SODAR-based estimation of TKE and momentum flux profiles in the atmospheric boundary layer: Test of a parameterization model

Summary A simple parameterization for the estimation of turbulent kinetic energy (TKE) and momentum flux profiles under near-neutral stratification based on sodar measurements of the vertical velocity variance has been tested using data from the LINEX-2000 experiment. Measurements included operation of a phased-array Doppler sodar DSDPA.90 and of a sonic anemometer USA-1 mounted at a meteorological tower at a height of 90m. Good agreement has been found between the TKE and momentum flux values derived from the sonic and sodar data (with correlation coefficients r>0.90 and a slope of the regression lines of about 1.01.1) suggesting the possible use of sodar measurements of _w ² to derive turbulence parameter profiles above the tower range.     

Effect of land management on ecosystem carbon fluxes at a subalpine grassland site in the Swiss Alps

Summary The influence of agricultural management on the CO₂ budget of a typical subalpine grassland was investigated at the Swiss CARBOMONT site at Rigi-Seebodenalp (1025m a.s.l.) in Central Switzerland. Eddy covariance flux measurements obtained during the first growing season from the mid of spring until the first snow fall (17 Mai to 25 September 2002) are reported. With respect to the 10-year average 1992–2001, we found that this growing season had started 10 days earlier than normal, but was close to average temperature with above-normal precipitation (100–255% depending on month). Using a footprint model we found that a simple approach using wind direction sectors was adequate to classify our CO₂ fluxes as being controlled by either meadow or pasture. Two significantly different light response curves could be determined: one for periods with external interventions (grass cutting, cattle grazing) and the other for periods without external interventions. Other than this, meadow and pasture were similar, with a net carbon gain of –128±17g Cm^–2 on the undisturbed meadow, and a net carbon loss of 79±17g Cm^–2 on the managed meadow, and 270±24g Cm^–2 on the pasture during 131 days of the growing season, respectively. The grass cut in June reduced the gross CO₂ uptake of the meadow by 50±2% until regrowth of the vegetation. Cattle grazing reduced gross uptake over the whole vegetation period (37±2%), but left respiration at a similar level as observed in the meadow.     

Environmental aerosols and their effect on the Earth’s local fair-weather electric field

Summary The Earths local fair-weather electric field is significantly affected by small ions present in the atmosphere. These ions are typically smaller than 0.001µm and occur in concentrations from 500 to 600cm^–3 in air. Attachment to larger aerosol particles may severely decrease the mobility of these atmospheric ions resulting in an increased local electric field. The number concentration of environmental aerosol particles in the size range 0.1 to 5.0µm was measured with two automatic laser scattering particle counters. The Earths electric field was monitored with an electric fieldmeter. Measurements were made in clean air and in an environment highly polluted by wood smoke. The electric field was found to be positively correlated to the aerosol number concentration. During one 24-hour period of measurement, the electric field increased from 180 to about 280Vm^–1 as the number concentration of aerosols larger than 0.1µm increased from about 2000 to 9000cm^–3. The number concentrations of aerosols larger than 0.1 and 0.3µm were both found to be positively correlated with the Earths electric field with correlation coefficients of 70% and 61%, respectively.Present address: School of Physical Sciences, Queensland University of Technology, Brisbane 4001, Australia.     

Black body temperature, orbital elements, the Milankovitch precession index, and the Seversmith psychroterms

