Characterization of Surface Layer Turbulence across a West African Tropical Climate Belt

This study surveyed the levels of boundary layer surface turbulence across a West African climate region. Five years (2011-2015) temperature and wind speed data at synoptic hours 0000 Hr, 0600 Hr, 1200 Hr and 1800 Hr within 0.125˚ grid resolution was sourced from Era-Interim Reanalysis platform at 1000 mbar pressure level. Using the Richardson (R i ) number technique, results showed that mechanical turbulence of R i range 0.04 - 0.57 dominates across the surface layer for study locations of Port Harcourt, Enugu, Jos, Kano and Maiduguri than thermal turbulence. However, the least turbulent area was the coastal zone of surveyed region. Results also indicated that the vertical height (L) at which thermal turbulence replaces mechanical turbulence across study locations ranged from 20 - 250 m with lowest replacement levels (20 - 50 m) occurring mainly in the coastal area of Port Harcourt during periods of dawn. The most turbulent periods in the southern coastal location of study region were during key rainy periods from June-August while that for the rest far northern inland areas occur during the dry season/early rainy periods i.e. November-May. The implication of the lower surface turbu-lence/replacement level within coastal domains most especially during periods of dawn is that emission releases near surface layer will be dispersed af-ter initial rise due to buoyancy at horizontal levels thereby increasing ground level pollutants concentration across sensitive receptors that are close to emission source. At heights of thermal turbulence replacement, emission releases will be transported vertically and then dispersed further away from emission sources, thus impacting sensitive receptors at farther distances. This is the atmospheric boundary layer dynamics that makes ground level pollution

across the surface layer for study locations of Port Harcourt, Enugu, Jos, Kano and Maiduguri than thermal turbulence. However, the least turbulent area was the coastal zone of surveyed region. Results also indicated that the vertical height (L) at which thermal turbulence replaces mechanical turbulence across study locations ranged from 20 -250 m with lowest replacement levels (20 -50 m) occurring mainly in the coastal area of Port Harcourt during periods of dawn. The most turbulent periods in the southern coastal location of study region were during key rainy periods from June-August while that for the rest far northern inland areas occur during the dry season/early rainy periods i.e. November-May. The implication of the lower surface turbulence/replacement level within coastal domains most especially during periods of dawn is that emission releases near surface layer will be dispersed after initial rise due to buoyancy at horizontal levels thereby increasing ground level pollutants concentration across sensitive receptors that are close to emission source. At heights of thermal turbulence replacement, emission releases will be transported vertically and then dispersed further away from emission sources, thus impacting sensitive receptors at farther distances. This is the atmospheric boundary layer dynamics that makes ground level pollution worse in the coastal city of Port Harcourt in recent times during periods of dawn. Efforts must be made by concerned Stakeholders towards ensuring that emissions are reduced during the periods of dawn within and around coastal environments.

Introduction
The surface layer is the layer in which humans, animals, and vegetation live and where most anthropogenic activities take place. It is the bottom part of the boundary layer that connects the atmosphere to ground surface and is very vital regardless of its lesser share within the overall atmosphere due to its significant turbulent exchanges [1]. The sharpest variations in meteorological variables with height occur within the surface layer and, consequently, the most significant exchanges of momentum, heat, and mass also occur in this layer [2]. Therefore, it is not surprising that surface layer has received far greater attention from meteorologists and climatologists than has the outer part of the atmospheric boundary layer (ABL). Turbulent exchanges processes in the ABL have profound effects on the evolution of local weather. Boundary layer fraction is primarily responsible for low-level convergence and divergence of flow in the regions of lows and high surface pressures, respectively. The frictional convergence in a moist boundary layer is also responsible for low-level convergence of moisture in low-pressure regions. The kinetic energy of the atmosphere is continuously dissipated by small-scale turbulence in the atmosphere. Almost one-half of this loss on an annual basis occurs within the ABL, even though the ABL comprises only a tiny fraction (less than 2%) of total kinetic energy of the atmosphere.
Due to geographical and micro-climatic differences, the level of surface layer turbulence varies from one climate zone to another [3]. This difference modifies ways lower atmospheric emissions are transported and dispersed within the boundary layer. The atmospheric surface layer is the active link between the atmosphere and surface and plays a major role in transporting suspended pollutant, water vapour and heat from ground surface. Accurate characterization of turbulent fluctuations in this layer is of great importance towards a successful modeling of large-scale meteorological processes. Reference [4]

