Solar Radiative Transfer Research Articles

Sensitivity Experiments on the Effects of Optical Properties of Dust Aerosols on Their Radiative Forcing under Clear Sky Condition

There are many uncertainties in the quantitative assessment of radiative effects due to atmospheric dust aerosol and the optical properties of dust particles contribute large to them. Numerical sensitivity experiments to evaluate the impacts of optical characteristics on the radiative forcing have been performed in this study. The experiments involve the effects of refractive indices, single scattering albedo, asymmetry factor and optical depth of the dust particles. An updated data set of refractive indices representing East Asian dust and the data set recommended by World Meteorology Organization (WMO) are used in our calculations for comparison. A k-distribution model for solar and thermal radiation transfer is employed in the calculation of radiative forcing. It is found that comparing with the WMO model, East Asian dust model has stronger scattering and weaker absorption at solar regime, which leads to higher negative forcing at the top of atmosphere (TOA) in this study. The more important is the signs of radiative forcing at TOA over certain regions could be reversed for the two dust models, which emphasizes the essentiality of accurate measurements of optical properties of dust aerosols for quantitatively estimating their radiative forcing.

Effects of bias in solar radiative transfer codes on global climate model simulations

Codes commonly used in climate and weather prediction models for calculating the transfer of solar radiation in the atmosphere show systematic differences amongst each other, and even the best of codes show systematic differences with respect to observations. A 1‐dimensional radiative‐convective equilibrium model is used to show the effects of such bias on the global energy balance and on the global response to a doubling of CO2. We find the main impact is in the energy exchange terms between the surface and atmosphere and in the convective transport in the lower troposphere, where it exceeds 10 W m−2. The impact on model response to doubling of CO2, on the other hand, is quite small and in most cases negligible.

Theoretical approach of a flat plate solar collector with clear and low-iron glass covers taking into account the spectral absorption and emission within glass covers layer

Accurate modeling of solar collector system using a rigorous radiative model is applied for the glass cover which represents the most important component of the system and greatly affects the thermal performance. The glass material is analyzed as a non-gray plane-parallel medium subjected to solar and thermal irradiations in one dimensional case using the radiation element method by ray emission model (REM 2). The optical constants of a clear and low-iron glass materials proposed by Rubin have been used. These optical constants, 160 values of real part n and imaginary part k of the complex refractive index of such materials, cover the range of interest for calculating the solar and thermal radiative transfer through the glass cover. The computational times for predicting the thermal behavior of solar collector were found to be prohibitively long for the non-gray calculation using 160 values of n and k for both glasses. Therefore, suitable semi-gray models have been proposed for rapid calculation. The temperature distribution within the glass cover shows a good agreement with that obtained with iterative method in case of clear glass. It has been shown that the effect of the non-linearity of the radiative heat exchange between the black plate absorber and the surroundings on the shape of the efficiency curve is important. Indeed, the thermal loss coefficient is not constant but is a function of temperature, due primarily to the radiative transfer effects. Therefore, when the heat exchange by radiation is dominant compared with the convective mode, the profile of the efficiency curve is not linear. It has been also shown that the instantaneous efficiency of the solar collector is higher in case of low-iron glass cover.

The role of cloud-scale resolution on radiative properties of oceanic cumulus clouds

Both individual and combined effects of the horizontal and vertical variability of cumulus clouds on solar radiative transfer are investigated using a two-dimensional ( x - and z -directions) cloud radar dataset. This high-resolution dataset of typical fair-weather marine cumulus is derived from ground-based 94 GHz cloud radar observations. The domain-averaged (along x -direction) radiative properties are computed by a Monte Carlo method. It is shown that (i) different cloud-scale resolutions can be used for accurate calculations of the mean absorption, upward and downward fluxes; (ii) the resolution effects can depend strongly on the solar zenith angle; and (iii) a few cloud statistics can be successfully applied for calculating the averaged radiative properties.

