MicrometeorologyDr. K. GhoshAgricultural Meteorology DivisionIndia Meteorological Department

Scales of atmospheric systems and network of observationsThe atmospheric systems vary widely in both space and time. Broadly we can categorise in to 5 categories. They are

Micro scaleMeso scaleSynoptic scaleMacro scalePlanetary scale

Micro scale:The formation of dew on the leaf of a plant and reduction in visibility over small areas are examples of micro scale system. To record and study these events we require observations with a spacing of few metres and the observation should be recorded almost continuously.

MicrometeorologyMicrometeorology is a branch of meteorology which deals with the atmospheric phenomena and physical processes taking place over limited region of the surface of the earth in the lowest layer of the atmosphere.The development of micrometeorology requires the following:-Accurate observations made in the layers of air adjacent to the surface.The study of the physical processes which give rise to the microclimate.

Importance of study of micrometeorology

It is the lowest atmosphere having many of its characteristic properties on which much of life depends.It is the region of turbulence at which the transport of heat energy, momentum and moisture takes place.It is the region at which exchange of CO2 between plant and animal life, the scattering of pollen and the lighter seeds, and the cycle of water occur.

Influence of soil surface on microclimateSoil is acting as a sink of solar heat energy during the day time and source to the surface at night.Soil heat is the most important factor that controls the intensity of biophysical, biochemical, microbiological and micrometeorological processes that take place in and above the soil.Soil temperature is influenced by the mixing and transport process of heat within the soil and by exchange of heat between the soil and atmosphere. Heat transfer within the soil can occur by conduction and convection.

Radiation budget at the land plant atmosphere interfaceThe energy balance at the surface is given by the equation,Rn = E + C + G + P + Swhere, Rn Net radiation, Latent heat of evaporation,E Evaporation,C Sensible heat flux,G Ground heat flux,P Energy for photosynthesis,S Storage term.The heat energy losses due to photosynthesis and storage within the canopy is about 2 to 3% of net radiation. The importance of the parameter Rn is that it is the fundamental quantity of energy available at the earths surface to drive the process of evaporation, photosynthesis etc.

The Net RadiationThe reflected portion of the total direct and diffused solar radiation (Rsw) is determined by the reflectivity r of the underlying surface, that is (Rsw) = r (Rsw)Thus, the shortwave radiation balance can be written as Rswbal = (Rsw) (Rsw) = (1 r) RswThe arrows indicate the direction of the radiation streams. The effective longwave balance is Rlwbal = (Rlw Rlw)The net of all the fluxes of radient energy Rn can, than, be written as Rn= Rswbal + Rlwbal = (1-r) (Rsw) + Rlwbal = (1-r) (Rsw) + Rlw Rlw = (1-r) (Rsw) + Rlw T4where, T is the temperature of the earths surface and is Stefan Boltzmans constant. The net radiation is the difference between total upward and downward radiation fluxes and is a measure of the energy available at the ground surface.

Light penetration into plant canopiesThe penetration of net radiation into plant canopies was introduced and described as a function of depth into the canopy expressed in terms of cumulative LAI.The penetration of light into plant canopies can be described or approximated in mathematical terms adopting the Beer-Bougher law as follows: where, Ke is an extinction coefficient for plant leaves and F is the downward cumulative leaf area index, I is the intensity of light at a particular height within the canopy and I0 is the intensity of light at the top of the canopy.The extinction coefficient can be defined as the ratio between the light loss through the leaf and the light at the top of the leaf. The extinction coefficient varies with the orientation of the leaves.Ke may range from 0.3 to 0.5 in upright leaf communities and from 0.7 to 1.0 in communities with horizontal leaves.

