DOCSLIB.ORG

The Influence of Soil Moisture on the Planetary Boundary Layer and On

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Influence of Soil Moisture on the Planetary Boundary Layer and On MARCH 2013 Z H O U A N D G E E R T S 1061 The Influence of Soil Moisture on the Planetary Boundary Layer and on Cumulus Convection over an Isolated Mountain. Part I: Observations XIN ZHOU AND BART GEERTS University of Wyoming, Laramie, Wyoming (Manuscript received 18 May 2012, in final form 29 August 2012) ABSTRACT Data collected around the Santa Catalina Mountains in Arizona as part of the Cumulus Photogrammetric, In Situ and Doppler Observations (CuPIDO) experiment during the 2006 summer monsoon season are used to investigate the effect of soil moisture on the surface energy balance, boundary layer (BL) characteristics, thermally forced orographic circulations, and orographic cumulus convection. An unusual wet spell allows separation of the two-month campaign in a wet and a dry soil period. Days in the wet soil period tend to have a higher surface latent heat flux, lower soil and air temperatures, a more stable and shallower BL, and weaker solenoidal forcing resulting in weaker anabatic flow, in comparison with days in the dry soil period. The wet soil period is also characterized by higher humidity and moist static energy in the BL, implying a lower cu- mulus cloud base and higher convective available potential energy. Therefore, this period witnesses rather early growth of orographic cumulus convection, growing rapidly to the cumulonimbus stage, often before noon, and producing precipitation rather efficiently, with relatively little lightning. Data alone do not allow discrimination between soil moisture and advected airmass characteristics in explaining these differences. Hence, the need for a numerical sensitivity experiment, in Part II of this study. 1. Introduction (e.g., Sellers et al. 1992). Thus, soil moisture is a critical parameter determining CBL characteristics (e.g., Zhang Warm-season deep convection in the western United and Anthes 1982). However, the impact of soil moisture States is strongly diurnally modulated, with cumulus on mountain-scale circulations and on the growth of convection initiating close to solar noon and a maximum orographic convection is less obvious. That is the topic in lightning and precipitation in the midafternoon of the present study. This paper (Part I) explores ob- (Watson et al. 1994b; Nesbitt and Zipser 2003; Carbone servations. A companion paper (Part II) uses numerical et al. 2002; Demko et al. 2009). Satellite imagery shows simulations. how this convection almost invariably initiates over This study should be framed in the broader question mountain ranges and not the surrounding lower terrain about the relationship between soil moisture and warm- (Banta and Schaaf 1987). Clearly the convection and season precipitation. Many numerical experiments have mountain-scale circulations that control this spatial been designed to address this question (e.g., Sun and distribution are tightly coupled to the land surface, es- Ogura 1979; Carlson et al. 1983; Rowntree and Bolton pecially the surface sensible heat flux that controls the 1983; Atlas et al. 1993; Koster et al. 2004). Observational daily evolution of the convective boundary layer (CBL). studies on this topic are relatively scarce, especially in The sensible heat flux mainly depends on the magnitude complex terrain. The simulated soil moisture feedback of the surface net radiation, and on soil moisture, which on precipitation over flat terrain can be positive or is the primary control in the partitioning of the avail- negative, mainly depending on scale, but also depending able energy between sensible and latent heat fluxes on numerical factors such as the surface and boundary layer schemes and the cumulus parameterization method (Pan et al. 1996; Gallus and Segal 2000; Hohenegger et al. Corresponding author address: Bart Geerts, Department of At- mospheric Science, University of Wyoming, 6034 Engineering 2009). Building, Laramie, WY 82071. The main positive feedback mechanism is that moist E-mail: [email protected] soil enriches BL air with moist static energy, which DOI: 10.1175/MWR-D-12-00150.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/01/21 05:36 AM UTC 1062 MONTHLY WEATHER REVIEW VOLUME 141 reduces convective inhibition (CIN) and increases con- vective available potential energy (CAPE; Betts et al. 1996; Eltahir 1998; Scha¨r et al. 1999; Pal and Eltahir 2001; Findell and Eltahir 2003a,b; Wood 1997; Hohenegger et al. 2009). Another positive feedback is that increased soil moisture decreases surface albedo (Idso et al. 1975) and thus increases net radiation [and boundary layer (BL) moist static energy] under clear skies (Eltahir 1998). Boundary layer clouds may complicate this re- sponse: the addition of water vapor into the CBL may increase low-level cloudiness and stability (Hohenegger et al. 2009), but the resulting surface net shortwave radiation reduction may be compensated by a reduced net