Neglect by GCMs of subgrid‐scale horizontal variations in cloud‐droplet effective radius: A diagnostic radiative analysis

AbstractOutput from a global climate model (GCM) that employed a low‐resolution two‐dimensional cloud‐system‐resolving model (CSRM) in each column is used to assess the radiative impact of neglecting subgrid‐scale horizontal variations in cloud‐droplet effective radius re. For this diagnostic study, only liquid‐phase variations in re are addressed; the ice‐cloud particle distributions are assumed to be constant. For reference calculations, values of re in the CSRM cells are computed assuming that the droplet‐number concentration Ncld and the effective variance of droplet‐size distribution are constant in a GCM cell. The independent‐column approximation is used to produce flux profiles for each GCM column. Three alternative methods of setting horizontally‐invariant re are examined, each of which resemble how re is set in one‐dimensional radiative‐transfer models. Relative to the reference calculations, the other methods lead to positive spurious radiative forcings at the surface and at the top of the atmosphere. These stem from overestimation of optical‐depth variability and, thus, reduced short‐wave albedo of clouds. Globally averaged, these forcings range from 1 W m−2 to 3 W m−2, with zonal‐mean biases reaching almost 15 W m−2. The most severe biases arise from use of constant values of re over the land and the ocean.In addition, radiative effects due to unacknowledged uncertainty in Ncld (or re) are assessed. It is shown that ad hoc, but not outlandish, estimates of unbiased uncertainty in Ncld impart biases on estimates of the earth's solar‐radiation budget (tantamount to a spurious radiative forcing). These arise through the chain of nonlinear relations that link Ncld to solar radiative transfer. Copyright © 2004 Royal Meteorological Society.

Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere

In the context of recent observations, microphysical models, and laboratory data, a photochemical model of Titan's atmosphere, including updated chemistry focusing on rate coefficients and cross sections measured under appropriate conditions, has been developed to increase understanding of these processes and improve upon previous Titan photochemical models. The model employs a two‐stream discrete ordinates method to characterize the transfer of solar radiation, and the effects of electron‐impact, cosmic‐ray deposition, and aerosol opacities from fractal and Mie particles are analyzed. Sensitivity studies demonstrate that an eddy diffusion profile with a homopause level of 850 km and a methane stratospheric mole fraction of 2.2% provides the best fit of stratospheric and upper atmosphere observations and an improved fit over previous Titan photochemical models. Lack of fits for C3H8, HC3N, and possibly C2H3CN can be resolved with adjustments in aerosol opacity. The model presents a benzene profile consistent with its detection in Titan's stratosphere [Coustenis et al., 2003], which may play an important role in the formation of Titan hazes. An electron peak concentration of 4200 cm−3 is calculated, which exceeds observations by 20%, considerably lower than previous ionosphere models. With adjustments in aerosol opacities and surface fluxes the model illustrates that reasonable fits to existing observations are possible with a single eddy diffusion profile, contrary to the conclusions of previous Titan models. These results will aid in the receipt and interpretation of data from Cassini‐Huygens, which will arrive at Titan in 2004 and deploy a probe into Titan's atmosphere in January 2005.

Investigation of Light Guides for Transfer of Solar Radiation from the Focal Volume of a Sunlight Collector to a Technological Zone

The efficiency of light guides manufactured at the State Optical Institute on exposure of the inlet end to concentrated solar radiation under the conditions of an SGU‐2 plant with a O1.5 m collector has been investigated. It has been established that all O2 mm light guides had a stability to concentrated solar radiation of the order of 600 W/cm2; the damage threshold of O8 mm multistrand light guides was about 450 W/cm2. It has been shown that TC1, TC3 and TC23 heavy crowns are the most stable to the action of 1500–1800 W/cm2 fluxes. The possibility of restoring the transmission of light guides exposed to ionizing radiations through the use of concentrated solar radiation has been confirmed.

Modeling aerosols on the mesoscale‐γ: Treatment of soot aerosol and its radiative effects

Aerosol particles modify the transfer of solar radiation in the atmosphere. To quantify this impact, the composition and the size distribution of the aerosol have to be known. In this study, simulations with a comprehensive model system including meteorological processes, gas phase chemistry, and aerosol dynamics and chemistry were carried out to investigate the impact of aerosol particles on global radiation. The model system is applied to a summer situation in the southwestern part of Germany. The aerosol model represents the aerosol population by several overlapping modes. It originally accounted for the species sulfate, nitrate, ammonium, and water and now has been extended to treat soot. Since the optical properties of soot particles depend critically on their mixing state, the transfer of soot from the external into the internal mixture is parameterized. On the basis of the simulated aerosol distributions the optical properties of the aerosol were derived using Mie calculations. These results serve as input data for the radiative transfer calculations, where we considered the wavelength interval from 280 nm to 700 nm. The results of the radiative transfer calculations show that for the summer situation presented, the aerosol in the boundary layer reduces the downward flux of the solar radiation by up to 25 W m−1. Sensitivity studies showed that up to 50% of this effect can be attributed to the impact of soot.

