Numerical studies of stratocumulus clouds have revealed an interesting phenomenon: Clouds respond to decay monotonically through a stability parameter of cloud-top entrainment instability, k, which is defined by the ratio of jumps of equivalent potential temperature and total water mixing ratio across a cloud top. Further, clouds evaporate completely when k is sufficiently large. A nearly linear relationship between the liquid water path (LWP) and k eventually forms. This paper explores the underlying mechanisms of such a relationship by focusing on a transient phase of large-eddy simulations (LESs), which have not yet been studied in great detail. As found in previous LES results, negative buoyancy due to evaporative cooling by mixing of air across a cloud top increases with an increase in k, and according to experimental observations, a larger k excites a stronger turbulent flow near the cloud top. As a result, during a transient phase, cloud water evaporates more rapidly. An in-depth analysis of the boundary-layer processes reveal the following: mixing occurs between an interfacial layer, which forms with time near the inversion, and the underlying cloud layer. Because the evolving interfacial layer becomes colder and more humid compared with the original inversion air, the negative buoyancy due to mixing decreases (~O (10−3) m2 s−3) by 5 h. The system eventually reaches a near equilibrium and a notable reduction of LWP ceases thereafter. Budget analysis of turbulent kinetic energy also reveals that the reduced buoyancy production, along with other terms such as shear production, compensates energy dissipation. The result implies that the state in the stratification of the interfacial layer is important in controlling the resultant LWP at the eventual state, even if the k does not vary notably. This study also investigates the time scales in terms of the rapidly decaying clouds and boundary-layer turbulence.
This paper presents a new method for identifying the type of solid hydrometeor mainly contributing to snowfall from the measured size and fall speed data. The main type is determined from the relationship between measured size and fall speed by considering the contributions of various hydrometeor types to precipitation, including graupel, graupel-like snow, aggregates at different riming stages, and small particles such as single snow crystals. The mass flux of each hydrometeor, defined as the product of its mass and fall speed, is needed to evaluate its contribution; however, it is practically difficult to measure. In this study, we estimate mass flux from the empirical relationships between size and mass and between size and fall speed. The mass flux distribution in the size-fall speed coordinates for all measured hydrometeors is found to accurately reflect the characteristics of types of hydrometeors and their contribution to observed precipitation. Considering these results, we introduce a new variable, the center of mass flux distribution (CMF), in the size-fall speed coordinates. The CMF, which is the average of size and fall speed weighted by the mass flux, can be obtained in the same way as the center of gravity in mechanics. We believe that it indicates the size and fall speed of the principal hydrometeors among all particles in the observation period. This new method allows the quantitative identification of the main hydrometeor types from the locations of CMFs in the coordinates of size and fall speed. We verify this method by its application to different types of observed snowfall events. Although there is some ambiguity in estimating the mass flux, the method is expected to be useful for identifying the main hydrometeor types in snowfall events and for quantitatively interpreting returned radar power.
In this study, a classification methodology of snow particle types, i.e., crystals, aggregates, rimed snow, and graupel, by using spatial variability of the equivalent radar reflectivity factor is proposed. The methodology is formulated on the basis of the analysis of vertically pointing Doppler radar, scanning dual-polarization weather radar, and supporting surface observations. It is argued that by using the proposed snow-type identification methodology, it is possible to guide the choice of the particular parameters of power law relations of equivalent radar reflectivity factor-liquid equivalent snowfall rate. The validity of the classification results are demonstrated by comparing the classification output to Vaisala WXT observations, which can be used to detect presence of high-density particles in snow. The performance of the proposed quantitative snowfall estimation algorithm is illustrated using an example of the data collected from the C-band operational Helsinki Vantaa radar and ground instruments (Vaisala PWD-11, Pluvio).
The initial value problem of vortex Rossby waves (VRWs) is analytically solved in a linearized barotropic system on an f plane. The basic axisymmetric vorticity q is assumed to be piecewise uniform in the radial direction so that the radial gradient dq/dr and the disturbance vorticity q are expressed in terms of Dirac delta functions. After Fourier transformation in the azimuthal direction with the wavenumber m, the linearized vorticity equation becomes a system of ordinary differential equations with respect to time; these can be analytically solved to give a closed-form solution with a prescribed initial value. For a monopolar q, the solution of q starting from the innermost radius exhibits the outward propagation of VRWs. As the outer disturbances are generated, the inner disturbance is diminished. On the other hand, in the case of a solution forced at the innermost radius, the inner disturbance is not diminished, and the outward propagation of VRWs forms a distribution of spiral-shaped disturbance vorticity. For a basic vorticity q with a moat, and if the radial distribution of q satisfies a certain additional condition, the solution of q with |m| ≠ 1 grows exponentially or linearly in time as a result of the interaction of counterpropagating VRWs near the moat. Although the solution of q with |m| = 1 cannot grow exponentially for any q, it can grow as a linear function of time. This linear growth may be regarded as a result of resonance between two internal modes of the system.
