DOCSLIB.ORG

Clouds in Current and in a Warming Climate Caroline Muller

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Clouds in Current and in a Warming Climate Caroline Muller Clouds in current and in a warming climate Caroline Muller To cite this version: Caroline Muller. Clouds in current and in a warming climate. Fundamental Aspects of Turbulent Flows in Climate, Volume 108, August 2017, 2020. hal-03023276 HAL Id: hal-03023276 https://hal.archives-ouvertes.fr/hal-03023276 Submitted on 25 Nov 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 Clouds in current and in a warming climate Caroline Muller CNRS, Laboratoire de M´et´eorologie Dynamique, Ecole´ Normale Sup´erieure Paris Lecturer picture from the summer school to be added 2 short lecture title 1.1 Introduction We see them in our everyday lives. They make skies and sunsets even more beauti- ful, inspiring painters all over the world. But what are clouds? What are the physical processes occuring within a cloud? Do they all look alike, or are there different types of clouds? Why? Beyond our small human scale, how are clouds distributed at large, planetary scales? How do they couple and interact with the large-scale circulation of the atmosphere? What do the physics of cloud formation tell us about the hydrolog- ical cycle, including mean and extreme precipitation, in our current climate and in a warming world? What role do they play in the global energetics of the planet, for instance by reflecting the incoming shortwave radiation from the sun, and by reducing the outgoing longwave radiation to space, because of their high altitudes and thus cold temperatures? These are the questions that will be addressed in these five lectures. The two first lectures will review well-understood aspects of cloud distribution and physics, which can also be found in various classical textbooks (which will be referred to at the relevant places). More specifically, the first lecture, Cloud fundamentals, will describe the fundamental properties of clouds: global distribution, cloud types, cloud visualization from satellites in space, and link with the large-scale circulation. The second lecture, Cloud formation and physics, will focus on the physics of clouds. We will briefly review the important results from atmospheric thermodynamics, and their implications for convective instability and cloud formation. Then the last three lectures will address three key open questions about clouds in our current and in a warming climate. The third lecture, Organization of deep convection at mesoscales, addresses the issue of spatial organization of convection (here and in the following, convection refers to overturning of air within which clouds are embedded). The most spectacular example of organized convection is arguably the tropical cyclone, with its eye devoid of deep clouds, surrounded by the eyewall where winds among the strongest on the planet are found. Despite the extreme weather and thus the strong societal impact of organized convection, the physical processes at stake are not fully understood. Recent advances on this topic will be presented. Clouds are also tightly linked with precipitation and the fourth lecture, Response of the hydrological cycle to climate change, will review recent results on the response of the hydrological cycle to climate change, including both mean and extreme precipitation. The fifth and last lecture, Clouds in a changing climate, will review the effect of clouds on climate sensitivity (climate sensitivity refers to the temperature increase at equilibrium in response to a doubling of carbon dioxide concentrations), how this effect is quantified, as well as recent theoretical advances on the response of clouds to climate change including the so-called “fixed anvil temperature" or FAT hypothesis. These last three topics are still very active areas of research, and for each we will review the state-of-the-art and present some recent advances made possible in recent years by the increased computational power as well as increased fundamental understanding of moist convection. Cloud fundamentals 3 1.2 Cloud fundamentals 1.2.1 Spatial distribution of clouds : overview What are clouds? The definition from the World Meteorological Organization of a cloud is a \hydrometeor consisting of minute particles of liquid water or ice, or of both, suspended in the atmosphere and usually not touching the ground. It may also include larger particles of liquid water or ice, as well as non-aqueous liquid or solid particles such as those present in fumes, smoke or dust." In other words, clouds are suspensions of liquid and ice water in the atmosphere. Note that, although there is water vapor in clouds - in fact there is water vapor everywhere in the troposphere-, this is not what we see. Indeed our eyes are not sensitive to water vapor, instead what makes a cloud visible are the liquid and/or ice particles that it contains. Looking at a daily