Effects of Surface Turbulence on Boundary Layer Climate
A vast portion of solar heat is transferred to the surface where it is transformed and transferred to various portions of the atmosphere by processes taking place in the layer [5]. One of these critical processes is turbulence. Surface layer characteristics as well as energy exchange processes that are significantly enhanced by turbulent mixing determine climate pattern of any locality [6]. The exchange of sensible and latent heat fluxes within earth surface and atmosphere are through turbulence transfers and the heat fluxes modify micro-climatic processes that take place between the atmosphere and ground surface [7]. While turbulence at surface layer is more or less unceasing, the micro-climate condition of any locality depends on intensity and variation of turbulence in modifying surface heat fluxes within boundary layer atmosphere. The energy of dynamic heat fluxes prevalent within the surface averages over 85% and this makes the surface layer play a major role in transport processes that occur between the planetary layer and upper part of the troposphere [8] [9]. Within the lower troposphere, turbulence controls vertical interactions of all climatic variables such as atmospheric stability, heat and moisture. Roughness of the surface layer affects airflow at the boundary layer as well as interchange of momentum and energy between ground surface and overlaying atmosphere [10]. This brands the surface layer as wind induced instability layer, which acts to transfer fluxes vertically. The magnitude of instability, nevertheless, depends on significant factors such as surface coarseness and atmospheric stability pattern. Since surface layer is the section near ground's surface, a variation in radiation warming or cooling will be initially noticed at the surface before upper atmosphere [11]. The existence or lack of instability at the surface layer determines levels to which creatures are exposed to weather extremes. Due to mixing ability of turbulence, modeling atmospheric planetary layers is relevant for many practical applications. Ranging up to 100 meters in altitude, the surface layer exhibits dynamic properties that influence to a large extent, human activities [12].

Study Area
The position of Nigeria is principally within lowland moist tropics north of the equator and branded by a high-temperature system [2] [13]. The atmosphere of study area is characterized by two air mass i.e. moist/warm (Ocean inclined) and dry/warm (Sahara desert inclined) air masses separated by a zone of discontinuity known as inter-tropical discontinuity (ITD). The ITD is deep-seated during DOI: 10.4236/acs.2020.103023 408 Atmospheric and Climate Sciences months of June-September in northern part of Nigeria and above 70% of mean annual rainfall both at the northern and southern areas of Nigeria is reached during these months [14]. The double maxima rainfall experienced in deep coastal south, for instance is as a result of ITD moving northwards during early part of the year, bringing rainfall to peak in June and then returning back southwards later in the year with another round of peak rainfall in September.
The high temperature range observed over Nigeria at its latitudinal position is primary because the astronomical variation of insolation is a function of latitude and the daily variation in elevation of the sun is large in low latitude and rather small in high latitudes [15]. High altitude locations such as Jos Plateaux, Adamawa highlands, Obudu and Mambila plateau have a cooler climate than their surrounding lowlands [16]. The lower temperature observed at highlands areas is due to temperature decrease with height and less air pressure that is not able to retain much heat and for longer period. The factors which influence the distribution of temperature at any location in Nigeria include: the amount of insolation received, nature of surface, distance from water bodies, relief, nature of prevailing winds and ocean currents. Therefore, the closer an area is to the ocean, the lower the temperature due to Land and Sea Breeze effect as well as associated trade wind during day periods. This is because diurnal temperature of closer land areas is being modified by Sea Breeze effect. Therefore under this condition, temperature increases as one move inland northwards. However, due to fact that land cools faster than the ocean at night, the reversed pressure gradient ensures that areas closer to the ocean are slightly warm at nights than far inland areas. Under this condition, air temperature of far inland becomes lower than areas close to the ocean at night due to distance from the ocean. Also, during wet   [15]. Figure 1 shows the map of Nigeria with study areas.

Materials and Method
Richardson number (R i ) establishes significance of turbulence in the boundary layer. It is a measure of relative influence of turbulent suppression by convective heat transfer compared with turbulence generated by mechanical shear. The numerator is associated to the disrupting forces that engender buoyancy and denominator associated to dynamic energy that terminates buoyancy. Richardson interpreted this as a characteristic of the proportion of work done on gravitational stability to energy transferred from mean to turbulent motion [17].
The turbulent fluxes of momentum and heat at surface layer are determined by vertical profiles of wind speed and temperature within maximum surface layer depth z 1 and z 2 . Applying logarithmic finite difference for wind speed and potential temperature profiles, the Richardson number (R i ) at geometric mean height (z m ), a dimensionless characteristic of atmospheric turbulence, is given by where, g: is acceleration due to gravity dθ/dz: is the vertical potential temperature gradient T a : is absolute temperature (K) du/dz: is the vertical wind speed gradient (m/s) z m : is the geometric mean which is given by: The factor, (du/dz) 2 is square of the rate in which wind speed varies with height and it is also relative to the rate at which mechanical shear in the atmosphere creates turbulence. The potential temperature (dθ/dz) can be replaced in Equation (4), given by: where "Г d " is the adiabatic lapse rate which is approximately given as "−0.01˚C/m" where "Z 1 " and "Z 2 " are reference heights for the initial and final temperature levels. This requires transformation of measured temperature to potential temperature using Equation ( where Z o is surface roughness length, L (m). The assumed length utilised for study areas are shown on Table 1. Table 2 shows the characteristics of turbulence flow for atmospheric boundary layer.  value of 0.25, turbulent flow is generated mostly by mechanical convection [21].
Large negative R ig values are generated due to thermal convection [22]. The data for this study were obtained from the European Centre for Medium Ranged Weather Forecast (ECMWF) Era-Interim Re-analysis data for periods 2011-2015 at 0.125˚ resolution for 6-hourly synoptic interval, i.e., 0000, 0600, 1200 and 1800. Meteorological variables such as wind speed, air temperature and relative humidity were acquired at pressure level of 1000 mbar. Table 3 shows the average surface layer R i values across the study areas and similar to long term turbulence parameter profiles generated from Western U.S.

