Search Results

You are looking at 1 - 10 of 57 items for

  • Author or Editor: Qiang Fu x
  • Refine by Access: All Content x
Clear All Modify Search
Qiang Fu

Abstract

The aspect ratio (AR) of a nonspherical ice particle is identified as the key microphysical parameter to determine its asymmetry factor for solar radiation. The mean effective AR is defined for cirrus clouds containing various nonspherical ice particles. A new parameterization of the asymmetry factor of cirrus clouds in terms of AR and mean effective size, Dge, is developed for solar radiation. It is based on geometric ray-tracing calculations for hexagonal ice crystals with a simple representation of particle surface roughness. The present parameterization well reproduces the asymmetry factors of complicated ice particles such as bullet rosettes, aggregates with rough surfaces, and fractal crystals and agrees well with observations. It thus can be properly applied to cirrus clouds containing various nonspherical ice particles. The asymmetry factor from this parameterization in the visible spectrum ranges from about 0.73 to more than 0.85.

Radiative transfer calculations show that for a cirrus cloud with an optical depth of 4 and a solar zenith angle of 60°, changes in AR from 1.0 to 0.5 or from 1.0 to 0.1 result in differences in reflected solar fluxes of about −30 or −70 W m−2, respectively. For the same cloudy conditions, the effect of ice particle surface roughness on the reflected solar flux is found to be about 20 W m−2.

Full access
Qiang Fu

Abstract

An accurate parameterization of the solar radiative properties of cirrus clouds is developed based on improved light scattering calculations. Here 28 ice crystal size distributions from in situ aircraft observations in both tropical and midlatitude regions are employed. In the single scattering calculations, the most recent measurements of the imaginary refractive indices of ice are used, thereby eliminating a large existing uncertainty. The single scattering properties of hexagonal ice crystals are calculated by using an improved geometric ray-tracing program that can produce accurate results for size parameters larger than 15.

A generalized effective size, Dge is defined to account for the ice crystal size distribution in the radiative calculations. Based on physical principles, the single scattering properties have been parameterized in terms of ice water content (IWC) and Dge. This allows the cirrus cloud single scattering properties to respond independently to changes in IWC or Dge. The generalized effective size can be related to the total cross-sectional area of ice particles per unit volume, a quantity directly measured by the 2D optical probe in in situ microphysical observations of cirrus clouds. The present parameterization of the extinction coefficient and the single scattering albedo in terms of IWC and Dge can be properly applied to cirrus clouds that contain various nonspherical particles, such as plates, columns, bullet rosettes, and aggregates, etc.

The present parameterization of the single scattering properties of cirrus clouds is evaluated by examining the bulk radiative properties for a wide range of atmospheric conditions. Compared with reference results, the typical relative errors due to the parameterization are ∼1.2%, ∼0.3%, and ∼2.9% in reflectance, transmittance, and absorptance, respectively. The accuracy of this parameterization guarantees its reliability in applications to climate models.

Cloud absorption plays an important role in cloud-radiation interactions and therefore in climate systems. Because of the large variation in the co-albedo of ice near the wavelength of 1.41 μm sum, one of the spectral divisions is chosen at 1.41 μm to predict cloud absorption properly. Furthermore, the averaging technique for single scattering albedo in spectral intervals associated with absorption bands is important for the parameterization of radiative properties of ice clouds.

Full access
Qiang Fu and Pu Lin

Abstract

One pronounced feature in observed latitudinal dependence of lower-stratospheric temperature trends is the enhanced cooling near 30° latitude in both hemispheres. The observed phenomenon has not, to date, been explained in the literature. This study shows that the enhanced cooling is a direct response of the lower-stratospheric temperature to the poleward shift of subtropical jets. Furthermore, this enhanced lower-stratospheric cooling can be used to quantify the poleward shift of subtropical jets. Using the lower-stratospheric temperatures observed by satellite-borne microwave sounding units, it is shown that the subtropical jets have shifted poleward by 0.6° ± 0.1° and 1.0° ± 0.3° latitude in the Southern and Northern Hemispheres, respectively, in last 30 years since 1979, indicating a widening of tropical belt by 1.6° ± 0.4° latitude.