Combined non-gray radiative and conductive heat transfer in solar collector glass cover

A rigorous approach for the radiative heat transfer analysis in solar collector glazing is developed. The model allows a more accurate prediction of thermal performance of a solar collector system. The glass material is analysed as a non-gray plane-parallel medium subjected to solar and thermal irradiations in the one-dimensional case using the Radiation Element Method by Ray Emission Model (REM by REM). This method is used to analyse the combined non-gray convective, conductive and radiative heat transfer in glass medium. The boundary surfaces of the glass are specular. The spectral dependence of the relevant radiation properties of glass (i.e. specular reflectivity, refraction angle and absorption coefficient) are taken into consideration. Both collimated and diffuse incident irradiation are applied at the boundary surfaces using the spectral solar model proposed by Bird and Riordan. The optical constants of a commercial ordinary clear glass material have been used. These optical constants (100 values) of real and imaginary parts of the complex refractive index of the glass material cover the range of interest for calculating the solar and thermal radiative heat transfer through the solar collector glass cover. The model allows the calculation of the steady-state heat flux and temperature distribution within the glass layer. The effect of both conduction and radiation in the heat transfer process is examined. It has been shown that the real and imaginary parts of the complex refractive index have a substantial effect on the layer temperature distribution. The computational time for predicting the combined heat transfer in such a system is very long for the non-gray case with 100 values of n and k. Therefore, a simplified non-gray model with 10 values of n and k and two semi-gray models have been proposed for rapid computations. A comparison of the proposed models with the reference non-gray case is presented. The result shows that 10 bandwidths could be used for rapid computation with a very high level of accuracy.

Three‐dimensional solar radiation effects on the actinic flux field in a biomass‐burning plume

Three‐dimensional (3‐D) solar radiative transfer models describe radiative transfer under inhomogeneous atmospheric conditions more accurately than the commonly used one‐dimensional (1‐D) radiative transfer models that assume horizontal homogeneity of the atmosphere. Here results of 3‐D radiative transfer simulations for a biomass‐burning plume are presented and compared with local one‐dimensional (l‐1‐D) simulations, i.e., 1‐D simulations in every column of the model domain. The spatial distribution of the aerosol particles was derived from a 3‐D atmospheric transport simulation. We studied the impact of 3‐D radiative effects on the actinic flux within the plume center. The differences in the actinic flux between results from the 3‐D and the l‐1‐D simulations are considerable, ranging from −40% to more than +200%, depending on the wavelength, solar zenith angle, and the absorbing properties of the aerosol. The reason for this discrepancy is the neglect of horizontal photon transport in the 1‐D simulation. These large 3‐D effects on the actinic flux have the potential to influence significantly the in‐plume photochemistry.

Assessing 1D Atmospheric Solar Radiative Transfer Models: Interpretation and Handling of Unresolved Clouds

Abstract The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended? One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport ...

Effect of Spatial Organization on Solar Radiative Transfer in Three-Dimensional Idealized Stratocumulus Cloud Fields