longwave radiation loss, unless the clouds are con- fined to the daytime (Scha¨r et al. 1999; Pal and Eltahir 2001). The main negative feedback mechanism is that moist soil reduces the sensible heat flux, and thus the daytime maximum temperature, the CBL depth, and the ability of air parcels to overcome CIN (Findell and Eltahir FIG. 1. Topography of the Santa Catalina Mountains, showing 2003a,b; Ek and Holtslag 2004; Hohenegger et al. 2009). the location of 10 ISFF stations (triangles are regular stations, di- amonds are stations with flux and soil measurements), the highest Another negative feedback relates to spatial heteroge- peak Mt. Lemmon (2791 m MSL), the flux tower at Mt. Bigelow neity: in a flat landscape with mesoscale soil moisture (2583 m MSL), the two stereo-cameras mounted on a rooftop at variations, the wetter soil regions will be relatively cool, the University of Arizona, and the National Weather Service resulting in mesoscale subsidence as BL air diverges KTUS radiosonde launch site in 2006. toward the drier, warmer patches, where the CBL be- comes deeper and deep convection may erupt (Walker and Rowntree 1977; Sun and Ogura 1979; Carlson et al. and orographic precipitation is complicated also be- 1983; Taylor et al. 2007; Nair et al. 2011). The flow to- cause deep convection produces outflow boundaries that ward warmer patches is density driven (solenoidal). The can trigger secondary convection (Demko and Geerts minimum scale supporting significant solenoidal BL 2010b), and because the complex terrain may aid or stifle circulations is at least an order of magnitude larger than mesoscale convective organization (Nesbitt et al. 2008; the CBL eddies [i.e., at least O(10 km)], depending Hauck et al. 2011). mainly on surface contrast, ambient wind, and BL depth To make progress on the topic of soil moisture impact (Chen and Avissar 1994). on warm-season precipitation in complex terrain, this These and other factors need to be considered in study focuses on the rather simpler component questions complex terrain. Elevated surface heating leads to sig- of how soil moisture affects CBL growth, mountain-scale nificant pressure perturbations (Geerts et al. 2008) re- convergence, and initial cumulus growth. Part I of this sulting in mountain-scale solenoidal BL circulations study is entirely observational. Its primary data source is (e.g., Banta 1986) especially when soils are dry (Ookouchi a network of surface weather and flux stations deployed et al. 1984). The initiation of cumulus convection over or for two months (July–August 2006) around a rather iso- near the high terrain is due to a combination of terrain- lated mountain in southern Arizona. induced convergent flow (Banta 1984; Kottmeier et al. A 5-day period of heavy rainfall occurred across 2008; Barthlott et al. 2011) and elevated heating and southeast Arizona in late July 2006, yielding widespread early elimination of CIN (Demko et al. 2009; Demko near-saturated soil conditions. About 118 mm of rain and Geerts 2010a,b). Clearly orographic convection fell between 27–31 July around the Santa Catalina would be most effective if the surrounding valley soil is Range (SCR; Fig. 1), mostly at night, causing some rare moist (with lush vegetation) and the mountain soil dry, local flooding (Damiani et al. 2008). This event was since the solenoidal circulations due to the terrain and driven by a synoptic-scale moisture surge, which is not due to soil moisture would have the same sign, but in uncommon in Arizona in summer (e.g., Adams and reality a history of precipitation mainly over mountains Comrie 1997). This spell was unusual because of a typically leads to an opposite soil moisture (and vege- series of nocturnal organized convective systems, aided tation) distribution. The relation between soil moisture by high surface humidity and a weak but persistent Unauthenticated | Downloaded 10/01/21 05:36 AM UTC MARCH 2013 Z H O U A N D G E E R T S 1063 upper-tropospheric cyclone. During this spell the air and humidity sensors and 3D sonic anemometers at a mass was as moist tropical as commonly observed far- height of 10 m above ground level (AGL). The latent ther south along the Sierra Madre Occidental in summer and sensible heat (LH and SH) fluxes are estimated (e.g., Higgins et al. 2006). The National Weather Service using the eddy correlation technique at 5-min intervals (NWS) forecast office at Tucson, Arizona (KTUS), re- based on 30-min running averages. The SH flux is pro- corded a total of 214 mm of rain in July and August portional to the buoyancy flux; that is, it includes the 0 0 2012, which is 193% of normal. While this unusual wet virtual temperature correction (w uy), where w is verti- spell makes it difficult to understand the more typical cal velocity and uy virtual potential temperature. The nature of land–atmosphere coupling in this region soil heat flux is computed using the gradient method. We (Small 2001), it does provide a strong regional-scale also use corresponding data at 10 m AGL from a flux signal to examine soil moisture impact.
