Eric Wilcox | Desert Research Institute (original) (raw)

Papers by Eric Wilcox

Journal of Climate, Nov 1, 2003

Testing general circulation model parameterizations against observations is traditionally done by... more Testing general circulation model parameterizations against observations is traditionally done by comparing simulated and observed, time-averaged quantities, such as monthly mean cloud cover, evaluated on a stationary grid. This approach ignores the dynamical aspects of clouds, such as their life cycle characteristics, spatial coverage, temporal duration, and internal variability. In this study, a complementary Lagrangian approach to the validation of modeled tropical cloudiness is explored. An automated cloud detection and tracking algorithm is used to observe and track overcast decks of cloud in a consecutive set of hourly Meteosat-5 images and the National Center for Atmospheric Research Community Climate Model version 3 (NCAR CCM3). The algorithm is applied to the deep convective cloud systems of the tropical Indian Ocean region during a 49-day period of the 1999 winter monsoon season. Observations of precipitation are taken from the Tropical Rainfall Measuring Mission (TRMM) satellite in addition to a Meteosat-5 infrared rainfall technique that is calibrated using the TRMM data.Clouds, defined as overcast decks, are observed spanning spatial scales from 25 km2 to greater than 107 km2, as well as temporal scales from 1 h to greater than 100 h. Semipermanent decks of anvil and cirrus cloud, with numerous regions of deep convection embedded within, dominate total cloud cover. Bridging between convective centers within the deck by cirrus clouds, particularly during the suppressed portion of the diurnal cycle of convection, may help to maintain the integrity of semipermanent overcast decks over long timescales. At scales greater than 106 km2 the size distribution of simulated clouds is biased such that the dominant scale of clouds is several million square kilometers larger than the dominant scale of observed clouds. Virtually all of the simulated precipitation occurs at rain rates lower than 2 mm h-1, while as much as 25% of observed precipitation occurs at rain rates higher than 2 mm h-1. Precipitation associated with deep convection in observed semipermanent cloud systems is organized into more localized mesoscale structures of adjacent convective cells attached to stratiform precipitation regions. A separate analysis of TRMM data by Wilcox and Ramanathan determined that such structures can exceed the size of grid cells in coarse-grid global models and have area-averaged rain rates up to and exceeding 2 mm h-1. These mesoscale convective systems are responsible for the extreme, episodic precipitation events that are not parameterized in the model. The simulated cloud systems gently precipitate throughout their duration and everywhere within their boundaries. The model lacks a process that acts to organize the convective cells within fewer grid cells, in addition to a representation of the observed stratiform precipitation structures. A modification to CCM3 is tested that is intended to account for the evaporation of upper-level precipitation in midlevel mesoscale downdrafts. The modification results in only a slight change in domain-averaged precipitation. However, it causes a significant shift in the distribution of precipitation toward higher rain rates that is more consistent with the distribution of TRMM observed rain rates. The modification demonstrates the sensitivity of the model to one important component of mesoscale organized convection.

Aerosol radiative forcing has likely impacted recent changes in regional and global precipitation... more Aerosol radiative forcing has likely impacted recent changes in regional and global precipitation. Purely scattering aerosols cool the surface, while absorbing aerosols both heat the atmosphere and cool the surface. These effects, particularly those of the absorbing aerosols, stabilize the atmosphere and reduce the need for precipitation and latent heating, however the regional response of precipitation to aerosols may be further complicated if aerosols modify the cloud field. During the dry season, the Amazon provides an ideal region to test the impacts of aerosols on regional hydrology. Absorbing smoke aerosols both severely impact the clear-sky radiative budget and significantly reduce cloud cover. In this study, we use long-term remote sensing and radiometric data together with a radiative transfer model and climate model simulations to investigate the links between aerosol radiative effects, the surface and atmosphere energy budgets, and possible links to precipitation. The changes in radiative fluxes at the surface owing to published estimates of decreases in cloud cover with increasing smoke aerosol are comparable in magnitude to the changes in the clear-sky radiative forcing attributable to the direct effect of the aerosols alone. The net radiative effect at the surface of combined changes in aerosols and clouds over the Amazon is a cooling only because the longwave and shortwave components of the change in cloud radiative forcing largely compensate. Reductions in precipitation may be significant depending, in part, upon the magnitudes of moisture recycling and the sensitivity of the Bowen Ratio to surface cooling.

