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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2018)
Global climate models robustly predict that global mean precipitation should increase at roughly 2-3% [Formula: see text], but the origin of these values is not well understood. Here we develop a simple theory to help explain these values. This theory combines the well-known radiative constraint on precipitation, which says that condensation heating from precipitation is balanced by the net radiative cooling of the free troposphere, with an invariance of radiative cooling profiles when expressed in temperature coordinates. These two constraints yield a picture in which mean precipitation is controlled primarily by the depth of the troposphere, when measured in temperature coordinates. We develop this theory in idealized simulations of radiative-convective equilibrium and also demonstrate its applicability to global climate models.
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2016)
    Cold pools of air, which are formed by evaporating precipitation, play a critical role in the triggering of new precipitation. Despite their recognized importance, little effort has been devoted to building simple models of their dynamics. Here, analytical equations are derived for the radius, height, and buoyancy of a cylindrical cold pool as a function of time, and a scale analysis reveals that entrainment is a dominant influence. These governing equations yield simple expressions for the maximum sizes and lifetimes of cold pools. The terminal radius of a cold pool is relatively insensitive to its initial conditions, with a typical maximum radius of about 14 times the initial radius, give or take a factor of 2. The terminal time of a cold pool, on the other hand, can vary over orders of magnitude depending on its initial potential and kinetic energies. These predictions are validated against large-eddy simulations.
      Cover page: On the sizes and lifetimes of cold pools
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2016)
      It is shown here that a wide, buoyant parcel of air at the surface accelerates far less rapidly than it does aloft. In particular, analytical formulae are derived for the effective buoyancy (i.e. the net vertical acceleration due to parcel buoyancy and environmental response) of idealized cylinders of diameter D and height H, located in free space and at the surface. These formulae quantify the decrease of effective buoyancy with increasing aspect ratio D/H, and show that this effect is more pronounced for surface cylinders, especially when D/H > 1. We gain intuition for these results by considering the pressure fields generated by these buoyant parcels, and we test our results with large-eddy simulations. Our formulae can inform parametrizations of the vertical velocity equation for clouds, and also provide a quantitative map of the 'grey zone' in numerical modelling between hydrostatic and non-hydrostatic regimes.
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2015)
        The Davies-Jones formulation of effective buoyancy is used to define inertial and buoyant components of vertical force and to develop an intuition for these components by considering simple cases. This decomposition is applied to the triggering of new boundary layer mass flux by cold pools in a cloud-resolving simulation of radiative-convective equilibrium (RCE). The triggering is found to be dominated by inertial forces, and this is explained by estimating the ratio of the inertial forcing to the buoyancy forcing, which scales as H/h, where H is the characteristic height of the initial downdraft and h is the characteristic height of the mature cold pool's gust front. In a simulation of the transition from shallow to deep convection, the buoyancy forcing plays a dominant role in triggering mass flux in the shallow regime, but the force balance tips in favor of inertial forcing just as precipitation sets in, consistent with the RCE results.
          Cover page: Effective Buoyancy, Inertial Pressure, and the Mechanical Generation of Boundary Layer Mass Flux by Cold Pools
          • Thesis
          • Peer Reviewed
          UC Berkeley Electronic Theses and Dissertations (2016)

          ‘Cold pools’ are pools of air that have been cooled by rain evaporation, and which subsequently slump down and spread out across the Earth’s surface due to their negative buoyancy. Such cold pools, which typically arise from rain produced by convection, also feed back upon convection by kicking up new convection at their edges.

          This thesis studies the interaction of cold pools and convection at two levels of detail: on one end, we study the dynamics and thermodynamics of a single, idealized cold pool, and on the other, we study the interplay between a steady-state ensemble of convection and the many cold pools that accompany it. A recurring notion is that of ‘effective buoyancy’, which is the net acceleration experienced by a density anomaly such as a cold pool, including the back-reaction of the environment (i.e. the ‘virtual mass effect’) which reduces the net acceleration from its Archimedean value. We derive analytical formulae for the effective buoyancy of cold pools and other roughly cylindrical density anomalies, and use the same framework to understand the forces at play when cold pools trigger new convection. We also analyze the sizes and lifetimes of cold pools, and examine the impact of cold pools on the organization (i.e. clustering) of convection.

            Cover page: Cold Pools, Effective Buoyancy, and Atmospheric Convection
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2019)
            The Fixed Anvil Temperature (FAT) hypothesis proposes that upper tropospheric cloud fraction peaks at a special isotherm that is independent of surface temperature. It has been argued that a FAT should result from simple ingredients: Clausius-Clapeyron, longwave emission from water vapor, and tropospheric energy and mass balance. Here the first cloud-resolving simulations of radiative-convective equilibrium designed to contain only these basic ingredients are presented. This setup does not produce a FAT: the anvil temperature varies by about 40% of the surface temperature range. However, the tropopause temperature varies by only 4% of the surface temperature range, which supports the existence of a Fixed Tropopause Temperature (FiTT). In full-complexity radiative-convective equilibrium simulations, the spread in anvil temperature is smaller by about a factor of 2, but the tropopause temperature remains more invariant than the anvil temperature by an order of magnitude. In other words, our simulations have a FiTT, not a FAT.
              Cover page: FAT or FiTT: Are Anvil Clouds or the Tropopause Temperature Invariant?
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2019)
              Tropical anvil clouds play a large role in the Earth's radiation balance, but their effect on global warming is uncertain. The conventional paradigm for these clouds attributes their existence to the rapidly declining convective mass flux below the tropopause, which implies a large source of detraining cloudy air there. Here we test this paradigm by manipulating the sources and sinks of cloudy air in cloud-resolving simulations. We find that anvils form in our simulations because of the long lifetime of upper-tropospheric cloud condensates, not because of an enhanced source of cloudy air below the tropopause. We further show that cloud lifetimes are long in the cold upper troposphere because the saturation specific humidity is much smaller there than the condensed water loading of cloudy updrafts, which causes evaporative cloud decay to act very slowly. Our results highlight the need for novel cloud-fraction schemes that align with this decay-centric framework for anvil clouds.
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