Load more

Recommended publications
  • Moist Static Energy Budget of MJO-Like Disturbances in The
    1 Moist Static Energy Budget of MJO-like 2 disturbances in the atmosphere of a zonally 3 symmetric aquaplanet 4 5 6 Joseph Allan Andersen1 7 Department of Physics, Harvard University, Cambridge, Massachusetts 8 9 10 Zhiming Kuang 11 Department of Earth and Planetary Science and School of Engineering 12 and Applied Sciences, Harvard University, Cambridge, Massachusetts 13 14 15 16 17 18 19 20 21 22 1 Corresponding author address: Joseph Andersen, Department of Physics, Harvard University, Cambridge, MA 02138. E-mail: [email protected] 1 1 2 Abstract 3 A Madden-Julian Oscillation (MJO)-like spectral feature is observed in the time-space 4 spectra of precipitation and column integrated Moist Static Energy (MSE) for a zonally 5 symmetric aquaplanet simulated with Super-Parameterized Community Atmospheric 6 Model (SP-CAM). This disturbance possesses the basic structural and propagation 7 features of the observed MJO. 8 To explore the processes involved in propagation and maintenance of this disturbance, 9 we analyze the MSE budget of the disturbance. We observe that the disturbances 10 propagate both eastwards and polewards. The column integrated long-wave heating is the 11 only significant source of column integrated MSE acting to maintain the MJO-like 12 anomaly balanced against the combination of column integrated horizontal and vertical 13 advection of MSE and Latent Heat Flux. Eastward propagation of the MJO-like 14 disturbance is associated with MSE generated by both column integrated horizontal and 15 vertical advection of MSE, with the column long-wave heating generating MSE that 16 retards the propagation. 17 The contribution to the eastward propagation by the column integrated horizontal 18 advection of MSE is dominated by synoptic eddies.
    [Show full text]
  • Chapter 3 Moist Thermodynamics
    Chapter 3 Moist thermodynamics In order to understand atmospheric convection, we need a deep understand- ing of the thermodynamics of mixtures of gases and of phase transitions. We begin with a review of some of the fundamental ideas of statistical mechanics as it applies to the atmosphere. We then derive the entropy and chemical potential of an ideal gas and a condensate. We use these results to calcu- late the saturation vapor pressure as a function of temperature. Next we derive a consistent expression for the entropy of a mixture of dry air, wa- ter vapor, and either liquid water or ice. The equation of state of a moist atmosphere is then considered, resulting in an expression for the density as a function of temperature and pressure. Finally the governing thermody- namic equations are derived and various alternative simplifications of the thermodynamic variables are presented. 3.1 Review of fundamentals In statistical mechanics, the entropy of a system is proportional to the loga- rithm of the number of available states: S(E; M) = kB ln(δN ); (3.1) where δN is the number of states available in the internal energy range [E; E + δE]. The quantity M = mN=NA is the mass of the system, which we relate to the number of molecules in the system N, the molecular weight of these molecules m, and Avogadro’s number NA. The quantity kB is Boltz- mann’s constant. 35 CHAPTER 3. MOIST THERMODYNAMICS 36 Consider two systems in thermal contact, so that they can exchange en- ergy. The total energy of the system E = E1 + E2 is fixed, so that if the energy of system 1 increases, the energy of system 2 decreases correspond- ingly.
