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J. Song
and
W. Gao

Abstract

A method was investigated to estimate broadband surface shortwave albedo from the narrowband reflectances obtained by the Advanced Very High Resolution Radiometers (AVHRRs) on board the polar orbiting satellites. Field experiments were conducted to measure simultaneously multispectral narrowband reflectances and broadband albedo over various vegetation and soil surfaces. These data were combined to examine the behavior of narrowband-to-broadband (NTB) conversion factors for different surfaces. Many previous studies have used constant NTB conversion factors for the AVHRR data. The results from this investigation indicate that the optimal NTB conversion factors for AVHRR channels 1 and 2 have a strong dependence on the amount of green vegetation within the field of view. Two conversion factors, f 1 and f 2, were established to quantify, respectively, 1) the relationship between the reflectance in the narrow red wave band and the total reflectance within the whole visible subregion (0.3–0.685 μm) and 2) the relationship between the reflectance in the narrow near-infrared wave band and the total reflectance within the whole near-infrared subregion (0.685–2.8 μm). Values of f 1 and f 2, calculated from field data, correlated well with the normalized difference vegetation index (NDVI), largely because of the unique characteristics of light absorption and scattering within the red and near-infrared wave bands by green leaves. The f 1–NDVI and f 2–NDVI relationships developed from this study were used to infer empirical coefficients needed to estimate surface albedo from AVHRR data. The surface albedo values calculated by the new method agreed with ground-based measurements within a root-mean-square error of 0.02, which is better than other methods that use constant empirical coefficients. Testing with albedo measurements made by unmanned aerospace vehicles during a field campaign also indicates that the new method provides an improved estimate of surface albedo.

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W. Gao
and
B. L. Li

Abstract

Wavelet analysis was applied to turbulence data for temperature and vertical velocity within and above a deciduous forest. This method appears to provide an objective technique for examining thermal and flow fields associated with coherent structures occurring near the forest. The two-dimensional unfolding in time and scale by the wavelet transform illustrates discrete warm and cool centers associated with organized updrafts and downdrafts, which have similar patterns but different magnitudes at different heights. Wavelet variances computed for temperature and velocity at different heights appear to have local maximum values corresponding to certain time scales, which are self-consistent and useful for objective determination of the principal time scale of the structures. Within the canopy, the principal time scales of the structures determined by this technique are 56–60 s and 40–44 s for the temperature and vertical velocity fields, respectively. These time scales are close to those determined by the multilevel detection scheme used in a previous analysis. The temperature structures above the canopy have a shorter duration, but the rate of the decrease in the time scale with increasing height appears to be proportional to the increase in mean wind speed. The horizontal size of the structure determined by the product of local wind speed and the detected principal time scales is in the range of 83–112 m. The time scale of the structures identified in vertical velocity appears to be consistently smaller than that in the thermal field. The canopy structures show a smooth connection in the scale change with circulations of lower frequency (about 5–7 min) and merge into updrafts and downdrafts of these larger-scale circulations.

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Bo-Cai Gao
and
W. J. Wiscombe

Abstract

A method for detecting cirrus clouds in terms of brightness temperature differences between narrowbands at 8, 11, and 12 µm has been proposed by Ackerman et al. In this method, the variation of emissivity with wavelength for different surface targets was not taken into consideration. Based on state-of-the-art laboratory measurements of reflectance spectra of terrestrial materials by Salisbury and D'Aria, it is found that the brightness temperature differences between the 8- and 11-µm bands for soils, rocks and minerals, and dry vegetation can vary between approximately −8 and +8 K due solely to surface emissivity variations. The large brightness temperature differences are sufficient to cause false detection of cirrus clouds from remote sensing data acquired over certain surface targets using the 8-11-12-µm method directly. It is suggested that the 8-11-12-µm method should be improved to include the surface emissivity effects. In addition, it is recommended that in the future the variation of surface emissivity with wavelength should be taken into account in algorithms for retrieving surface temperatures and low-level atmospheric temperature and water vapor profiles.