This study investigates the sensitivity of aerosol concentrations in the atmosphere to the spatia... more This study investigates the sensitivity of aerosol concentrations in the atmosphere to the spatial and temporal distribution of precipitation by integrating a high resolution, 3-hourly, multi-platform satellite precipitation dataset into the wet deposition scheme of the MATCH chemical transport model. Precipitation is the dominant contributor to the removal of aerosol, acting through both the removal of cloud drops seeded by aerosol particles, as well as the collision of raindrops with particles below cloud. To date, budget studies of aerosol species have relied upon numerical representations of precipitation typical of climate models, which in the tropics are primarily parameterizations of deep, moist convection. Significant biases in the spatial and temporal distributions of precipitation in a modern convection parameterization are apparent when compared with satellite observations of precipitation, for example underestimating the occurrence of extreme precipitation events and overestimating the occurrence of drizzle events. Furthermore, assumptions are often made within scavenging parameterizations about the fraction of each model grid cell occupied by precipitation, which have received limited validation. A series of month-long simulations are made with MATCH applying a steady, uniform aerosol source from the Indian Subcontinent and subjecting the aerosol separately to scavenging based on model simulated precipitation and satellite observed precipitation. The sensitivity of the horizontal and vertical distribution of aerosol over the Indian Ocean basin to biases in grid cell mean rain rate, the frequency of occurrence of precipitating grid cells, and the grid cell fraction containing precipitation are investigated. Also considered is the amount of aerosol that manages to avoid scavenging and is thereby made available for transport to remote regions, such as across the ITCZ to the Southern Indian Ocean, and to the upper-troposphere.

Environmental Research Letters, 2014

Mechanistic relationships exist between variability of dust in the oceanic Saharan air layer (OSA... more Mechanistic relationships exist between variability of dust in the oceanic Saharan air layer (OSAL) and transient changes in the dynamics of Western Africa and the tropical Atlantic Ocean. This study provides evidence of possible interactions between dust in the OSAL region and African easterly jet-African easterly wave (AEJ-AEW) system in the climatology of boreal summer, when easterly wave activity peaks. Synoptic-scale changes in instability and precipitation in the African/Atlantic intertropical convergence zone are correlated with enhanced aerosol optical depth (AOD) in the OSAL region in response to anomalous 3D overturning circulations and upstream/downstream thermal anomalies at above and below the mean-AEJ level. Upstream and downstream anomalies are referred to the daily thermal/dynamical changes over the West African monsoon region and the Eastern Atlantic Ocean, respectively. Our hypothesis is that AOD in the OSAL is positively correlated with the downstream AEWs and negatively correlated with the upstream waves from climatological perspective. The similarity between the 3D pattern of thermal/dynamical anomalies correlated with dust outbreaks and those of AEWs provides a mechanism for dust radiative heating in the atmosphere to reinforce AEW activity. We proposed that the interactions of OSAL dust with regional climate mainly occur through coupling of dust with the AEWs.

Tellus B, 2004

A B S T R A C T Deposition of atmospheric aerosols by precipitation is investigated by assimilati... more A B S T R A C T Deposition of atmospheric aerosols by precipitation is investigated by assimilating near-global 3-hourly precipitation estimates from satellites at 0.25 • resolution into a global chemical transport model. Simulations of 21 d duration during February 2002 are performed where satellite observations of surface rain rate are incorporated into the computation of the aerosol removal rate. A steady source of a tracer simulating a soluble aerosol species is imposed on the bottom layer of the model over the Indian Subcontinent at a rate of 0.08 µg m −2 s −1 . The impacts of wet deposition by observed precipitation patterns upon regional-scale and global-scale transport of aerosols are examined. The results are compared with a model simulation where wet deposition has been turned off and a simulation which uses modelsimulated precipitation to compute wet deposition. Observed precipitation patterns are responsible for reducing aerosol amounts over the Indian Ocean basin by about 25% compared with a case with no wet deposition. Over the remainder of the Northern Hemisphere the aerosol amounts are reduced by nearly 80%. Precipitation in the region of the intertropical convergence zone (ITCZ) acts as an effective barrier to the transport of aerosol to the Southern Hemisphere. Virtually all of the wet deposition over the Indian Ocean occurs in grid cells experiencing rain rates of less than 1 mm h −1 , even though these events account for only 20% of the total precipitation observed in the region. Light-rain events to the north of the ITCZ occur frequently enough, and scavenge aerosol efficiently enough, to remove much of the aerosol before southward-moving air masses reach the ITCZ. Model-simulated precipitation over the Indian Ocean occurs more frequently than observed by satellite, by as much as 50%, and is characterized by more drizzle events and fewer heavyrain events than observed. The differences in simulated and observed rain rate distributions result in only a 6% difference in aerosol amount over the Indian Ocean, and an 8% difference over the remainder of the Northern Hemisphere. Satellite observations also reveal that rain events covering more than 80% of the horizontal area of a model grid cell occur up to an order of magnitude more frequently than predicted by a common parametrization of precipitation scavenging. A test of wet deposition based on the observed spatial coverage of surface precipitation suggests that incorrect assumptions about the physical sizes of rain events may result in as much as a factor of 2 over-estimate of the amount of aerosol transported out of the South Asia region to the remainder of the Northern Hemisphere in global atmospheric transport models.