Full access
J. Li and Qiang Fu

Abstract

A scheme that can handle cloud infrared scattering based on the absorption approximation is developed. In a two-stream mode, the new scheme produces more accurate results than those from the modified two-stream discrete ordinate method. For low and middle clouds, the two-stream version of the scheme produces a flux error less than 1 W m−2 and a heating rate error less than 0.5 K day−1. With high clouds, the errors in calculated fluxes and heating rates are less than 1.4 W m−2 and 1.5 K day−1, respectively. The four-stream version of the proposed scheme is slightly inferior to the four-stream discrete ordinate method. However, as opposed to the discrete ordinate technique, this scheme treats cloud-free layers the same as the absorption approximation. Therefore, numerically, it is much more efficient. Considering the radiative transfer module only, in a two-stream mode, the new scheme, which considers multiple scattering, uses only about 50% more CPU time than the absorption approximation method for a 100-layer column atmosphere with 20 cloudy layers.

Full access
Qiang Fu and Shawn Hollars

Abstract

The parameterization of in-cloud water vapor pressure below 0°C is examined using in situ aircraft observations from Canadian National Research Council (NRC) Convair-580 flights during the Surface Heat Budget of the Arctic Ocean (SHEBA)/First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment– Arctic Cloud Experiment (FIRE–ACE) campaign. The accuracy of in-cloud water vapor measurements is evaluated against the saturated water vapor pressure in liquid water clouds as derived from measured temperatures, which have a mean bias of about −1%. This study reveals that the parameterization used in the ECMWF cloud scheme, which employs a temperature-weighted average of the values with respect to ice and liquid water underestimates the saturated water vapor by ∼9% when applied to all in-cloud data from the campaign. It is found that a parameterization that relates the weighting to the cloud liquid and ice water contents agrees well with the observations. This study also reveals that it is incorrect to assume that water vapor is in equilibrium with liquid water in mixed-phase clouds.

Full access
Qiang Fu and Celeste M. Johanson

Abstract

Retrievals of tropospheric temperature trends from data of the Microwave Sounding Unit (MSU) are subject to biases related to the strong cooling of the stratosphere during the past few decades. The magnitude of this stratospheric contamination in various retrievals is estimated using stratospheric temperature trend profiles based on observations. It is found that from 1979 to 2001 the stratospheric contribution to the trend of MSU channel-2 brightness temperature is about −0.08 K decade−1, which is consistent with the findings of Fu et al. In the retrieval method developed by Fu et al. based on a linear combination of MSU channels 2 and 4, the stratospheric influence is largely removed, leaving a residual influence of less than ±0.01 K decade−1. This method is also found to be more accurate than the angular scanning retrieval technique of Spencer and Christy to remove the stratospheric contamination.

Full access
Stephen Po-Chedley and Qiang Fu

Abstract

The University of Alabama at Huntsville (UAH), Remote Sensing Systems (RSS), and the National Oceanic and Atmospheric Administration (NOAA) have constructed long-term temperature records for deep atmospheric layers using satellite Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit (AMSU) observations. However, these groups disagree on the magnitude of global temperature trends since 1979, including the trend for the midtropospheric layer (TMT). This study evaluates the selection of the MSU TMT warm target factor for the NOAA-9 satellite using five homogenized radiosonde products as references. The analysis reveals that the UAH TMT product has a positive bias of 0.051 ± 0.031 in the warm target factor that artificially reduces the global TMT trend by 0.042 K decade−1 for 1979–2009. Accounting for this bias increases the global UAH TMT trend from 0.038 to 0.080 K decade−1, effectively eliminating the trend difference between UAH and RSS and decreasing the trend difference between UAH and NOAA by 47%. This warm target factor bias directly affects the UAH lower tropospheric (TLT) product and tropospheric temperature trends derived from a combination of TMT and lower stratospheric (TLS) channels.