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W. Gao
and
M. L. Wesely

Abstract

Turbulent fluxes of chemically reactive trace gases in the neutral atmospheric boundary layer (ABL) were simulated with a one-dimensional, coupled diffusion-chemistry model. The effects of rapid chemical reactions were included with a suite of second-order turbulence equations in which additional chemical terms were used to describe contributions to flux by rapid chemical production and loss. A total of 69 chemical reactions were incorporated to describe basic atmospheric photochemistry coupled with chemistry for isoprene and its oxidation products. Daytime flux Profiles of O3, NO, N02, OH, isoprene, and other depositing gases were simulated with assumed rates of NO emission from soil, isoprene emission rates appropriate for a deciduous forest, and initial concentrations of various chemical species typical of a remote area. Results show that chemical reactions can influence vertical fluxes by producing sources or sinks in the atmosphere and by changing mean concentrations. Magnitudes of NO and NO2 fluxes decrease with height at a much greater rate than predicted by a nonreactive model. The NO emitted from soil can quickly be converted to N02, and the upward NO flux can decrease by as much as 80% at a height of 100 m. The magnitude of NO2 flux decreases sharply with height because of the NO-to-NO2 conversion, but NO2 deposition near the surface tends to be enhanced by an increase in NO2 concentration near the surface NO emission source. The Profile of 03 flux simulated with forced entrainment at the top of the ABL closely matches the profile derived from a field experiment, and the flux throughout the ABL increases slightly because mean O3 concentrations are increased by chemical production associated with isoprene emissions. Simulated profiles of isoprene flux closely agree with results of a nonreactive model and appear to be controlled primarily by surface emission and vertical turbulent mixing. Chemical reactions appear to have a substantial effect on vertical concentration gradients, diffusivities, and deposition velocities for NO2, NO3, and N2O5. The reactions have a negligible effect on the deposition velocities for O3, HCHO, CH3OOH, HNO2, H2O2, and HNO3.

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Wen-wen Tung
,
Mitchell W. Moncrieff
, and
Jian-Bo Gao

Abstract

The multiscale tropical deep convective variability over the Pacific Ocean is examined with the 4-month high-resolution deep convection index ( I T BB ) derived from satellite imagery. With a systemic view, the complex phenomenon is described with succinct parameters known as generalized dimensions associated with the correlation structures embedded in the observed time series, with higher-order dimensions emphasizing extreme convective events. It is suggested that convective activities of lifetimes ranging from 1 h to ∼21 days have interdependence across scales that can be described by a series of power laws; hence, a spectrum of generalized dimensions, that is, the I T BB time series is multifractal. The spatiotemporal features of the I T BB time series is preliminarily examined by changing the spatial domain from 0.1° × 0.1° to 25° × 25°. The multifractal features are weakened with increasing strength of spatial averaging but cannot be eliminated. Furthermore, the I T BB data has the property of long-range dependency, implying that its autocorrelation function decays with a power law in contrast to the zero or exponentially decaying autocorrelation functions for white and commonly used red noise processes generated from autoregressive models. Physically, this means that intensified convection tends to be followed by another intensified event, and vice versa for weakened events or droughts. Such tendency is stronger with larger domain averaging, probably due to more complete inclusion of larger-scale variability that has more definite trends, such as the supercloud clusters associated with the Madden–Julian oscillation (MJO). The evolution of cloud clusters within an MJO event is studied by following the MJO system across the analysis domain for ∼21 days. Convective activities along the front, center, and rear parts of the MJO event continuously intensify while approaching the date line, indicating multifractal features in the range of 1 h to about 5–10 days. Convective activity along the front and rear edges of the MJO event are more intermittent than in the center. The multifractal features of the I T BB time series can be approximated by the random multiplicative cascade processes, suggesting likely mechanisms for the multiscale behavior and casting concern on the predictability time scale of the observed phenomena.

