Air sea interaction laws and mechanisms part 2
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Air sea interaction laws and mechanisms part 2
Chapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2he peculiar distribution of “total static energy" or “moist static energy" CpT + gz + Lvq, that they found in the tropical atmosphere (Figure 4.1). Between the energy-rich mixed layer below, and the equally energetic air high in the troposphere, a considerable energy deficit is evident, greatest jus Air sea interaction laws and mechanisms part 2t above the Trade Inversion. They realized that the high values of total static energy aloft cannot be the result of simple area-wide mixing or advectAir sea interaction laws and mechanisms part 2
ion from below, especially if radiant cooling is taken into account. As Johnson (1969) points out in his review, Riehl and Malkus (1958) proposed thatChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2e large cross-section of the clouds." Release of latent heat turns the central cores of cumulonimbus into hot towers and generates fast ascent in them. In spite of their “large" individual cross section, their aggregate area is small, compared to the area of the global tropical ocean. Mass balance d Air sea interaction laws and mechanisms part 2ictates that the total upward mass transport in all the hot towers return to low levels. This takes place in slow subsidence outside hot towers (i.e..Air sea interaction laws and mechanisms part 2
over most of the tropical ocean). Hot towers form the upward leg of an overturning atmospheric circulation.Cumulonimbus clouds spawning showers are fChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2 ground level they have a “cauliflower" look, with many glistening w hite billows that often grow into an anvil-shaped top, where they spread out their cloud mass horizontally in the upper troposphere, even intruding on the lower4.1 Thermodynamics of Atmospheric Hot TowersFigure 4.i Average total st Air sea interaction laws and mechanisms part 2atic energy versus height in the tropical atmosphere (full line). Points show mean values at a single location. Gan Island (0 41 'S, 73WE) in the IndiAir sea interaction laws and mechanisms part 2
an Ocean. From Riehl and Malkus (1958).From the point of view of air-sea interaction, hot towers perform the important function of drying the air. As Chapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2heat of evaporation remains, leaving the total static energy essentially unchanged, while humidity drops from a typical mixed layer value in the tropics of 20 g/kg to much lower values in the upper troposphere, typically 4g/kg. As upper tropospheric air descends in the subsidence regions surrounding Air sea interaction laws and mechanisms part 2 hot towers, its low humidity remains conserved. As we have discussed in the last chapter, this supply of dry air to the atmospheric mixed layer is thAir sea interaction laws and mechanisms part 2
e key driving force of sea level latent heat flux.Oceanic equivalents of hot towers arise from surface cooling, and could be more appropriately calledChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2differs in many respects from the atmospheric one. There is no analogue of latent heat release in the ocean, nor of dry ing the air. and all of the cooling as well as heating take place very close to the sea surface. Therefore winds, not heating from below, drive the ascending leg of the overturning Air sea interaction laws and mechanisms part 2 circulation, recognizable as upwelling. Nevertheless, the overturning circulation of the ocean is instrumental in large poleward heat transfer, and pAir sea interaction laws and mechanisms part 2
lays a major role in the global heat balance.We first discuss atmospheric hot towers and the part they play in air-sea interaction.4.1Thermodynamics oChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2en such a distribution, observed on a few occasions in the Atlantic Trade Wind Experiment (ATEX). at the research vessel ’‘Meteor." on rainy days (Figures 3.11 and 3.12). The lifting condensation level was low, the transition layer underneath the subcloud mixed layer marked only by a small drop in h Air sea interaction laws and mechanisms part 2umidity, the equivalent potential temperature more or less constant above the transition layer, lacking the sharp knee that identifies the Trade InverAir sea interaction laws and mechanisms part 2