    [Show full text]
  • The Moist Static Energy Budget and Wave Dynamics Around the ITCZ in Idealized WRF Simulations
    The moist static energy budget and wave dynamics around the ITCZ in idealized WRF simulations Scott W. Powell and David S. Nolan RSMAS/University of Miami; Miami, FL Eighth Conference on Coastal Atmospheric and Oceanic Prediction and Processes, 89th AMS Annual Meeting; Phoenix, AZ January 12, 2009 This research has been supported by a NSF/REU grant under proposal ATM-0354763. A shallow meridional circulation (SMC) in Eastern Pacific ● EPIC (2001) field program ● Zhang, et al. (2004) indicates that a meridional flow out of the ITCZ was present in observational data. a) Eight-flight mean b) Oct. 2, 2001 A shallow meridional circulation (SMC) in Eastern Pacific ● Nolan, et al. (2007) used a full physics model to show such a circulation in a tropical half-channel. ● Regional model by Wang, et al. (2005) and radiative cooling ● “Sea-breeze” circulation ● Newer simulations with linear and hyperbolic temperature profiles on a full SRF above channel. Boundary layer boundary layer inflow Linear temperature profile across equator Log10 RR(+1) (mm/hr), 03-21:00 max=1.71e+00 min=1.00e-02 int=4.00e-01 y(km) x (km) Moist static energy (MSE) budget Moist static energy: dry static energy plus the product of the latent heat of vaporization and water vapor mixing ratio: s = CpT + Ф + Lvr, Cp = specific heat of air at constant pressure T = absolute temperature Ф = geopotential Lv = latent heat of vaporization r = mixing ratio MSE Flux due to Advection: ΦMSE = ρ*v*s, ρ = density v = meridional wind velocity Calculation of the MSE budget: Advection ULO MLI SRF SMC Level MSE Flux*: 2S MSE Flux*: 2N Upper level return flow -2.26E+9 -2.05E+9 Mid-level inflow 7.50E+8 4.42E+8 *Fluxes in units J*m-1*s-1.
    [Show full text]
  • 1 Theory of Tropical Moist Convection
    OUP UNCORRECTED PROOF – FIRST PROOF, 30/10/2019, SPi 1 Theory of tropical moist convection David M. Romps Department of Earth and Planetary Science, University of California, Berkeley, CA Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA Romps, D., Theory of tropical moist convection. In: Fundamental aspects of turbulent flows in climate dynamics. Edited by Freddy Bouchet, Tapio Schneider, Antoine Venaille, Christophe Salomon: Oxford University Press (2020). © Oxford University Press. DOI: 10.1093/oso/9780198855217.003.0001 1 Theory of Tropical Moist Convection David M. Romps Department of Earth and Planetary Science, University of California, Berkeley, CA Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 1.1 Introduction These lecture notes cover the theory of tropical moist convection. Many simplifications are made along the way, like neglecting rotation and treating the atmosphere as a two- dimensional fluid or even reducing the atmosphere to two columns. We can gain an immense amount of insight into the real atmosphere by studying these toy models, including answers to the following questions: what is the dominant energy balance in the tropical free troposphere; what sets the temperature structure of the tropical free troposphere; what happens to the pulse of heating deposited into the atmosphere by a rain cloud; why does the tropical atmosphere have the relative-humidity profile that it does; and what sets the amount of energy available to storms? These notes attempt to give the first-order answers to these questions in a format that is accessible to beginning graduate students. We begin with a discussion of atmospheric energy.
    [Show full text]
  • An Analysis of the Moisture and Moist Static Energy Budgets in AMIP Simulations
    An Analysis of the Moisture and Moist Static Energy Budgets in AMIP Simulations by Kristine Adelaide Boykin Bachelor of Science Physics Southern Polytechnic State University 2004 A thesis submitted to the Department of Marine and Environmental Systems at Florida Institute of Technology in partial fulfilment of the requirements for the degree of Master of Science in Meteorology Melbourne, Florida September, 2016 © Copyright 2016 Kristine Adelaide Boykin All Rights Reserved The author grants permission to make single copies_____________________________ We the undersigned committee hereby recommend that the attached document be accepted as fulfilling in part of the requirements for the degree of Master of Science of Meteorology. An Analysis of the Moisture and Moist Static Energy Budgets in AMIP Simulations by Kristine Adelaide Boykin _____________________________________________ P. K. Ray, Ph.D., Major Advisor Assistant Professor, Marine and Environmental Systems _____________________________________________ G. Maul, Ph.D. Professor of Oceanography _____________________________________________ M. Bush, Ph.D. Professor of Biological Sciences _____________________________________________ T. Waite, Ph.D., Department Head Marine and Environmental Systems Abstract An Analysis of the Moisture and Moist Static Energy Budgets in AMIP S by Kristine Adelaide Boykin Major Advisor: P. K. Ray, Ph.D. An analysis of the second phase of the Atmospheric Model Intercomparison Project (AMIP) simulations has been conducted to understand the physical processes that control precipitation in the tropics. This is achieved primarily through the analysis of the moisture and moist static energy budgets. Overall, there is broad agreement between the simulated and observed precipitation, although specific tropical regions such as the Maritime Continent poses challenges to these simulations. The models in general capture the latitudinal distribution of precipitation and key precipitating regions including the Inter- Tropical Convergence Zone (ITCZ) and the Asian Monsoon.