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Ismail Yucel
,
W. James Shuttleworth
,
X. Gao
, and
S. Sorooshian

Abstract

This study investigates the extent to which assimilating high-resolution remotely sensed cloud cover into the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) provides an improved regional diagnosis of downward shortwave surface radiation fluxes and precipitation and enhances the model's ability to make short-range prediction. The high-resolution (4 km × 4 km) clear- and cloudy-sky radiances derived using a cloud-screening algorithm from visible band Geostationary Operational Environmental Satellite (GOES) data were used in the University of Maryland Global Energy and Water Cycle Experiment's Surface Radiation Budget (UMD GEWEX/SRB) model to infer the vertically integrated cloud mass via cloud optical thickness. Three-dimensional cloud fields were created that took their horizontal distribution from the satellite image but derived their vertical distribution, in part, from the fields simulated by MM5 during the time step immediately prior to assimilation and, in part, from the observed cloud-top height derived from the infrared band of GOES. Linear interpolation was used to derive 1-min cloud images between 15-min GOES samples, and the resulting images were ingested every minute. Comparisons were made between modeled and observed data taken from the Arizona Meteorological Network (AZMET) in southern Arizona for model runs with and without cloud ingestion. Cloud ingestion substantially improved the ability of the MM5 model to capture temporal and spatial variations in surface fields associated with cloud cover. Experiments in which the model was operated in forecast mode suggest that cloud ingestion gave some limited enhancement in MM5 short-term prediction ability for up to 3 h. However, an analysis suggests that, in order to get additional forecasting capability, it will be necessary to modify the atmospheric dynamics and thermodynamics in the model to be consistent with the ingested cloud fields.

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Kun Gao
,
Da-Lin Zhang
,
Mitchell W. Moncrieff
, and
Han-Ru Cho

Abstract

A mesoβ-scale momentum budget and its effect on larger-scale mean flow in a midlatitude mesoscale convective system are investigated using a numerical simulation of an intense squall line that occurred during 10–11 June 1985 PRE-STORM. It is found that the momentum generation normal to the line associated with the latent heating and cooling contributes most significantly to the momentum budget and determines the mesoβ-scale internal structure and evolution of the squall line. The momentum generation along the line contributes to the initial development of a mesovortex but has little effect on the final vertical structure of the along-line flow. Both vertical and horizontal momentum advection have significant contributions, particularly to the vertical mixing of the along-line flow; and this component of the horizontal momentum is locally transported down-gradient. It is also found that for midlatitude convective systems, convectively generated downdrafts can play as prominent a role as updrafts in vertically transporting horizontal momentum within both convective and stratiform regions.

The momentum flux associated with the mesoβ-scale circulations of the simulated squall line is found to agree with previous observational investigations, namely, normal to the line the squall system transports horizontal momentum in a countergradient sense while parallel to the line the transport is downgradient. Implications with respect to the convective momentum parameterization are discussed in the context of the mesoβ-scale momentum budget.

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J. Li
,
S. Sorooshian
,
W. Higgins
,
X. Gao
,
B. Imam
, and
K. Hsu

Abstract

Diurnal variability is an important yet poorly understood aspect of the warm-season precipitation regime over southwestern North America. In an effort to improve its understanding, diurnal variability is investigated numerically using the fifth-generation Pennsylvania State University (PSU)–NCAR Mesoscale Model (MM5). The goal herein is to determine the possible influence of spatial resolution on the diurnal cycle.

The model is initialized every 48 h using the operational NCEP Eta Model 212 grid (40 km) model analysis. Model simulations are carried out at horizontal resolutions of both 9 and 3 km. Overall, the model reproduces the basic features of the diurnal cycle of rainfall over the core monsoon region of northwestern Mexico and the southwestern United States. In particular, the model captures the diurnal amplitude and phase, with heavier rainfall at high elevations along the Sierra Madre Occidental in the early afternoon that shifts to lower elevations along the west slopes in the evening. A comparison to observations (gauge and radar data) shows that the high-resolution (3 km) model generates better rainfall distributions on time scales from monthly to hourly than the coarse-resolution (9 km) model, especially along the west slopes of the Sierra Madre Occidental. The model has difficulty with nighttime rainfall along the slopes, over the Gulf of California, and over Arizona.