Full access
Qiang Fu and K. N. Liou

Abstract

The correlated k-distribution method for radiative transfer in nonhomogeneous atmospheres is discussed in terms of the physical and mathematical conditions under which this method is valid. Two correlated conditions are necessary and sufficient for the exact transformation of the wavenumber integration to an integration over the cumulative probability (g), a monotonically increasing and smooth function in the absorption coefficient space. These conditions involve the use of a reference condition to define the absorption coefficient and an assumption concerning the ordering of the absorption coefficient. The correlated conditions are exact in the context of a single line, periodic lines, and the strong- and weak-line limits. In realistic atmospheres, these assumptions are best for adjacent levels but produce increasing blurring or deviations for distant levels.

We investigate the blurring of the correlated assumptions on the computations of fluxes and heating rates based on “exact” line-by-line results, using a variety of atmospheric profiles and spectral intervals containing principal absorbing gases. In the thermal infrared, errors in fluxes are less than 0.2% for H2O, CO2, CH4, and N2O, and ∼2% for O3. Errors in heating rates are less than 0.01 K day−1 for these gases below ∼30 km. Larger errors of ∼0.1 K day−1 can occur at some levels above this height. For H2O lines in the solar region, errors in fluxes and heating rates are within 0.05% and 0.01 K day−1, respectively. Based on numerical experimentation, we find that the number of g values ranging from 1 (for weak bands) to ∼10 (for strong bands) are usually sufficient to achieve acceptable accuracy for flux and heating rate calculations.

The correlated k-distribution method differs fundamentally from the traditional approach that employs scaling approximations and band models to separate height and wavenumber integrations for transmittance calculations. The equivalent k values for various gases computed from this approach can be directly incorporated in the multiple-scattering program involving cloud and aerosol particles.

Full access
Tyler J. Thorsen and Qiang Fu

Abstract

A feature detection and extinction retrieval (FEX) algorithm for the Atmospheric Radiation Measurement Program’s (ARM) Raman lidar (RL) has been developed. Presented here is Part II of the FEX algorithm: the retrieval of cloud and aerosol extinction profiles. The directly retrieved extinction profiles using the Raman method are supplemented by other retrieval methods developed for elastic backscatter lidars. Portions of features where the extinction-to-backscatter ratios (i.e., lidar ratios) can be obtained are used to infer the lidar ratios for the regions where no such estimate can be made. When neither directly retrieved nor an inferred value can be determined, a climatological lidar ratio is used. This best-estimate approach results in the need to use climatological lidar ratios for less than about 5% of features, except for thin cirrus at the ARM tropical western Pacific Darwin site, where above 12 km, about 20% of clouds use a climatological lidar ratio. A classification of feature type is made, guided by the atmosphere’s thermodynamic state and the feature’s scattering properties: lidar ratio, backscatter, and depolarization. The contribution of multiple scattering is explicitly considered for each of the ARM RL channels. A comparison between aerosol optical depth from FEX and that from collocated sun photometers over multiple years at two ARM sites shows an agreement (in terms of bias error) of about −0.3% to −4.3% (relative to the sun photometer).

Full access
Stephen Po-Chedley and Qiang Fu

Abstract

The main finding by Po-Chedley and Fu was that the University of Alabama in Huntsville (UAH) microwave sounding unit (MSU) product has a bias in its NOAA-9 midtropospheric channel (TMT) warm target factor, which leads to a cold bias in the TMT trend. This reply demonstrates that the central arguments by Christy and Spencer to challenge Po-Chedley and Fu do not stand. This reply establishes that 1) Christy and Spencer found a similar, but insignificant, bias in the UAH target factor because their radiosonde data lack adequate sampling and measurement errors were considered twice; 2) the UAH individual satellite TMT difference between NOAA-9 and NOAA-6 reveals a bias of 0.082 ± 0.011 in the UAH NOAA-9 target factor; 3) comparing the periods before and after NOAA-9 is not an adequate method to draw conclusions about NOAA-9 because of the influence of other satellites; 4) using the Christy and Spencer trend sensitivity value, UAH TMT has a cold bias of 0.035 K decade−1 given a target factor bias of 0.082; 5) similar trends from UAH and Remote Sensing Systems (RSS) for the lower tropospheric temperature product (TLT) do not indicate that the UAH TMT and TLT NOAA-9 target factor is unbiased; and 6) the NOAA-9 warm target temperature signal in UAH TMT indicates a problem with the UAH empirical algorithm to derive the target factor.

Full access