    [Show full text]
  • Pyatmosthermo Documentation Release 0.1
    pyAtmosThermo Documentation Release 0.1 Ravi Shekhar February 06, 2016 Contents i ii pyAtmosThermo Documentation, Release 0.1 This library allows calculation of moist atmospheric thermodynamic quantities. For example, the following snippet will calculate moist static energy from mixing ratio of 0.001 kg/kg, 298 Kelvin, and 200 meters height. Thus, MSE has the value 304161.6 J/kg. import atmos_thermo h= atmos_thermo.h_from_r_T_z(0.001, 298, 200) This code calculates the saturation vapor pressure and produces 3533.7 Pa. e_saturation= atmos_thermo.esat_from_T(300) All functions are designed around using numpy arrays transparently, and using numpy arrays is vastly faster than passing in values one by one. Contents: Contents 1 pyAtmosThermo Documentation, Release 0.1 2 Contents CHAPTER 1 atmos_thermo package 1.1 Submodules 1.2 atmos_thermo.atmoslib module atmos_thermo.atmoslib.Lv_from_T(T) Calculate latent heat of vaporization for water, Lv. Args: T: temperature (Kelvin) Returns: Latent heat of vaporization [J / kg K] atmos_thermo.atmoslib.T_from_p_theta(p, theta, p0=100000.0) Calculate the temperature of air. Calculate the temperature (T) given pressure (p) and potential temperature (theta). Formula is : T = theta * (p / p0) ** (R_d / C_pd) Args: p: total pressure (Pascals) theta: potential temperature (Kelvin) p0: reference pressure used in calculating potential temperature Returns: temperature in Kelvin atmos_thermo.atmoslib.Tv_from_r_T(r, T) Calculate the virtual temperature. Calculate the virtual temperature (T_v) given temperature (T) and mixing ratio (r). Args: r: mixing ratio (dimensionless) T: temperature (Kelvin) Returns: Virtual Temperature in Kelvin atmos_thermo.atmoslib.e_from_r_p(r, p) Calculate partial pressure of water vapor. Calculate partial pressure of water vapor (e) from mixing ratio (r) and total pressure (p).
    [Show full text]
  • Weakening of Upward Mass but Intensification of Upward Energy While the Mass Transport Weakens
    RESEARCH LETTER Weakening of Upward Mass but Intensification of 10.1029/2018GL081399 Upward Energy Transport in a Warming Climate Key Points: • Isentropic stream function shows Yutian Wu1 , Jian Lu2 , and Olivier Pauluis3 a robust expansion toward smaller pressure and broadening of MSE in a 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA, 2Division of Atmospheric Sciences warming climate and Global Change, Pacific Northwest National Laboratory, Richland, WA, USA, 3Courant Institute of Mathematical • While upward mass transport Sciences, New York University, New York, NY, USA decreases in global average, upward MSE transport increases, mostly due to the eddies • An increase of effective MSE range Abstract How the atmospheric overturning circulation is projected to change is important for is found globally, especially in the understanding changes in mass and energy budget. This study analyzes the overturning circulation by subtropics sorting the upward mass transport in terms of the moist static energy (MSE) of air parcels in an ensemble of coupled climate models. It is found that, in response to greenhouse gas increases, the upward transport of Supporting Information: MSE increases in order to balance the increase in radiative cooling of the mass transport. At the same time, • Supporting Information S1 the overall mass transport decreases. The increase in energy transport and decrease in mass transport can be explained by the fact that the MSE of rising air parcels increases more rapidly than that of subsiding air, Correspondence to: thus allowing for a weaker overturning circulation to transport more energy. Y. Wu, [email protected] Plain Language Summary The atmospheric circulation transports energy upward to balance the energy loss due to radiative cooling.