Summary The temperature T of a black or gray body orbiting the Sun can be expressed in terms of spherical harmonics in latitude and longitude, its Keplerian orbital elements, and a variable describing rotation about its axis. Assuming that the Earth is a black or gray body without thermal inertia, the resulting equation for T exhibits previously unrecognized odd-degree zonal terms dubbed Seversmith psychroterms. They cause a hemispheric temperature difference which depends upon e sin _S, where e is the orbital eccentricity and _S is the Suns argument of perigee measured in an Earth-centered frame. The hemisphere containing perihelion is the cooler. For a gray body with the Earths average albedo of 0.3, an emissivity of unity, and an obliquity of 23.5°, the pole-to-pole temperature difference for the combined first and third degree spherical harmonic psychroterms can reach 3.4K for the present eccentricity of 0.016, and 12.9K for the maximum eccentricity of 0.06. While a thermally inertia-less black or gray body with boiling hot subsolar points and nights at absolute zero are poor models for the Earth, the Seversmith psychroterms will survive in more realistic models (although with smaller amplitudes) because the Earth radiates nonlinearly in T. The psychroterms acts in the direction opposite to the Milankovitch precession index, which also depends on e sin _S: by warming the cool northern summers, the psychroterms make it harder for the traditional Milankovitch mechanism to operate. The Seversmith psychroterms could in fact be responsible for the ice sheets that cycle with e sin _S, instead of the Milankovitch mechanism. By cooling the southern hemisphere for thousands of years when perihelion is in the south, the psychroterms may somehow cause the southern hemisphere to control the northern ice sheets associated with the 23kyr and 19kyr periods (kyr=10³ years), possibly through ice-albedo feedback in the sea ice surrounding Antarctica. Two other unexpected features besides the psychroterms are: while the average insolation increases with increasing e, the average temperature of the Earth paradoxically decreases; and the equator-to-pole temperature difference of 21K on a gray body with an albedo equal to 0.3 and an emissivity of unity is actually smaller than the observed difference of 28K on the real Earth.     

Propagation of the sea-surface temperature anomalies in the Tropical Atlantic

Summary The evolving modes of the sea-surface temperature (SST) in the Tropical Atlantic on the short interannual (IA) timescale were obtained by performing the extended empirical orthogonal function (EEOF) analyses on this variable separately for the 106-year (1871–1976) and 20-year (1881–1900; 1901–1920; 1921–1940; 1941–1960) periods. The equatorial and inter-hemispheric patterns manifest in the first EEOF mode of each analysis as part of the short IA evolution of the SST anomalies in the Tropical Atlantic. Another outstanding feature of the first EEOF mode of each analysis concerns the propagations of the SST anomalies in the meridional direction within the 20°N–20°S band and in the zonal direction in the sector 40°W–20°W. For all analyses, the SST anomalies propagate northward from the equator to 15°N and southward from 20°N to 15°N, with the same sign anomalies merging approximately at 15°N. On the other hand, the SST anomalies propagate westward in the sector 40°W–20°W with a propagation rate close to that of the phase speed of the fastest baroclinic Rossby wave in the ocean. So, the observed propagations of the SST anomalies in the 20°N–20°S band might result from the combined effect of the surface oceanic currents in this band and the baroclinic Rossby waves in the ocean.     

Amazonian forest dieback under climate-carbon cycle projections for the 21st century

Summary The first GCM climate change projections to include dynamic vegetation and an interactive carbon cycle produced a very significant amplification of global warming over the 21st century. Under the IS92a business as usual emissions scenario CO₂ concentrations reached about 980ppmv by 2100, which is about 280ppmv higher than when these feedbacks were ignored. The major contribution to the increased CO₂ arose from reductions in soil carbon because global warming is assumed to accelerate respiration. However, there was also a lesser contribution from an alarming loss of the Amazonian rainforest. This paper describes the phenomenon of Amazonian forest dieback under elevated CO₂ in the Hadley Centre climate-carbon cycle model.     

Recent warming in a small region with semi-oceanic climate, 1949–1998: what is the ground truth?

Summary Trends of monthly air temperature extremes were investigated in five meteorological stations of the Grand-Duchy of Luxembourg during the period 1949–1998. The application of an innovative homogenization method based on the concept of relative homogeneity to climatic time series allows identifying multiple break points, as well as correcting data series in an objective and robust statistical way. The rise of maximum temperature (Tmax) has occurred at a rate of 1.5 times that of the minimum temperature (Tmin) in winter (+1.4°C versus +0.9°C) and summer (+1.4°C versus +0.8°C). No trend in temperature extremes was found in autumn, while spring was affected by a small warming (+0.3°C) of Tmin and no change in Tmax resulting in a decrease of the diurnal temperature range (DTR) (–0.3°C). In spring, a strong positive linear relationship between Tmin warming and local terrain slope could be found. Comparison to new-gridded large-scale climatologies indicates generally close agreement to temperature trends during the 1949–1998 period, while a lower local warming was observed in summer during the post-1975 period following the changing-point year of atmospheric circulation over North-western Europe. This study shows that the question of data homogeneity is not trivial and should receive careful attention before quantifying historical temperature trends and identifying their spatial patterns at regional scale.     