Physical Review Letters, 1996

Large negative magnetoresistance and anomalous magnetic properties are found in amorphous Si dope... more Large negative magnetoresistance and anomalous magnetic properties are found in amorphous Si doped with magnetic rare earth ions near the metal-insulator transition. The resistivity below 50 K rises orders of magnitude above that of comparable composition nonmagnetic alloys and is strongly reduced by a magnetic field. Magnetization measurements show noninteracting moments at high temperature which develop antiferromagnetic interactions below 50 K. We suggest that these results are due to formation below 50 K of a dense concentration of magnetic polarons which localize conduction electrons.

Journal of Geophysical Research, 2009

... Atlantic southern African region investigated in this study, a systematic difference between ... more ... Atlantic southern African region investigated in this study, a systematic difference between AMSR-E and MODIS LWP retrievals is found for stratocumulus clouds over three biomass burning months in 2005 and 2006 that is consistent with above-cloud absorbing aerosols. ...

Journal of Climate, 2001

Clouds exert a thermodynamic forcing on the ocean-atmosphere column through latent heating, owing... more Clouds exert a thermodynamic forcing on the ocean-atmosphere column through latent heating, owing to the production of rain, and through cloud radiative forcing, owing to the absorption of terrestrial infrared energy and the reflection of solar energy. The Tropical Rainfall Measuring Mission (TRMM) satellite provides, for the first time, simultaneous measurements of each of these processes on the spatial scales

Journal of Climate, 2007

The frequency distributions of surface rain rate are evaluated in the Tropical Rainfall Measuring... more The frequency distributions of surface rain rate are evaluated in the Tropical Rainfall Measuring Mission (TRMM) and Special Sensor Microwave/Imager (SSM/I) satellite observations and the NOAA/GFDL global atmosphere model version 2 (AM2). Instantaneous satellite rain-rate observations averaged over the 2.5°latitude ϫ 2°longitude model grid are shown to be representative of the half-hour rain rate from single time steps simulated by the model. Rain-rate events exceeding 10 mm h Ϫ1 are observed by satellites in most regions, with 1 mm h Ϫ1 events occurring more than two orders of magnitude more frequently than 10 mm h Ϫ1 events. A model simulation using the relaxed Arakawa-Schubert (RAS) formulation of cumulus convection exhibits a strong bias toward many more light rain events compared to the observations and far too few heavy rain events. A simulation using an alternative convection scheme, which includes an explicit representation of mesoscale circulations and an alternative formulation of the closure, exhibits, among other differences, an order of magnitude more tropical rain events above the 5 mm h Ϫ1 rate compared to the RAS simulation. This simulation demonstrates that global atmospheric models can be made to produce heavy rain events, in some cases even exceeding the observed frequency of such events. Additional simulations reveal that the frequency distribution of the surface rain rate in the GCM is shaped by a variety of components within the convection parameterization, including the closure, convective triggers, the spectrum of convective and mesoscale clouds, and other parameters whose physical basis is currently only understood to a limited extent. Furthermore, these components interact nonlinearly such that the sensitivity of the rain-rate distribution to the formulation of one component may depend on the formulation of the others. Two simulations using different convection parameterizations are performed using perturbed sea surface temperatures as a surrogate for greenhouse gas-forced climate warming. Changes in the frequency of rain events greater than 2 mm h Ϫ1 associated with changing the convection scheme in the model are greater than the changes in the frequency of heavy rain events associated with a 2-K warming using either model. Thus, uncertainty persists with respect to simulating intensity distributions for precipitation and projecting their future changes. Improving the representation of the frequency distribution of rain rates will rely on refinements in the formulation of cumulus closure and the other components of convection schemes, and greater certainty in predictions of future changes in both total rainfall and in rain-rate distributions will require additional refinements in those parameterizations that determine the cloud and water vapor feedbacks.