Dual-wavelength polarization lidar measurements of aerosols and clouds were conducted over Kochi (33.6°N, 133.5°E) during the warm-season field campaign of the Japanese Cloud Seeding Experiment Precipitation Augmentation project in June-July of 2008 to 2010. Lidar-derived aerosol optical properties were compared with the microphysical properties obtained with aircraft-based instruments to evaluate the utility of the lidar data for characterizing the background aerosol, which critically affects the success of cloud seeding to enhance precipitation. The results showed that the particle backscattering coefficient at 532 nm correlated well with the number concentration of aerosols with diameter (Dp) exceeding 0.3 μm (correlation coefficient, r = 0.89), whereas the correlation of the backscattering coefficient with the number concentration of cloud condensation nuclei (CCN), activated at a water saturation of 0.7% or 1.0%, was lower (r = 0.74) because of the low sensitivity of the lidar system to the small CCN particles (Dp ≈ 0.04 μm). In lidar data collected on 1 July 2008, the depolarization ratio (δ) was high (20%), and the backscatter wavelength exponent (å) was low (< 0.5) between altitudes of 4 and 6 km, and they were low (δ = 2.5%) and moderate (å = 0.7) between 0.5 and 1.0 km, suggesting the presence of supermicrometer-sized, non-spherical particles in the upper altitude range and a predominance of submicrometer-sized particles and/or spherical particles in the lower altitude range. These values were consistent with aircraft measurements, indicating the presence of supermicrometer-sized mineral particles in the upper altitude range and a predominance of submicrometer-sized sulfates and supermicrometer-sized sea-salt droplets at lower altitude. Our results demonstrate the utility of lidar data for aerosol characterization, although the further improvement of CCN characterization by lidar is necessary.
The rain microphysical budget associated with precipitation in the tropical deep convective regime is investigated through the analysis of grid-scale data from a 1.5-km-mesh two-dimensional cloud-resolving model simulation forced by large-scale forcing from tropical ocean global atmosphere coupled-ocean atmosphere response experiment. The rain grids are partitioned into several types based on the rain microphysical budget, and relationships between rainfall types and vertical profiles of vertical momentum, water vapor, and cloud hydrometeors are examined. Over 67% of the total rainfall is associated with the net rain source, in which the collection of cloud water by rain is greater than the melting of precipitation ice to rain in the presence of upward motions throughout the troposphere. Over 26% of the total rainfall is related to downward motions in the lower troposphere, leading to the melting of precipitation ice as a major term in the production of precipitation. About 15% of the total rainfall corresponds to dynamic hydrometeor advection only.
This study examines the significance of aerosol serving as cloud condensation nuclei (CCN) in modulating strengths of tropical maritime convection. Through a Tropical Ocean Global Atmosphere Couple Ocean-Atmosphere Response Experiment (TOGA COARE) case study using a cloud-resolving model (the Goddard Cumulus Ensemble Model with a horizontal mesh interval of 750 m) and a detailed spectral bin microphysical scheme, it is found that low aerosol concentration acts to reduce convection strengths. Over the tropical western Pacific where low-level water vapor is abundant and a ubiquitous weak stable level exists near 0°C, the low background maritime aerosol concentration is conducive for forming cumulus congestus. Sensitivity tests show that the main mechanism of convection damping in a clean maritime environment is through reduced condensational growth, although the freezing of supercooled water, cloud top evaporation, and rain evaporation also contribute to the simulated effects. Considering the importance of congestus in tropical dynamics and the Madden-Julian oscillation (MJO) lifecycle, we further propose a hypothesis that aerosol-cloud-precipitation interactions in an ultraclean marine environment may serve as a damping mechanism for tropical convection.
During the late 1970s, the El Niño-Southern Oscillation (ENSO) experienced a notable regime change, manifested by a change in amplitude, dominant ENSO period, and sea surface temperature anomaly (SSTA) propagation characteristics. The present study shows that these features of the ENSO regime change are associated with property changes of the canonical ENSO, i.e., cold-tongue (CT) type ENSO. Another signature of the ENSO regime change is manifested in the frequent occurrence of a warm-pool (WP) type ENSO that accompanies SSTAs centered over the central Pacific near the WP edge and exhibits characteristics differing from those of the CT ENSO. The distinct manifestations of the two types of ENSO detected in this ENSO regime change are clearly identifiable with the removal of the strong background decadal signal. Since the late 1970s, the WP ENSO has featured a weak eastward (westward) propagation of the SSTA center in the developing (decaying) phase, which makes no net contribution to the observed eastward propagation, and a 2-3 yr period compared to the 4-5 yr period of the CT ENSO. Observations strongly suggest that the WP and CT ENSO are independent quasi-biennial and quasi-quadrennial modes, respectively, of the tropical Pacific climate variability. Our observations also suggest that these two ENSO modes have coexisted actively since the late 1970s when either El Niño or La Niña can be separated into the two types.
A previous study proposed two methods for calculating the upper bound of the growth of disturbances from barotropic instability of a zonal flow in a two-dimensional incompressible fluid on a rotating sphere. The study conjectured that these two upper bounds are equivalent. One method was based on the conservation of the domain-averaged pseudomomentum density, and the other solved a minimization problem under the constraints of the conservations of all Casimir invariants and the total absolute angular momentum. In this study, this conjecture is verified, i.e., a proof is presented for their equivalence by developing an annealing-like procedure to reach the absolute vorticity profile that corresponds to the upper bound. The procedure also provides a more efficient method to calculate the upper bound.