Assessment of climatic suitability for the expansion of Solenopsis invicta Buren in Oklahoma using three general circulation models

Summary This study assessed the climatic suitability for the expansion of Solenopsis invicta Buren (red imported fire ant) in Oklahoma under the present climate and with a doubling of atmospheric CO₂ using three general circulation models (GCMs) (GFDL R30, OSU, UKMO). Oklahoma was chosen as the geographical focus because it has a dense network of meteorological stations and lies on the edge of the current biogeographic range of S. invicta. Meteorological data were spatially referenced with model data in GIS to produce a series of images of selected suitability indicators: (1) mean annual precipitation >510mm; (2) less than seven consecutive days with mean air temperature <1.1C; and (3) mean winter air temperature >9.4C. These indicator images were combined to produce suitability maps for the potential range of S. invicta. Under current climatic conditions, roughly three-quarters of Oklahoma is suitable for potential invasion by S. invicta. The GFDL R30, OSU, and UKMO show that the area suitable for colonization increases by approximately 26, 26, and 36%, respectively. In terms of actual land area, the increase with a warmer, wetter climate ranges from 35,300km² to 47,600km². The destructiveness of S. invicta on human livelihood necessitates a better understanding of the future expansion of the species for an uncertain future climate.     

Modification Of The Standard ∈-Equation For The Stable Abl Through Enforced Consistency With Monin–Obukhov Similarity Theory

A condition is derived for consistency of the standard-equation with Monin–Obukhov (MO) similarity theory of thestably-stratified surface layer. The condition is derivedby extending the procedure used to derive the analogous condition forneutral theory to stable stratification. It is shown that consistencywith MO theory requires a function of flux Richardson number, Ri_f, to be absorbed into either of two closure parameters, c ₁ or c ₂.Inconsistency, on the other hand, results if constant values of these are maintained for all Ri_f, as is done in standardapplication of the equation, and the large overpredictions ofturbulence found in such application to the one-dimensionalstable atmospheric boundary layer (1D-SBL) are traced to thisinconsistency. Guided by this, we formulate a MO-consistent-equation by absorbing the aforementioned function intoc ₁, and combine this with a Level-2.5 second-orderclosure model for vertical eddy viscosity and diffusivities.Numerical predictions of the 1D-SBL by the modified model converge to a quasi-steady state, rectifying the predictive failure of the standard -equation for the case.Quasi-steady predictions of non-dimensional variables agree stronglywith Nieuwstadt's theory. Qualitative accuracy of predictionsis inferred from comparisons to field data, large-eddy simulationresults and Rossby-number similarity relationships.     

Seasonal variations of anthropogenic sulfate aerosol and direct radiative forcing over China

Summary Regional climate model (RegCM2) and sulfur transport model (NJUADMS) were combined to simulate the distribution of anthropogenic sulfate aerosol burden over China, where a look up table method was applied to illustrate sulfate formation from SO₂-oxidation. Direct radiative forcing of sulfate aerosol was further estimated using the scheme suggested by Charlson et al (1991). Investigations show that the annual average total sulfate column over mainland China is 2.01mg/m² with high value in East and Central areas (more than 7mg/m²). The annual average direct radiative forcing of China is about –0.85W/m². The forcing can reach –7W/m² in Central and East China during the winter season. Total sulfate column shows significant seasonal variations with winter maximum-summer minimum in the Southern part of China and spring maximum-autumn minimum in the northern part of China. Strong seasonal cycles of direct radiative forcing are also found due to the influence of total sulfate column, cloud, relative humidity and the reflectivity of underlying surfaceReceived May 16, 2001; accepted August 5, 2002 Published online: May 8, 2003     

Infrared observations and numerical modelling of grassland fires in the Northern Territory, Australia