    [Show full text]
  • Water Vapor Mixing Ratio Is Given by Dw = Dws
    1. Saturated Adiabatic Processes If vertical ascent continues above the LCL, condensation will occur and the latent heat of phase change will be released. This latent heating will cause the temperature to decrease more slowly with pressure above the LCL than below. This means that the lapse rate above the LCL will be smaller than the lapse rate below the LCL. To describe the behavior of the parcel we use a Pseudoadiabatic process or irreversible saturated adiabatic process where all the condensed water or sublimated ice falls out of the air parcel as soon as it is produced (if we were to include the presence of the condensate it would unnecessarily complicate the process representation). Because the water component accounts for only a small fraction of the system’s mass, the pseudo adiabatic process is nearly identical to a reversible saturated adiabatic process. Through any point of an aerological diagram there is one, and only one, pseudoadiabat. During this process, the change of water vapor mixing ratio is given by dw = dws 1) DERIVATION OF THE EXPRESSION FOR PSEUDOADIABATIC LAPSE RATE Our objective is to find how the temperature changes as we move up the pseudoadiabat (Γs = −dT=dz). We begin with the First Law of Thermodynamics: cpdT − αdp = dq = −Ldws (1) The heat exchange between the air parcel and its environment is zero, but heat is released within the air parcel by the phase change from water vapor to liquid water (ice). As vapor is converted to liquid water, w = ws decreases and dq = −Ldws remember dws < 0 @w @w c dT − αdp = −L s dT + s dp (2) p @T @p We can neglect the weak dependence of ws on pressure, and use αdp = −gdz @w c + L s dT = −gdz (3) p @T L @ws 1 + dT = −Γddz (4) cp @T dT 1 = −Γd (5) dz 1 + L @ws cp @T dT − = Γ (6) dz s (7) This means that the pseudoadiabatic lapse rate (Γs) is not constant, but a function of altitude.
    [Show full text]
  • Interactions Among Cloud, Water Vapor, Radiation and Large-Scale
    Interactions among Cloud, Water Vapor, Radiation and Large-scale Circulation in the Tropical Climate Part 1: Sensitivity to Uniform Sea Surface Temperature Changes Kristin Larson* and Dennis L. Hartmann Department of Atmospheric Sciences University of Washington Seattle, Washington To Be Submitted to Journal of Climate Shorter, August 7, 2002 *Corresponding author address: Ms. Kristin Larson, Department of Atmospheric Sciences, University of Washington, BOX 351640, Seattle, WA 98195-1640. E-mail: [email protected] Abstract: The responses of tropical clouds and water vapor to SST variations are investi- gated with simple numerical experiments. The Pennsylvania State University (PSU) / National Center for Atmospheric Research (NCAR) mesoscale model is used with doubly periodic bound- ary conditions and a uniform constant sea surface temperature (SST). The SST is varied and the equilibrium statistics of cloud properties, water vapor and circulation at different temperatures are compared. The top of the atmosphere (TOA) radiative fluxes have the same sensitivities to SST as in observations averaged from 20 °N - 20 °S over the Pacific, suggesting that the model sensitivities are realistic. As the SST increases, the temperature profile approximately follows a moist adia- batic lapse rate. The rain rate and cloud ice amounts increase with SST. The average relative humidity profile stays approximately constant, but the upper tropospheric relative humidity increases slightly with SST. The clear-sky mean temperature and water vapor feedbacks have similar magnitudes to each other and opposite signs. The net clear-sky feedback is thus about equal to the lapse rate feedback, which is about -2 W m-2 K-1. The clear sky OLR thus increases with SST, but the high cloud top temperature is almost constant with SST, and the high cloud amount increases with SST.
