mass-balance assessment; mapping flow of mountain glaciers; and capturing the geomorphic traces of past ice flow. We review the basic technique development, the measurement characteristics, and the extensive set of results yielded by these measurements.

Molnar P.,University of Colorado at Boulder | Molnar P.,Cooperative Institute for Research in Environmental science | Rajagopalan B.,Cooperative Institute for Research in Environmental science | Rajagopalan B.,University of Colorado at Boulder
Geophysical Research Letters | Year: 2012

Many geologic observations suggest that the eastern portion of the Tibetan Plateau has expanded eastward and grown in height over the past 15-10Ma, and other observations suggest that climate over the northwestern Indian subcontinent ("NW India") became more arid between ∼11 and 7Ma. We suggest that they are linked: higher terrain increased orographic precipitation over eastern Tibet, which diabatically heated the atmosphere there; that heating then forced subsidence to the west via the mechanism proposed by Rodwell and Hoskins; and subsidence suppressed rainfall. Simple Gill-model calculations for a heat source displaced from the equator show such subsidence, and a regression of July-August outgoing long-wave radiation (OLR) over southeastern Tibet with rainfall shows a negative relationship between eastern Tibet and NW India, consistent with the proposed link between growth of eastern Tibet and diminished South Asian monsoon rainfall over the northwestern Indian subcontinent. Copyright 2012 by the American Geophysical Union.

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