A comparison of surface wind data from three NCAR Integrated Sounding System (ISS) stations and the Quick Scatterometer (QuikSCAT) to the model reveals a low bias in the strength of the Gulf of California low-level jet, even at high resolution. The model results indicate that outflow from convection over northwestern Mexico can modulate the low-level jet, though the extent to which these relationships occur in nature was not investigated.

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Lisi Pei
,
Nathan Moore
,
Shiyuan Zhong
,
Anthony D. Kendall
,
Zhiqiu Gao
, and
David W. Hyndman

Abstract

Irrigation’s effects on precipitation during an exceptionally dry summer (June–August 2012) in the United States were quantified by incorporating a novel dynamic irrigation scheme into the Weather Research and Forecasting (WRF) Model. The scheme is designed to represent a typical application strategy for farmlands across the conterminous United States (CONUS) and a satellite-derived irrigation map was incorporated into the WRF-Noah-Mosaic module to realistically trigger the irrigation. Results show that this new irrigation approach can dynamically generate irrigation water amounts that are in close agreement with the actual irrigation water amounts across the high plains (HP), where the prescribed scheme best matches real-world irrigation practices. Surface energy and water budgets have been substantially altered by irrigation, leading to modified large-scale atmospheric circulations. In the studied dry summer, irrigation was found to strengthen the dominant interior high pressure system over the southern and central United States and deepen the trough over the upper Midwest. For the HP and central United States, the rainfall amount is slightly reduced over irrigated areas, likely as a result of a reduction in both local convection and large-scale moisture convergence resulting from interactions and feedbacks between the land surface and atmosphere. In areas downwind of heavily irrigated regions, precipitation is enhanced, resulting in a 20%–100% reduction in the dry biases (relative to the observations) simulated over a large portion of the downwind areas without irrigation in the model. The introduction of irrigation reduces the overall mean biases and root-mean-square errors in the simulated daily precipitation over the CONUS.

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W. Gao
,
R. L. Coulter
,
B. M. Lesht
,
J. Qiu
, and
M. L. Wesely

Abstract

The authors compared methods for estimating surface fluxes under clear-sky conditions over a large heterogeneous area from a limited number of ground measurements and from satellite observations using data obtained from the southern Great Plains Cloud and Radiation Testbed (CART) site, an area of approximately 350 km × 400 km located in Kansas and Oklahoma. In situ measurements from 10 energy balance Bowen ratio (EBBR) stations showed large spatial variations in latent and sensible heat fluxes across the site because of differences in vegetation and soil conditions. This variation was reproduced by a model for parameterization of subgrid- scale (PASS) surface fluxes that was developed previously and extended in the present study to include a distribution of soil moisture inferred from combined visible and thermal infrared remote sensing data. In the framework of the PASS model, the satellite-derived normalized difference vegetation index and surface temperature were used to derive essential surface parameters including surface albedo, surface conductance, soil heat flux ratio, surface roughness length, and soil moisture, which were then used to calculate a surface energy budget at satellite-pixel scales with pixel-specific surface meteorological conditions appropriately distributed from their mean values using a distribution algorithm. Although the derived soil moisture may be influenced by various uncertainty factors involved in the satellite data and the model, spatial variations of soil moisture derived from the multichannel data from the Advanced Very High Resolution Radiometers on the NOAA-14 satellite appeared to have some correlation (the correlation coefficient is as large as 0.6) with the amount of accumulated previous rainfall measured at the 58 Oklahoma Mesonet stations located within the CART area. Surface net radiation, soil heat flux, and latent and sensible heat fluxes calculated at a spatial resolution of 1 km (the size of a satellite pixel) were evaluated directly by comparing with flux measurements from the EBBR stations and indirectly by comparing air temperature and humidity inferred from calculated sensible and latent heat fluxes with corresponding values measured at 1.5 m above the 58 meteorological stations. In calculating regional fluxes, biases caused by the sampling uncertainty associated with point measurements may be corrected by application of the satellite data.

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