weather map of deep clouds, see e.g. the website (EUMETSAT, 2017), confirms the diversity of cloud dynamics (see figure 1.1 for a snapshot). This map shows a one-year long evolution of weather and clouds derived from infrared satellite measurements, which are sensitive to cloud top temperatures, thus to high and deep clouds reaching cold high altitudes. The distribution of clouds is not uniform, and varies greatly with latitude. The rich dynamics of deep clouds include small-scale, \pop-corn" like, deep convec- tion in the tropics, notably convection associated with the diurnal cycle above tropical continents (e.g. central Africa close to the equator). In the tropics, corresponding to the rising branch of the Hadley circulation known as the inter-tropical convergence zone (ITCZ), deep clouds yield cold cloud-top temperatures. Deep clouds can span the whole depth of the troposphere, from the top of the planetary boundary layer, to the tropopause, around 15 km at tropical latitudes. Also notable at mid latitudes (around ±45◦ latitude) are large-scale cloud sys- tems associated with extra-tropical cyclones and their associated frontal systems. Deep clouds are embedded in frontal low and high pressure systems typical of those lati- tudes. More precisely clouds and high water vapor concentrations can be found on the eastward side of cold fronts. Smaller cloud areal coverage is found in the subtropics (centered around ±30◦ latitude), i.e. in the subsiding branch of the Hadley circulation, as can also be seen in the time-mean spatial distribution of cloud amounts and cloud top temperatures figure 1.2. The data is from the ISCCP project, whose website provides useful information and data on clouds (ISCCP, 2009; Rossow and Schiffer, 1999). In the subtropics, warmer cloud-top temperatures are consistent with the occurence of shallow clouds (typically one to two kilometers in height). Globally, clouds cover a large fraction of our planet, about two thirds (figure 1.2). The zonal mean makes clear that high cloud coverage is found in the tropics (close to the equator, within 10◦ latitude or so) and at mid latitudes (centered around ±45◦ latitude). The largest cloud cover is over the southern ocean, and more generally larger cloudiness is found over oceans than over land. Clouds are thus key actors of the climate system, as they cover a significant fraction of the planet. Indeed, depending on their thickness and their height, clouds can interact with the Earth radiation budget in different ways. Clouds cool the planet by reflecting 4 short lecture title Fig. 1.1 Snapshot from the 2017 \Year of Weather" online one-year long animation of high clouds from satellites. c EUMETSAT 2017. ISCCP-D2: 1983/07-2009/12 Mean Annual Zonal Means ISCCP-D2: 1983/07-2009/12 Mean Annual Zonal Means Mean cloud amount % Mean cloud amount % Cloud top temperature K Cloud top temperature K Fig. 1.2 Time-averaged (1983 to 2009 estimated from ISCCP) distribution of cloud amount (fractional area covered by clouds, left) and cloud top temperatures (right) as a function of latitude and longitude. Also shown are zonal means as a function of latitude (blue curves). From ISCCP data (Rossow and Schiffer, 1999). some of the incoming shortwave radiation from the sun. Clouds also warm the planet by trapping some of the longwave radiation emitted at the Earth surface, thus reducing the radiative cooling to space. Those radiative effects of clouds make them crucial ingredients of the global energetics. We will come back to this in the last lecture x1.6. Consistently, understanding clouds, their coupling with a large-scale circulation, and how it impacts climate sensitivity, has been identified as one of the World Cli- mate Research Programme (WCRP) grand challenge (Bony, Stevens, Frierson, Jakob, Cloud fundamentals 5 Kageyama, Pincus, Shepherd, Sherwood, Siebesma, Sobel et al., 2015). WCRP grand challenges represent areas of emphasis in scientific research in the coming decade, identifying specific barriers preventing progress in a critical area of climate science. 1.2.2 Cloud classification Clouds are classified based on their appearance and altitude (Houze Jr, 2014). The classification is based on five latin roots: • Cumulus: heap, pile; • Stratus: flatten out, cover with a layer; • Cirrus: lock of hair, tuft of horsehair; • Nimbus: precipitating cloud; • Altum: height. Cumulus refers to clouds with some vertical extent, typically resembling \cotton balls". Stratus refers to clouds with some horizontal extent, typically with a flat large horizon- tal cover. Cirrus characterizes the thin, filamentary aspect of the highest clouds almost entirely composed of ice crystals. Nimbus simply indicates precipitation reaching the ground, and altum is used to characterize mid-level clouds.
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]
  • 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.
    [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]