近地面微氣象學

1. 近地面微氣象學 (Micrometeorology near the Ground) 授課老師: 游政谷 Instructor: Cheng-Ku Yu Micrometeorology(3-8)

2. The surface energy budget Principle of the conservation of energy at the surface can be expressed as RN = H + HL + HG RN is the net radiation H and HL are the sensible and latent heat fluxes to or from the air HG is the ground heat flux to or from the submedium (sign convention: the radiative flux directed toward the surface is positive, while other energy fluxes directed away from the surface are positive and vice versa) During the daytime, the surface receives radiative energy (RN> 0), which is partitioned into sensible and latent heat fluxes to the atmosphere and the heat flux to the submedium. Typically, H, HL, and HG are all positive over land surfaces during the day. Fig. 2.6

3. At night, the surface loses energy by outgoing radiation, which is compensated by gains of heat from air and soil media and from the latent heat of condensation released during the process of dew formation. Fig. 2.7 The magnitudes of the nighttime fluxes are generally much smaller than the magnitudes of the daytime fluxes, except for HG. The magnitudes of HG do not differ widely between day and night, although the direction or sign obviously reverses during the morning and evening transition periods. Fig. 2.8 For extensive water surfaces (large lakes, seas, and oceans), the combined value of HL and HG balances most of the net radiation (RN), while H plays only a minor role (H << HL, or B <<1). ------ water surface temperature does not respond readily to solar heating due to the large heat capacity and depth of the subsurface mixed layer of a large lake or ocean

4. Fig. 2.8 Observed diurnal energy budget over a dry bare surface (Vehrencamp 1953) Note that HL = 0

5. The specific heat (c) of a material is defined as the amount of heat absorbed or released in raising or lowering the temperature of a unit mass of the material by one degree. The product of mass density and specific heat is called the heat capacity (C). Table 2.1 Molecular thermal properties of natural materials

6. Energy budget of a layer ----The earth’s surface has horizontal inhomogeneities ----The earth’s surface may be partially transparent to radiation (e.g., water) In many practical situations, it will be more appropriate to consider the energy budget of a finite interfacial layer instead of an ‘ideal’ surface. This layer must have finite mass and heat capacity which would allow the energy to be stored in or released from the layer over a given time interval. The conservation of energy for the layer: RN = H + HL + HG + ΔHs where ΔHs is the changes in the energy storage per unit time, per unit horizontal area, over the whole depth of the layer Fig. 2.9 and Fig. 2.10 The rate of heat storage in the oceanic mixed layer plays an important role in the energy budget. This layer acts as a heat sink (ΔHs> 0) by day and a heat source (ΔHs < 0) at night.

7. Example problem: Over the tropical oceans the Bowen ratio is typically 0.1. Estimate the sensible and latent heat fluxes, as well as the rate of evaporation from the ocean surface. Assume the net radiation received just above the surface is 400 W/m2, the heat flux to water below 50 m is negligible, the rate of warming of the 50-m-deep oceanic mixed layer is 0.05 oC/day, and the ocean surface temperature is 30 oC. Ans. H = 25.4 W/m2, HL = 253.7 W/m2, and the rate of evaporation = 1.03 x 10-4 kg/m2s

8. Chap. 3 Soil Temperatures and Heat Transfer Surface temperature (Ts): refers to the temperature at the air-soil interface Near-surface air temperature: refers to the temperature measured at standard meteorological stations at the height of 1-2 meter. The direct in situ measurement of surface temperature is made very difficult ----- extremely large temperature gradient near the surface in both the air and the soil media ----- the finite dimensions of the temperature sensor The surface temperature is often determined by extrapolation of measured temperature profiles in soil and air. Subsurface soil temperatures are easier to measure than the surface temperature; the amplitude of diurnal variation of soil temperatures decreases exponentially with depth and becomes insignificant at a depth of the order of 1 m or less.