Summary This paper reports on a small-scale pilot experiment held early in the dry season near Darwin, Australia, in which fine-scale observations of several prescribed fires were made using infrared digital video. Infrared imaging is used routinely to locate fires as infrared radiation suffers little attenuation as it propagates through the smoke that normally obscures visible imagery. However, until now, little use has been made of digital video imagery in analyzing the convective-scale structure of prescribed (or wild) fires. The advantage of digital video imagery is that the individual frames can be objectively analyzed to determine the convective motion in the plane viewed by the camera. The infrared imagery shows mostly rising plumes, much like convective clouds. The flow is highly convective, and the vertical transport of heat is confined to relatively narrow thermals. The updrafts range from a few ms^–1 to around 15ms^–1. A numerical model is used to simulate one of the prescribed fires at very high-resolution. For the most part, the model predictions compare well to the observations. The model produces plumes that are around 7m high, and spaced around 5m apart, which is similar to that observed. The model correctly predicts the mean rate of spread of the fire to be 1.3ms^–1. Perhaps the most serious limitations to using infrared observations of the type presented here are the difficulties in interpreting precisely the relationship between the observed infrared temperature field and the air temperature calculated by the model, and the exact connection between the infrared camera derived flow field and that calculated by the model.     

A study on the number of snow days over Eurasia, Indian rainfall and seasonal circulations

Summary In this paper, the relationship between seasonal mean (June, July, August and September) monsoon circulation features and the midlatitude circulations in winter and spring seasons have been examined during contrasting years of more (less) number of snow days in winter/spring followed by deficient (excess) Indian Summer Monsoon Rainfall (ISMR) using NCEP/NCAR reanalyzed data for the period 1966–1994. The Historical Soviet Daily Snow Depth (HSDSD) version II data set has been used to calculate the number of days of snow over west and east Eurasia separately under three classes: class 1 for SD>5cm, class 2 for SD>10cm and class 3 for SD>50cm where SD stands for snow depth. Correlation coefficients are computed between the anomaly in the number of days of snow depth under the above three classes during winter/spring over west and east Eurasia and the subsequent ISMR. HSDSD data show that difference in the number of days of SD>10cm in two extreme years is most prominent in the west Eurasia in the months of January and April. Also the anomaly in the number of days of snow in January and April over west Eurasia has correlation coefficients of –0.69 and –0.56 with the following ISMR, respectively at 0.1% significance level when the SD is more than 10cm at all the stations. Results also show that low-level atmospheric temperature difference between two extreme years of snow days in winter is up to 10°C and the cooling persists up to spring season with a difference of 2°C. This cooling persistence may give rise to anomalous cyclonic circulations over the midlatitudes and tropics which may be responsible for weakening the monsoon circulation over India during the year of more snow days over west Eurasia.     

Dynamical and microphysical characteristics of Arctic clouds using integrated observations collected over SHEBA during the April 1998 FIRE.ACE flights of the Canadian Convair

Summary The purpose of this study is to better understand the dynamical and microphysical processes within Arctic clouds, which occurred in April 1998 over the Surface Heat Budget of the Arctic Ocean (SHEBA) ship during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment Arctic Cloud Experiment (FIRE.ACE). The observations from the four cases in the present study were collected by instruments mounted onboard the National Research Council (NRC) Convair, as well as, the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) satellite, the SHEBA surface based NOAA Doppler radar (35GHz, Ka-Band), and the NOAA depolarization lidar (0.523µm) measurements. The aircraft observations were collected at 32Hz (3-m scale). The Meteorological Services of Canada (MSC) lidar (1.064µm) was operated onboard the Convair-580. The AVHRR observations, representing a 5-km horizontal resolution, were used to estimate particle size, phase, and optical thickness. Constant altitude flight legs were made at about 100m over the ocean surface. Vertical air velocity (w), reflectivity and Doppler velocity, and backscatter and depolarization ratio values were used to define the size of the important dynamical structures. Ice crystal number concentration (N_i), ice water content (IWC), droplet number concentration (N_d), liquid water content (LWC) and characteristic particle size and shape were summarized for each case. The effective radius (r_eff) values for liquid clouds obtained from in-situ and AVHRR observations were found comparable. The large variability in IWC can be due to undetected ice crystals at small size ranges. Mixed phased conditions in the AVHRR retrievals complicated the comparisons with in-situ data. N_i was found to be directly related to the history of the air-parcel dynamics e.g., w. The variability and differences in the parameters obtained from various platforms can be attributed to their instrumental capabilities, resolution, as well as the cloud development.     