sion. These are the earmarks of a hot tower.At constant oe. unit mass of the moist air contains constant thermal plus potential energy, so that its enChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2 entropy only by an insignificant amount, however, so that the pressure and temperature changes in the moist air during ascent are nearly adiabatic, or “pseudoadiabatic.” The energy balance of this process at any stage of the ascent is then:Tds = CpdT + gdz + Lvdq = 0-4.1condensation of vapor during Air sea interaction laws and mechanisms part 2 ascent, dq < 0, providing the energy for an increase of the potential temperature, do = dT + (g/Cp)dz > 0, while dOe = 0.It might surprise the readerAir sea interaction laws and mechanisms part 2
that Equation 4.1 implies zero heat gain or loss in a hot lower from an external source, notably from the divergence of radiant heat flux. Clear air Chapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2ter droplets effectively block any radiation, short wave or long wave. We have seen above how effective stratiform cloud is in this regard, recalling Figure 3.17 of the previous chapter. Another point is the fast upward motion: at the typical convection speed of 5 m s-1, moist air rises from the Lif Air sea interaction laws and mechanisms part 2ting Condensation Level (LCL) to the top of the troposphere in less than an hour. Even at the typical clear air cooling rate of 2 K day-1 the temperatAir sea interaction laws and mechanisms part 2
ure drop in a rising parcel would be less than 0.05 K. making an insignificant change in oe.The blockage of radiation is one consequence of the condenChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2le meteorological forecasts focus on the rain, what matters for air-sea interaction is the drying out of the air.4.1.1 The Drying-out Process in Hot TowersThermodynamic equilibrium between liquid water and water vapor, or ice and water vapor, limits the partial pressure of vapor that can be present Air sea interaction laws and mechanisms part 2in moist air to a “saturation" pressure that depends on temperature alone. In a rising parcel of originally unsaturated air. both the partial pressureAir sea interaction laws and mechanisms part 2
of vapor and the temperature drop.4.1 Thermodynamics of Atmospheric Hot Towerspartial pressure: Above this level the moist air would become supersatuChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2ortions, and thus on specific humidity q. If the partial densities of dry air and vapor are A/ and the definition of specific humidity is q = pvỊ(pd + A>). An alternative measure of humidity is the mixing ratio /• = Pv! Pd- Al the small vapor partial pressures of interest these two measures are near Air sea interaction laws and mechanisms part 2ly equal, but r proves easier to work with in thermodynamic argument. The connection is Ợ = r/(r + 1).According to Dalton’s law, the total pressure pAir sea interaction laws and mechanisms part 2
in a mixture of gases is the sum of partial pressures, so that if e is the partial pressure of water vapor, p — e is the partial pressure of the dry aChapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2)/RdT, with Rv = 461.5 J kg-' K"1 and Rd = 287 J kg-' K"', gas constants of waler vapor and dry air. Putting E = Rd/RI — 0.622 for the ratio of the gas constants, we arrive then al the relationship of the mixing ratio to the partial pressures, r = ee/(p — e). or el p — r/(r 4- s) = r/E. With el p sm Air sea interaction laws and mechanisms part 2all, the gas law for the mixture is to a good approximation p = Pd 4- Pv = pl RdT. Emanuel (1994) lists the exact relationships.The relationship of prAir sea interaction laws and mechanisms part 2
essures to mixing ratio remains true at saturation, so that the saturation mixing ratio is r* = ee*Ị(p — €*). a function of temperature as well as of Chapter 4Hot TowersThe graphic term “hot tower" has originated with Riehl and Malkus (1958), to describe their conceptual model intended to explain th Air sea interaction laws and mechanisms part 2ibrium follow the Clausius-Clapeyron equation:3(7 - 273 K)[J kg"1]. In vapor-ice equilibrium the same equation applies but Lv has to be replaced by Ly. the latent heat of sublimation. Lx = 2.834 X Air sea interaction laws and mechanisms part 2 106 J kg *.Integration of Equation 4.2 yields the functional relationship of e* to temperature. This is useful for calculating saturation partial preAir sea interaction laws and mechanisms part 2
ssure differences over small ranges of temperature. For the calculation of (’* at a specific temperature a more convenient approximate formula is dueGọi ngay
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