To relate the error associated with 1D radiative calculations to the geometrical scales of cloud organization and/or in-cloud optical inhomogeneities, a new idealized methodology, based on a Fourier statistical technique, has been developed. Three-dimensional cloud fields with variability over a selected range of horizontal spatial scales and consistent vertical structure can be obtained and controlled by a small number of parameters, which relate directly to the dynamical and thermodynamical meteorology of the situation to be examined. This initial study deals with marine stratocumulus. Two experiments are conducted: an overcast situation and a broken cloud case with maximum cloud cover of 80%. For each experiment, five cloud fields are generated with the dominant organizational scale changing from 1.4 to 22 km, while all other quantities—such as cloud cover, cloud liquid water, and total water variance—remain constant. For each scene, three radiative calculations are performed for solar zenith angles of 08 and 608: a plane parallel (PP) calculation, similar to that commonly implemented in general circulation models; an independent pixel approximation (IPA); and a full 3D calculation. The ‘‘PP bias’’ (IPA-PP) is used to assess cloud optical homogeneity approximation, while the ‘‘IPA bias’’ (3D-IPA) measures the impact of horizontal photon transport. For the overcast scenes, the neglect of horizontal photon transport was found to be unimportant, and the IPA calculation gives accurate results. For the broken cloud case, this was only true for clouds with dominant horizontal spatial scales exceeding 10 km. With a scale of 2 km or less, the IPA bias in reflectivity, transmissivity, and absorption could exceed 5%. This indicates that even for shallow cloud systems, cloud geometry can play an important role. The sign of the bias depends critically on the solar angle, with IPA over- (under) estimating reflectivity for high (low) sun angles. The PP bias in reflectivity was also found to be around 5% for both cases, comparable to the IPA bias, and smaller than previous estimates for this cloud type. Additional sensitivity tests prove this to be due to the vertical cloud structure. Vertically resolving the subcloud adiabatic liquid profile leads to a more opaque cloud upper boundary, reducing photon penetration into the cloud layer, and thus also PP biases. Additionally, for the broken cloud case it was found that changes in cloud fraction with height are translated by cloud overlap rules into effective horizontal variability in the liquid water path, further reducing biases. Taking a vertically uniform slab with identical integrated properties led to much larger PP biases comparable to previous estimates. Thus, models with sufficient vertical resolution are likely to suffer from smaller PP biases than previously estimated.

Monte Carlo Simulation of Solar Reflectances for Cloudy Atmospheres

Abstract Monte Carlo simulations of solar radiative transfer were performed for a well-resolved, large, three-dimensional (3D) domain of boundary layer cloud simulated by a cloud-resolving model. In order to represent 3D distributions of optical properties for ∼2 × 106 cloudy cells, attenuation by droplets was handled by assigning each cell a cumulative distribution of extinction derived from either a model or an assumed discrete droplet size spectrum. This minimizes the required number of detailed phase functions. Likewise, to simulate statistically significant, high-resolution imagery, it was necessary to apply variance reduction techniques. Three techniques were developed for use with the local estimation method of computing reflectance ρ. First, small fractions of ρ come from numerous, small contributions of ζ computed at each scattering event. Terminating calculation of ζ when it falls below ζmin ≈ 10−3 was found to impact estimates of ρ minimally but reduced computation time by ∼10%. Second, large f...

Solar radiative transfer for stratiform clouds with horizontal variations in liquid‐water path and droplet effective radius

AbstractEffects of horizontal variations in liquid‐water path (LWP) and droplet effective radius (re) on solar radiative‐transfer properties of stratiform clouds are addressed. To this end, a broadband Monte Carlo solar radiation code and in situ aircraft observations of cloud properties collected in the Radiation, Aerosol and Cloud Experiment are used. The experiments confirm that the radiative impact of cloud horizontal inhomogeneity is dominated by variations in LWP, which make cloud short‐wave albedo smaller than that for a corresponding plane‐parallel horizontally homogeneous cloud. When horizontal variations in re are accounted for, however, the impact due to horizontal variations in LWP is usually reduced. When re and LWP are highly correlated, as is typically the case, the impact on albedo due to LWP variations alone can be reduced easily by 30% when variations in re are included. Uncertainty factors involved in the calculations are also discussed. Copyright © 2003 Royal Meteorological Society.