The performance of an E−l scheme for the atmospheric boundary layer in a mesoscale model with grid spacing as small as 1 km

Summary ¶A new E–l boundary layer scheme is tested within the U.S. Navys COAMPS model. The goal is to give COAMPS the capability to simulate mesoscale cellular convection. The new scheme is aimed to be consistent with both classic results for clear entrainment and recent calibrations, derived from large-eddy simulations, for entrainment into smoke clouds and water clouds. A parameter is included in the scheme that allows sub-grid transport to be reduced so that, when the model has 2km grid spacing or less, more of the transport is forced to occur in resolved convection. At 2km grid spacing, the scheme allows COAMPS to simulate the break up of a stratocumulus cloud deck into mesoscale cellular convection.Received June 7, 2002; accepted August 13, 2002 Published online: February 20, 2003     

CO2 efflux from agricultural soils in Eastern Germany – comparison of a closed chamber system with eddy covariance measurements

Summary In order to quantify the effects of temperature and soil water content on soil respiration, during June and July 2002 CO₂ soil efflux was measured with a closed chamber (non-steady state, flow through) system in the field. The amount of CO₂ emission was highly dependent on the land-use in the observation area, which consisted of meadow soil and brownfield. The CO₂ emission from the brownfield ranged from 0.9 to 5.5µmol CO₂ m^–2s^–1, and that for meadow soil from 1.1 to 12.6µmol CO₂ m^–2s^–1. Soil respiration, as a function of soil temperature (T_soil), relative soil water content (RSWC), soil pH, and the soil carbon/nitrogen ratio (C/N), was analysed by a modified closed non-linear regression model. Between 63% and 81% of the variation of soil CO₂ emission could be explained with changes of T_soil, RSWC, pH, and C/N for the individual chambers on the brownfield.Subsequent analysis involved a comparison of the soil chamber results with eddy covariance (EC) measurements of one week, and included a footprint analysis to account for the influence of the different land use types on the measurements. For this, EC data (143 measurements after quality check) were restricted to those originating from the brownfield area with more than 90% of the flux. For a second comparison, the net ecosystem exchange (NEE) was calculated for different parts of the meadow using the SVAT model PROXEL. Together with the respiration from the brownfield, a weighted average of model NEE was produced using the flux contribution determined by the footprint model. Acceptable agreement (r²=0.69) was found between the modelled data and individual EC measurements, except during situations where the performance of the footprint model was disturbed by internal boundary layer effects.     

Application of a “Big-Tree” model to regional climate modeling: a sensitivity study

Summary ¶In order to better understand land-atmosphere interactions and increase the predictability of climate models, it is important to investigate the role of forest representation in climate modeling. Corresponding to the big-leaf model commonly employed in land surface schemes to represent the effects of a forest, a so called big-tree model, which uses multi-layer vegetation to represent the vertical canopy heterogeneity, was introduced and incorporated into the National Center for Atmospheric Research (NCAR) regional climate model RegCM2, to make the vegetation model more physically based. Using this augmented RegCM2 and station data for China during 1991 Meiyu season, we performed 10 experiments to investigate the effects of the application of the big-tree model on the summer monsoon climate.With the big-tree model incorporated into the regional climate model, some climate characteristics, e.g. the 3-month-mean surface temperature, circulation, and precipitation, are significantly and systematically changed over the model domain, and the change of the characteristics differs depending on the area. Due to the better representation of the shading effect in the big-tree model, the temperature of the lower layer atmosphere above the plant canopy is increased, which further influences the 850hPa temperature. In addition, there are significant decreases in the mean latent heat fluxes (within 20–30W/m²) in the three areas of the model domain.The application of the big-tree model influences not only the simulated climate of the forested area, but also that of the whole model domain, and its impact is greater on the lower atmosphere than on the upper atmosphere. The simulated rainfall and surface temperature deviate from the originally simulated result and are (or seem to be) closer to the observations, which implies that an appropriate representation of the big-tree model may improve the simulation of the summer monsoon climate.We also find that the simulated climate is sensitive to some big-tree parameter values and schemes, such as the shape, height, zero-plane displacement height and mixing-length scheme. The simulated local/grid differences may be very large although the simulated areal-average differences may be much lower. The area-average differences in the monthly-mean surface temperature and heat fluxes can amount to 0.5°C and 4W/m², respectively, which correspond to maximum local/grid differences of 3.0°C and 40W/m² respectively. It seems that the simulated climate is most sensitive to the parameter of the zero-plane displacement among the parameters studied.     