    [Show full text]
  • The Effective Static Stability Experienced by Eddies in a Moist Atmosphere
    JANUARY 2011 O ’ G O R M A N 75 The Effective Static Stability Experienced by Eddies in a Moist Atmosphere PAUL A. O’GORMAN Massachusetts Institute of Technology, Cambridge, Massachusetts (Manuscript received 26 April 2010, in final form 28 June 2010) ABSTRACT Water vapor directly affects the dynamics of atmospheric eddy circulations through the release of latent heat. But it is difficult to include latent heat release in dynamical theories because of the associated nonlinearity (precipitation generally occurs where there is upward motion). A new effective static stability is derived that fundamentally captures the effect of latent heat release on moist eddy circulations. It differs from the usual dry static stability by an additive term that depends on temperature and a parameter measuring the up–down asymmetry of vertical velocity statistics. Latent heat release reduces the effective static stability experienced by eddies but cannot reduce it to zero so long as there are nonprecipitating regions of the eddies. Evaluation based on reanalysis data indicates that the effective static stability in the lower troposphere ranges from ;80% of the dry static stability at high latitudes to ;25% in the tropics. The effective static stability provides a solution to the longstanding problem of how to adapt dry dynamical theories to the moist circulations in the atmosphere. Its utility for climate change problems is illustrated based on simulations with an idealized general circulation model. It is shown to help account for changes in the thermal stratification of the extratropical troposphere, the extent of the Hadley cells, the intensity of extra- tropical transient eddies, and the extratropical eddy length.
    [Show full text]
  • Pseudoadiabatic Processes Dr
    ESCI 341 – Atmospheric Thermodynamics Lesson 16 – Pseudoadiabatic Processes Dr. DeCaria References: Madden, R.A. and F.E. Robitaille, 1970: A comparison of the equivalent potential temperature and the static energy, J. Atmos. Sci., 27, 327-329. Betts, A.K., 1974: Further comments on ‘A comparison of the equivalent potential temperature and the static energy, J. Atmos. Sci., 31, 1713- 1715. Brunt, D., 1934: Physical and Dynamical Meteorology, Cambridge University Press, 411 pp. SPECIFIC HEAT OF MOIST AIR Water vapor is a triatomic molecule, bent at an angle of 109. The specific heat of water vapor cannot be found by through a simple rule such as 7/2 Rv (This is because determining the energy associated with the vibrational modes is not a straightforward calculation). The heat capacities for water vapor are nearly double those for dry air, as shown below. Dry Air Water Vapor 1 1 cv (J-kg -K ) 717 1410 1 1 cp (J-kg -K ) 1005 1850 The specific heat of a mixture of ideal gases is given by i c pi c pm . i For a mixture of dry air and water vapor this becomes d c p vc pv c p rc pv c pm . d v 1 r Since r is so small, we can often neglect the contribution of the water vapor to the specific heat, and just use the specific heat of the dry air, cpm cp (a similar argument applies for cvm). THERMODYNAMIC EQUATION FOR A MOIST AIR PARCEL We will consider a moist air parcel that consists of three components: Dry air of mass md Water vapor with mass mv Liquid water with mass ml The total entropy of the air parcel is S Sd Sv Sl where the subscripts d, v, and l refer to dry air, water vapor, and liquid water.
    [Show full text]
  • Planetary Boundary Layer Turbulence
    Planetary Boundary Layer Turbulence In previous chapters boundary layers were evident as thin layers across which thermal, dy- namic, and material properties of a free interior flow make a transition to their boundary values and/or boundary fluxes. In those cases the thinness of the boundary layer was typically a statement of the smallness of the relevant diffusivity; e.g., the depth h of the laminar Ekman boundary layer scales as ν1=2 (Shear Turbulence) and the thermal boundary layer in convection scales as either κ2=3 or κ4=7 in soft or hard turbulence regimes (Convective Turbulence). Boundary layers are also evident in geophysical flows, for instance at the bottom of the atmosphere, and at the top and bot- tom of the ocean. And, they serve essentially the same purpose: to match a free or interior flow to the conditions imposed at the surface, i.e., no-slip, fixed temperature, specified material properties, or specified boundary fluxes. Unlike boundary layers in many engineering flows, these planetary boundary layers (or PBLs) are almost always turbulent. The turbulence expresses the instability of the laminar boundary layer solutions, thus giving rise to an effective (eddy) diffusivity that allows the flow to make the transition between its surface and free-flow properties over a much deeper layer than would occur with only molecular diffusion and no turbulent mixing. Through this layer turbulent vertical fluxes, 0 0 0 0 w uh and w c , are expected to be significant in shaping the mean vertical profiles, uh(z) and c(z), where c is any material property.
    [Show full text]