Journal of Climate, 2003

Geophysical Research Letters, 2011

Geophysical Research Letters, 2010

Geophysical Research Letters, 2006

1] Aerosols over the Northeastern Pacific Ocean enhance the cloud drop number concentration and r... more 1] Aerosols over the Northeastern Pacific Ocean enhance the cloud drop number concentration and reduce the drop size for marine stratocumulus and cumulus clouds. These microphysical effects result in brighter clouds, as evidenced by a combination of aircraft and satellite observations. In-situ measurements from the Cloud Indirect Forcing Experiment (CIFEX) indicate that the mean cloud drop number concentration in low clouds over the polluted marine boundary layer is greater by 53 cm À3 compared to clean clouds, and the mean cloud drop effective radius is smaller by 4 mm. We link these in-situ measurements of cloud modification by aerosols, for the first time, with collocated satellite broadband radiative flux observations from the Clouds and the Earth's Radiant Energy System to show that these microphysical effects of aerosols enhance the top-of-atmosphere cooling by À9.9 ± 4.3 W m À2 for overcast conditions. Citation: Wilcox, E. M., G. Roberts, and V. , Influence of aerosols on the shortwave cloud radiative forcing from North Pacific oceanic clouds: Results from the Cloud Indirect Forcing Experiment (CIFEX), Geophys.

Geophysical Research Letters, 2012

Atmospheric Research, 2014

The objective of this review is to discuss physical processes over a wide range of spatial scales... more The objective of this review is to discuss physical processes over a wide range of spatial scales that govern the formation, evolution, and dissipation of marine fog. We consider marine fog as the collective combination of fog over the open sea along with coastal sea fog and coastal land fog. The review includes a history of sea fog research, field programs, forecasting methods, and detection of sea fog via satellite observations where similarity in radiative properties of fog top and the underlying sea induce further complexity. The main thrust of the study is to provide insight into causality of fog including its initiation, maintenance, and destruction. The interplay between the various physical processes behind the several stages of marine fog is among the most challenging aspects of the problem. An effort is made to identify this interplay between processes that include the microphysics of fog formation and maintenance, the influence of large-scale circulations and precipitation/clouds, radiation, turbulence (air-sea interaction), and advection. The environmental impact of marine fog is also addressed. The study concludes with an assessment of our current knowledge of the phenomenon, our principal areas of ignorance, and future lines of research that hold promise for advances in our understanding.

Atmospheric Chemistry and Physics, 2010

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly ... more Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion

Atmospheric Chemistry and Physics, 2012

Page 1. ACPD 11, 20947–20972, 2011 Radiative forcing of smoke aerosols over clouds EMWilcox Title... more Page 1. ACPD 11, 20947–20972, 2011 Radiative forcing of smoke aerosols over clouds EMWilcox Title Page ... smoke aerosols over clouds EM Wilcox Desert Research Institute, Reno, Nevada, USA Received: 4 June 2011 – Accepted: 8 July 2011 – Published: 25 July 2011 ...

Annales Geophysicae, 2009

Aerosol perturbations over selected land regions are imposed in Version-4 of the Goddard Earth Ob... more Aerosol perturbations over selected land regions are imposed in Version-4 of the Goddard Earth Observing System (GEOS-4) general circulation model (GCM) to assess the influence of increasing aerosol concentrations on regional circulation patterns and precipitation in four selected regions: India, Africa, and North and South America. Part 1 of this paper addresses the responses to aerosol perturbations in India and Africa. This paper presents the same for aerosol perturbations over the Americas. GEOS-4 is forced with prescribed aerosols based on climatological data, which interact with clouds using a prognostic scheme for cloud microphysics including aerosol nucleation of water and ice cloud hydrometeors. In clear-sky conditions the aerosols interact with radiation. Thus the model includes comprehensive physics describing the aerosol direct and indirect effects on climate (hereafter ADE and AIE respectively). Each simulation is started from analyzed initial conditions for 1 May and was integrated through June-July-August of each of the six years: 1982-1987 to provide a 6-ensemble set. Results are presented for the difference between simulations with double the climatological aerosol concentration and one-half the climatological aerosol concentration for three experiments: two where the ADE and AIE are applied separately and one in which both the ADE and AIE are applied. The ADE and AIE both yield reductions in net radiation at the top of the atmosphere and surface while the direct absorption of shortwave radiation contributes a net radiative heating in the atmosphere. A large net heating of the atmosphere is also apparent over the subtropical North Atlantic Ocean that is attributable to the large aerosol perturbation imposed over Africa. This atmospheric warming and the depression of the surface pressure over North America contribute to a northward shift of the inter-Tropical Convergence Zone over northern South