The convective boundary layer over pasture and forest in Amazonia

Summary The coupling between different types of surface (tropical forest or grass) and the Convective Boundary Layer (CBL) has been investigated using observational (rawinsoundings) data collected over Rondônia in southwest Amazonia. The data reported here support the notion that deforestation may modify the dynamics of the boundary layer, in particular during the dry season. In this period the sensible heat fluxes are very high over pasture, creating a CBL around 550m deeper compared to that over the forest. The measurements showed the height of the fully developed CBL for pasture to be 1650m, compared to around 1100m for forest. During the wet season the height of the CBL is lower than during the dry season and has the same height (around 1000m) for forest and pasture sites. The CBL over pasture is hotter and drier than over forest during the dry season, but during the wet season the air temperatures and humidities are similar. Comparing the CBL growth during the dry and wet season, there is evidence that the CBL properties over the forest are not dependent on the surface characteristics, but over the pasture they are.     

Measured and modelled contributions to UV exposures by the albedo of surfaces in an urban environment

Summary The increases in the erythemal UV exposures to horizontal planes and to inclined planes over three surfaces that are found in an urban environment (water, concrete and sand) due to the albedo of these surfaces have been estimated. For the cloud free case, the additional daily estimated UV exposures to a horizontal plane have a maximum value of 222 (Jm^–2)_ER, where the index after the unit is there to indicate that it refers to a biologically effective exposure. In comparison, the daily erythemal UV exposures over a year to a horizontal plane ranged from 425 to 8,321 (Jm^–2)_ER. For a vertical receiving plane that is rotating about a vertical axis, the additional erythemal daily UV exposures for the sub-tropical latitude location of this research for the ranges of solar azimuth angles encountered over the days in each season ranged from 16 to 311 (Jm^–2)_ER, 29 to 566 (Jm^–2)_ER and 46 to 905 (Jm^–2)_ER for water, concrete and sand respectively. The estimated error is ±20% and the calculations are based on clear-sky conditions. The additional erythemal UV averaged over each of the seasons was higher for the receiving plane inclined at 45° below the horizontal plane. In a similar fashion, the vertical surface has the higher additional erythemal UV exposures compared to the surfaces inclined at an angle above the horizontal.     

Rain height in relation to 0°C isotherm height for satellite communication over the Indian Subcontinent

Summary ¶The 0°C isotherm height, a parameter needed for the estimation of attenuation of microwave and millimetre wave for earth-space communication, has been estimated for different stations spread over India. The variations of 0°C isotherm height for different seasons over these stations are presented. Attenuations of radio wave due to rain at frequencies 10GHz and above have also been estimated for few stations using the 0°C isotherm height so derived. The results are useful for radio systems designers.     

Transition from the Southern to the Northern Hemisphere summer of zones of active convection over the Congo Basin

Summary This study used monthly rainfall totals for the period 1961 to 1988 and pentad OLR values for the period 1974 to 1991 to study the structure and transition of active convention across the Congo Basin (10°S–5°N, 15°E–35°E) from the southern to the northern hemisphere summer. This involved the examination of map patterns and cross-sections of monthly rainfall and pentad OLR data.The results from the study indicated that there were two seasons observed over the Congo Basin; one is the wet season lasting from September to April and the other a dry season covering the rest of the year. The onset of the wet season takes place rapidly with active convection spreading very quickly to the south near latitude 20°S. This is due to the formation of the meridional (north–south) branch of the ITCZ over this region.This study has confirmed that the annual rainfall over the Congo Basin is reliable with the coefficient of variation of less than 30%. The wet seasons (e.g., SON and DJF) also show reliable rainfall occurrence but the dry season (e.g., JJA) has low reliability.The anomalously wet seasons are characterised by a relatively slow transition rate (1° latitude per pentad) of zones of active convection resulting in a late withdrawal of the rainy season while the dry seasons show a rapid transition rate with an early withdrawal of zones of active convection.High-rainfall months (>200mm) are concentrated within the Southern Hemisphere summer months. These high-rainfall months progress from the equator to the southern latitude following generally the movement of the overhead sun.The results further revealed that the years 1987/1984 had the lowest/highest mean OLR values over the Congo Basin within the period 1979 to 1991. The rates of transition of the zones of low OLR values were 0.9/5.0 degrees of latitude during 1987/1984, respectively.Received June 18, 2002; revised September 30, 2002; accepted November 21, 2002 Published online: June 12, 2003