9. Fig. 3.1 Observed diurnal course of subsurface soil temperatures at various depths in a sandy loam soil with bare surface (West 1952)

10. In addition to the “diurnal waves” present in soil temperature in the top layer, daily or weekly averaged temperatures show a nearly sinusoidal “annual wave” which penetrates to much greater depths(~10 m) in the soil. Fig. 3.2 Annual temperature waves in the weekly averaged subsurface soil temperatures at two depths in a sandy loam soil (West 1952)

11. Thermal properties of soils Through solid media, heat is transferred primarily through conduction, which involves molecular exchanges. The rate of heat transfer or heat flux in a given direction is found to be proportional to the temperature gradient in that direction, i.e., the heat flux in the z-direction H = -k(әT/әz) (Fig. 3.3) in which k is known as the thermal conductivity of the medium The ratio of thermal conductivity to heat capacity is called the thermal diffusivity αh. The Equation above can also be written as H/ρ c = - αh(әT/әz) where αh = k/ ρ c = k/C

12. Table 2.1 Molecular thermal properties of natural materials Note that air has the lowest heat capacity and the lowest thermal conductivity of all the natural materials, while water has the highest heat capacity Addition of water to an initially dry soil increases its heat capacity and conductivity markedly, because it replaces air (a poor heat conductor) in the pore space.

13. Fig. 3.4 Schematic of heat transfer in a vertical column of soil below a flat, horizontal surface

14. Chap. 4 Air Temperature and Humidity in the PBL • Factors influencing air temperature in the PBL: • Type of air mass and its temperature just above the PBL, which depend on the synoptic situation and the large-scale circulation pattern (Fig. 4.1) • Thermal characteristics of the surface and submedium, which influence the diurnal range of surface temperatures • Net radiation at the surface and its variation with height, which determine the radiative warming or cooling of the surface and the PBL • Sensible heat flux at the surface and its variation with height, which determine the rate of warming or cooling of the air due to convergence or divergence of sensible heat flux • әT/әt = -(әH/әz)/ρcp • Latent heat exchanges during evaporation and condensation processes at the surface and in air, which influence the surface and air temperatures, respectively. • Mean vertical motions forced by topography often lead to rapid changes in temperature and humidity of air, as well as to local cloud formation and precipitation, as moist air rises over the mountain slopes (Fig. 4.2) • Warm or cold air advection as a function of height in the PBL

15. Fig. 4.1 Surface weather map in East Asia(00 UTC 17 Nov. 2003)

16. Fig. 4.2 Three types of moist convection initiation mechanism by topography (Banta et al. 1990)

17. Fig. 4.3 Plume visualization of side view of plume dispersion Neutral Condition Stable Condition

18. Fig. 4.4 Schematic of the various stability categories on the basis of virtual temperature gradient

19. Fig. 4.5 Nonlocal stability characterization for the various hypothetical virtual potential temperature profiles (Stull 1988)

20. Fig. 4.6 Diurnal variations of potential temperature profiles

21. Fig. 4.7 Diurnal variation of specific humidity

22. Fig. 4.8 Diurnal variation of near-surface air temperature in clear and overcast days

23. Fig. 4.9 Diurnal variation of water vapor pressure at three heights

24. Chap. 5 Wind Distribution in the PBL • Some factors influencing wind distribution: • Large-scale horizontal pressure and temperature gradients in the lower atmosphere, which drive the PBL flow. • The surface roughness characteristics, which determine the surface drag and momentum exchange in the lower part of the PBL. • The diurnal cycle of heating and cooling of the surface, which determines the thermal stratification of the PBL. • The PBL depth, which determines wind shears in the PBL. • Presence of clouds and precipitation in the PBL, which influence its thermal stratification. • Surface topographical features, which give rise to local and mesoscale circulations

25. Fig. 5.1 Schematic of relationship between geostrophic winds and isobars

26. Fig. 5.2 Schematic of relationship between thermal winds and isotherms

27. Fig. 5.3 Relationship between the geostrophic winds for different orientations of surface isobars and isotherms

28. Fig. 5.4 The balance of forces at different heights within a barotropic PBL (a) at the surface(b) surface layer (c) middle PBL(d) top of PBL

29. Fig. 5.5 Vertical profile of wind speed and direction and potential temperature in BL

30. Fig. 5.6 Vertical profiles of winds, potential temperature and Ri number