Understanding the relationship between radiation and crop production requires knowledge of radiation distribution within the crop canopy based on the transmissibility of the leaf, leaf arrangement and inclination, plant density, plant height and the angle of the sun.Transmissibility varies slightly with the age of the leaf. The transmissibility of a young leaf is relatively high. With the maturity of the leaf, it declines but rises again as the leaf turns yellow.The transmissibility of a leaf is directly related to its chlorophyll content. The logarithm of transmissibility decreases linearly with an increase in chlorophyll content. Transmissibility of leaves of deciduous trees, herbs, and grasses ranges from 5 to 10%, whereas, that of evergreen plants varies from 2 to 8%.If the leaves that transmit 10% of the radiation were horizontally displayed in continuous layers, only 1% of light, could penetrate the second layer.However, leaves are rarely displayed horizontally. In full sunlight, the optimum leaf inclination for efficient light use is 810. For the best results and in an ideal arrangement of the plant canopy, the lower 13% of the leaves should be oriented at an angle of 0 to 300, middle 37% of the leaves should be at 30 to 600 and the upper 50% leaves should be at 60 to 900 with the horizontal.When the plant height increases, the interception of light by the canopy also increases with only a small variation at different times of the day.The interception of light is minimum at noon and maximum during morning and evening hours.

Different heat fluxes in micrometeorological studiesThe source of heat fluxes is the solar radiation and it is transported by means of turbulent mixing, conduction, convection and diffusion processes. Different fluxes are Soil Heat Flux The rate of heat flux into soil is determined by the temperature gradient and the thermal conductivity (K). The latter is defined as the quantity of heat flowing in unit time through a unit cross section of soil in response to a specified temperature gradient. K has units of Wm-1k-1. The soil heat flux S, or heat flow into or out of the soil, is given by

where, dT/dz is the temperature gradient within the soil. All energy fluxes to the surface will be considered positive and all away from the surface will be negative.Thermal conductivity depends on porosity, moisture content, and organic matter content of the soil. At similar moisture contents conductivity decreases from fine sand to silt loam to clay soil because of the increasing porosity in this sequence of textures.

Sensible Heat FluxSensible heat flux results from the difference in the temperature of the surface and overlying air.The transfer of this energy is mainly through molecular conduction in the surface layer and turbulent mixing in the layer above the surface layer.Normally during day time heat will be transferred from the warm ground or crop surface to the cooler air above. At night, when the air is warm and the surface is cool, the converse situation prevails and heat will be transferred to the surface.Sensible heat transfer in the atmospheric surface layer The vertical flux of sensible heat H can be estimated by

where, a is the air density, Cp is the specific heat of air at constant pressure, Kh is the turbulent exchange coefficient, and /z is the vertical gradient of potential temperature.In the first 2-3 m above the ground, /z can be approximated by /z, the vertical gradient of actual temperature.

Resistance approach for estimating sensible heat fluxThe flux of sensible heat may be conceived as a process analogous to the flow of electrical current. In an analogy to Ohms Law, we can write

Where Ts and Ta are the surface and air temperatures, respectively, and ra is the aerial (aerodynamic or boundary layer) resistance to the flow of sensible heat. Sensible heat flux will increase with decreasing aerial resistance ra.

Latent Heat fluxLatent heat flux results from the evaporation at the surface and later condensation within the atmosphere.Latent heat flux = Latent heat of evaporation x Rate of evaporationEvaporation from water surface/wet soil depends upon (1) Temperature of the surface,(2) Vapour pressure deficit of adjacent air layer and(3) Quality of water.

Bowen RatioBowen ratio is defind as the ratio of sensible heat flux to latent heat flux lost by a surface to the atmosphere by the process of conduction and turbulence. Bowen (1926) proposed that

where, is Bowen ratio, Ts is the temperature of the surface and Ta is air temperature, es is vapour pressure of the surface, and e is vapour pressure of air.Bowen ratio is negative when heat is transferred from air to crop, and positive when heat transfer is from crop to air.Bowens ratio approach fails only when the value is less than -0.5. For well-watered crops, the value of the Bowen ratio is usually in the neighbourhood of 0.1 during the day when the evaporation is large. Average value of Bowen ratio for the ocean surface is +0.1.

Wind profile near the groundUse of wind profile studies The wind profile data provide a measurement of momentum flux and are essential for evaluating the transfer of water vapour and CO2 by the so called aerodynamic approach.The wind profile data also aid in estimating the wind speed at one height from the measurement of another.