Quantifying the magnitude of anomalous solar absorption

The data set from ARESE II, sponsored by the Atmospheric Radiation Measurement Program, provides a unique opportunity to understand solar absorption in the atmosphere because of the combination of three sets of broadband solar radiometers mounted on the Twin Otter aircraft and the ground based instruments at the Atmospheric Radiation Measurement (ARM) Southern Great Plains facility. In this study, we analyze the measurements taken on two clear‐sky days and three cloudy days and model the solar radiative transfer in each case with two different models. On the two clear days, the calculated and measured column absorptions agree to better than 10 W m−2, which is about 10% of the total column absorption. Because both the model fluxes and the individual radiometer measurements are accurate to no better than 10 W m−2, we conclude that the models and measurements are essentially in agreement. For the three cloudy days, the model calculations agree very well with each other and on 2 of the 3 days agree with the measurements to 20 W m−2 or less out of a total column absorption of more than 200 W m−2, which is again an agreement at better than 10%. On the third day, the model and measurements agree to either 8 or 14% depending on which value of surface albedo is used. Differences exceeding 10% represent a significant absorption difference between model and observations. In addition to the uncertainty in absorption due to surface albedo, we show that including aerosol with an optical depth similar to that found on clear days can reduce the difference between model and measurement by 5% or more. Thus we conclude that the ARESE II results are incompatible with previous studies reporting extreme anomalous absorption and can be modeled with our current understanding of radiative transfer.

Solar radiative biases in deep convective regimes: Possible implications for dynamical feedback

There is increasing evidence that interactions between radiation and convectively determined water vapour and cloud fields can have a significant dynamical impact on the tropical circulation. However, the use of a one-dimensional (1D) independent column method for the calculation of solar fluxes in cloud-resolving or mesoscale models could possibly misrepresent this feedback. This is investigated by calculating 1D and full three-dimensional (3D) solar fluxes through a 3D field of deep convective clouds generated using a high-resolution cloud-resolving model, and diagnosing the clear-sky subsidence velocities required to balance this if a state of radiative convective equilibrium were to exist. The mean clear-sky solar radiative heating rate is found to change by up to 15%. At low sun angles the shading of the clear-sky regions implies a reduction of net clear-sky heating rates. In contrast, at higher solar elevations the scattering effect dominates shading and the clear-sky heating rates are enhanced, implying an enhancement of the diurnal cycle. However, since the 1D/3D heating-rate differences in the clear sky increase smoothly from zero at the tropopause to their maximum value near the surface, the vertical gradient and thus the impact on the vertical profile of entrainment and detrainment into the convective regions is negligibly small. This implies that the strong radiative dynamical feedbacks so far documented in cloud-ensemble models cannot be discounted on the grounds of their use of an independent column calculation for the solar radiative transfer. That said, the substantial bias noted in surface downward solar fluxes could result in a substantial impact in convective organization if an interactive land surface were used as a lower boundary condition. Copyright © 2003 Royal Meteorological Society

대기 복사 모형에 의한 남극 세종기지에서의 복사학적 특징 : 지표면에서 UV-A와 Erythemal UV-B 자외선 양 변화

A solar radiation model was used to investigate the UV radiation at the surface offing Sejong Station in West Antarctica. The results calculated by this model were compared with the values measured by UV-Biometer and UV-A meter during 1999-2000. In this study, the parameterization of solar radiative transfer process was based on Chou and Lee(1996). The total ozone amounts measured by Breve. Ozone Spectrophotometer and the aerosol amounts by Nakajima et al.(1996) was used as the input data of the solar radiative transfer model. And the surface albedo is assumed to be 0.20 in summer and 0.85 in winter. The sensitivity test of solar radiative transfer model was done with the variation of total ozone, aerosol amount, and surface albedo. When the cosine of solar zenith angle is 0.3, Erythemal UV-B radiation decreased 73% with the 200% increase of total ozone from 100 DU to 300 DU, but the decrease of UV-A radiation is about 1%. Also, for the same solar zenith angle, UV-A radiation was decreased 31.0% with the variation of aerosol optical thickness from 0.0 to 0.3 and Erythemal UV-B radiation was decreased only 6.1%. The increase of Erythemal W-B radiation with the variation of surface albedo was twice that of UV-A increase. The surface Erythemal UV-B and UV-A radiation calculated by solar raditive transfer model were compared with the measured values fer the relatively clear day at King Sejong Station in West Antarctica. The model calculated Erythemal UV-B radiation at the surface coincide well with the measured values except for cloudy days. But the difference between the model calculated UV-A radiation and the measured value at the surface was large because of cloud scattering effect. So, the cloud property data is needed to calculate the UV radiation more exactly at King Sejong Station in West Antarctica.