Analysis of Richardson Number (Ri) for Study Areas
(     R i values were lesser than Richardson Termination Level (T iT ) of 1. It is noted that any flow above the Termination Level becomes lamina [22].
Results also indicated that turbulent eddies varied across the study locations and this is due to the peculiar boundary layer microclimatic characteristics over turbulence. Reference [24] noted that higher winds are allied with low relative humidity while light winds are usually allied with high relative humidity. The least turbulent area of Port Harcourt indicates the prevalence of the humid air mass which accumulates the characteristics of the tropical wet climate condition due to the closeness from the source (Ocean). The humid boundary layer environment is also enhanced by the rate of evapotranspiration resulting from the wet surface as well as the several water bodies surrounding the coastal location. This humid air over Port Harcourt (than the rest study locations) moderates wind shear that lessens mechanical turbulence at the surface layer. Study results showed that the surface layer over Port Harcourt is most turbulent during the first bi-modal periods of rainy season i.e. (June-August). This is indicative of the increased surface layer wind speeds in Port Harcourt during this season due to the rain bearing air mass that sweeps the area. During this season, the ITD is entirely over Nigeria and there is increased instability due to convective activities across the atmospheric boundary layer of Port Harcourt. The average wind speed of Port Harcourt surface layer up to 50 m is from 0 -2.2 m/s [2]. Higher values of wind velocity are observed during the June-August season mainly during the afternoon periods. Reference [25] emphasized that atmospheric circulation in the tropics is largely related with low wind speeds of ranges less than 3 m/s. It was stated that these minimal wind speeds impact significantly on surface layer free convention during the afternoon periods and strong stable conditions in the night time. Port Harcourt is very stable during the early hours of dawn i.e. Pasquill stability (class F) and slightly-moderately unstable (Class C-B) during the daytime periods [26]. The diurnal turbulent trend anomaly noticed between Kano and Maiduguri that are of close proximity in the northern fringes of Nigeria (Figures 2-5) could be linked to the slight latitudinal difference which affects air temperature as well as Lake Chad River close to Maiduguri that tends to increase the moisture content of the atmospheric environment.

The Exchange of Mechanical Turbulence/Thermal
Turbulence over Study Areas    transforms slowly and increasingly from frictional regime at the surface to a buoyancy regime aloft. It has been shown that mechanical turbulence dominates the surface layers of the entire areas as indicated by the results from the analysed R i values which were small positives (see Table 2) ranging from 0.04 -0.57  Reference [15] noted that if the specific heat of any surface is high, more energy will have to be absorbed by the surface to increase surface air temperature.

The Implication of Surface Layer Turbulence on Pollutants Dispersion in the Coastal Area of Port Harcourt
When pollutants from any emission sources are released within the planetary has shown that horizontal dispersion of air pollutants will be more prominent within the surface layer in the areas. This means that increased emissions from sources at minimal surface layer turbulence will increase pollutants concentrations across sensitive receptors. In as much as the boundary layer is quite variable diurnally in thickness with respect to time from some meters to few kilometers, the impacts of emissions releases from an environment to another will be dependent on the boundary layer mixing height as propagated by the surface layer.
The dominant mechanical turbulence within the surface layer in Port Harcourt which is lesser than the rest study areas as well as its thermal replacement at shorter vertical distance shows that emissions dispersion will be slower at the surface layer and amplified at the mid layer within the Port Harcourt boundary layer than the rest study areas. This is because thermal turbulence increases mixing and hence enhances air pollutants transportation to farther distances away from emission sources. This study results revealed that surface layer turbulence due to wind shear is lowest in Port Harcourt during early hours of dawn i.e. 06:00 hour than the rest study areas. This means that major emission releases from very close distances from the City center during these periods will constitute serious threat to City dwellers. This phenomenon explains the hazards of air pollution that ravage the City areas over the years during the early morning

Conclusion
Pollutants dispersion within the atmospheric boundary layer for any location depends largely on responsive turbulence characteristics of surface layer generated by either thermal or mechanical forcings. This study analyses the turbu-