Properties of wind profile close to the surfaceAt or near the earths surface horizontal wind velocity is equal to zero; and it increases with height above the surface.Wind increases exponentially with height and wind gradient depends upon wind velocity and surface condition.

Laminar sublayerIn considering the flow of air over the earths surface, there is a very thin layer of air immediately above the surface where the transfer processes are controlled primarily by molecular diffusion. This layer is called the laminar sublayer.The laminar sublayer may be only a few millimeters thick and may, sometimes, be even thinner, especially under windy conditions. The thickness of Laminar sublayer depends upon Prevailing wind speedRoughness of the surfaceAbove the laminar sublayer is the turbulent surface layer (or simply the surface layer) which extends up to 50 100 m and is dominated by strong mixing or eddying motion.The wind structure in this layer is primarily determined by the nature of the underlying surface and the vertical gradient of the air temperature.The effect of earths rotation, the coriolis force, is small and may be neglected as the frictional effects of the surface dominate.The planetary boundary layer, which envelops the surface layer and extends to about 1 km above the surface, is a zone of transition from the disturbed flow near the surface to the frictionless or smooth flow of the free atmosphere (Sutton, 1953).

Wind profile equation for short cropAt or near the surface of the ground, the horizontal wind velocity is zero and increases with the height above the surface. The wind gradient above the ground depends on the wind speed as well as the surface conditions. The wind profile over short crops may be expressed by the logarithmic equation:

where u is the wind velocity at the height z; K is Van Karmans constant having the value of 0.4; is air density; is the shearing stress and z0 is the roughness parameter.According to this equation, the wind speed near the ground increases exponentially with height over a very smooth surface.

Wind profile equation for tall cropsThe wind structure over a tall crop is different from that over a short crop. The wind profile changes abruptly at a height slightly below the canopy.Above that height, the logarithmic relationship seems to hold; below it, the wind speed is greatly reduced.Therefore, the wind profile equation for tall crops should be modified to the following form:

where, d is known as the zero plane displacement.

Roughness and zero plane displacementRoughnessOver a moderately rough surface (e.g. short grass), the logarithmic wind profile holds true only above a hypothetical height z0, known as the roughness of the surface. For short vegetation, or relatively smooth surfaces, the roughness parameter is relatively constant over a range of wind speeds. Typical values of roughness are as follows:

In general, roughness increases with the height of the vegetation.

SurfaceRoughness (cm)Ice0Sand0 to 0.1Open water0.02 to 0.6Snow surface0.1 to 0.6Short grass0.6 to 4.0Long grass4.0 to 10.0

Zero plane displacementThe zero plane displacement is roughly the order of the depth of the layer of air trapped among the plants. In other words, it is a datum level, above which the normal turbulent exchange takes place freely. The zero plane displacement may also be regarded as the sink for momentum.

Both z0 and d are geometric constants of the surface. Under neutral conditions, the relationship between the wind velocity and the logarithm of height is linear, and the intercept is the roughness. However, over a tall crop, the relationship is curvilinear, and the departure from the straight line is known as the zero plane displacement.

Significance of RoughnessThe roughness of a surface has several implications in the micrometeorological study of plant environment.Firstly, other things being equal, an increase in roughness will cause lowering of maximum temperature during the day time and a rise in the minimum temperature at night.Secondly, the rougher the surface, the greater the mixing and swirling. According to the theory of turbulent transport, the rate of mixing, expressed as a coefficient of diffusion, does not depend upon the wind speed, but upon the rate of change with height of the wind speed. Over a rough surface, heat and water vapour are readily transferred, even though the wind speed may be fairly low. Therefore, other things being equal, the evapotranspiration of a rough surface will exceed that of a smooth surface, especially in areas of strong advection.Thirdly, it would be difficult to determine the transfer of water vapour, CO2 and other properties by the aerodynamic method for a crop whose roughness and zero plane displacement vary greatly.