Area-Average Solar Radiative Transfer in Three-Dimensionally Inhomogeneous Clouds: The Independently Scattering Cloudlet Model

A new conceptual and computational basis is described for renormalizing the single-scatter and extinction properties (optical depth, single-scatter albedo, and scattering phase function or asymmetry parameter) of a three-dimensionally inhomogeneous cloud volume or layer so as to describe a radiatively equivalent homogeneous volume or layer. The renormalization may allow area-averaged fluxes and intensities to be efficiently computed for some inhomogeneous cloud fields using standard homogeneous (e.g., plane parallel) radiative transfer codes. In the Independently Scattering Cloudlet (ISC) model, macroscopic “cloudlets” distributed randomly throughout a volume are treated as discrete scatterers, analogous to individual cloud droplets but with modified single-scatter properties due to internal multiple scattering. If a volume encompasses only cloudlets that are optically thin, the renormalized single-scatter properties for the volume revert to the intrinsic values and the homogeneous case is recovered. Although the ISC approach is based on a highly idealized, and therefore unrealistic, geometric model of inhomogeneous cloud structure, comparisons with accurate Monte Carlo flux calculations for more realistic random structures reveal surprising accuracy in its reproduction of the relationship between area-averaged albedo, direct transmittance, diffuse transmittance, and in-cloud absorptance. In particular, it describes the approximate functional dependence of these characteristics on the intrinsic single-scatter albedo when all other parameters are held constant. Moreover, it reproduces the relationship between renormalized single-scatter albedo and renormalized optical thickness derived independently, via a perturbative analysis, by other authors. Finally, the ISC model offers a reasonably intuitive physical interpretation of how cloud inhomogeneities influence area-averaged solar radiative transfer, including the significant enhancement of in-cloud absorption under certain conditions.

Stochastic radiative transfer in multilayer broken clouds. Part I: Markovian approach

A two-part paper describes the statistical treatment of solar radiative transfer in multilayer broken clouds. The proposed approach is a logical development of the statistical ones originally suggested for a single-layer broken clouds. This first part introduces a new statistically inhomogeneous Markovian model that allows one to properly account for different combinations of the random and maximum overlap of broken clouds at distinct vertical layers. The statistically inhomogeneous Markovian model and the stochastic radiative transfer equation have been used to derive equations for the mean radiance of solar radiation. It was demonstrated that in extreme cases the obtained equations agree with corresponding equations previously derived for (i) the statistically homogeneous broken clouds and (ii) the vertically inhomogeneous overcast clouds.

Chlorophyll modulation of mixed layer thermodynamics in a mixed-layer isopycnal General Circulation Model — An example from Arabian Sea and equatorial Pacific

Western tropical Indian Ocean, Arabian Sea, and the equatorial Pacific are known as regions of intense bio-chemical-physical interactions: the Arabian Sea has the largest phytoplankton bloom with seasonal signal, while the equatorial Pacific bloom is perennial with quasi-permanent upwelling. Here, we studied three dimensional ocean thermodynamics comparing recent ocean observation with ocean general circulation model (OPYC) experiment combined with remotely sensed chlorophyll pigment concentrations from the Coastal Zone Color Scanner (CZCS). Using solar radiation parameterization representing observations that a higher abundance of chlorophyll increases absorption of solar irradiance and heating rate in the upper ocean, we showed that the mixed layer thickness decreases more than they would be under clear water conditions. These changes in the model mixed layer were consistent with Joint Global Ocean Flux Study (JGOFS) observations during the 1994-1995 Arabian Sea experiment and epi-fluorescence microscopy (EFM) on samples collected during Equatorial Pacific Ocean Climate Study (EPOCS) in November, 1988. In the Arabian Sea, as the chlorophyll concentrations peak in October (3 mg/m3) after the summer plankton bloom induced by coastal upwelling, the chlorophyll induced biological heating enhanced the sea surface temperature (SST) by as much as 0.6‡C and sub-layer temperature decreases and sub-layer thickness increases. In the equatorial Pacific, modest concentrations of chlorophyll less than 0.3 mg/m3 is enough to introduce a meridional differential heating, which results in reducing the equatorial mixed layer thickness to more than 20 m. The anomalous meridional tilting of the mixed layer bottom enhances off equatorial westward geostrophic currents. Consequently, the equatorial undercurrent transports more water from west to east. We proposed that these numerical model experiments with use of satellite andin situ ocean observations are consistent under three dimensional ocean circulation theory combined with solar radiation transfer process.