Cumulus Activity Research Articles

RACORO continental boundary layer cloud investigations: 3. Separation of parameterization biases single‐column model CAM5 simulations of shallow cumulus

AbstractClimatically important low‐level clouds are commonly misrepresented in climate models. The FAst‐physics System TEstbed and Research (FASTER) Project has constructed case studies from the Atmospheric Radiation Measurement Climate Research Facility's Southern Great Plain site during the RACORO aircraft campaign to facilitate research on model representation of boundary‐layer clouds. This paper focuses on using the single‐column Community Atmosphere Model version 5 (SCAM5) simulations of a multi‐day continental shallow cumulus case to identify specific parameterization causes of low‐cloud biases. Consistent model biases among the simulations driven by a set of alternative forcings suggest that uncertainty in the forcing plays only a relatively minor role. In‐depth analysis reveals that the model's shallow cumulus convection scheme tends to significantly under‐produce clouds during the times when shallow cumuli exist in the observations, while the deep convective and stratiform cloud schemes significantly over‐produce low‐level clouds throughout the day. The links between model biases and the underlying assumptions of the shallow cumulus scheme are further diagnosed with the aid of large‐eddy simulations and aircraft measurements, and by suppressing the triggering of the deep convection scheme. It is found that the weak boundary layer turbulence simulated is directly responsible for the weak cumulus activity and the simulated boundary layer stratiform clouds. Increased vertical and temporal resolutions are shown to lead to stronger boundary layer turbulence and reduction of low‐cloud biases.

Relationship between Cumulus Activity and Environmental Moisture during the CINDY2011/DYNAMO Field Experiment as Revealed from Convection-Resolving Simulations

Relationship between Cumulus Activity and Environmental Moisture during the CINDY2011/DYNAMO Field Experiment as Revealed from Convection-Resolving Simulations

A China-Japan Cooperative JICA Atmospheric Observing Network over the Tibetan Plateau (JICA/Tibet Project): An Overviews

A China-Japan Cooperative JICA Atmospheric Observing Network over the Tibetan Plateau (JICA/Tibet Project): An Overviews

Estudo numérico da interação entre convecção rasa e radiação com ênfase no ciclo diurno do balanço de energia à superfície na Amazônia

Estuda-se neste artigo o efeito da convecção rasa no ciclo diurno do balanço de energia à superfície na Amazônia. O enfoque foi dado ao papel do sombreamento, devido aos cúmulos rasos na configuração do forçamento convectivo à superfície. Um esquema para o cálculo da cobertura de cúmulos rasos foi implementado no modelo BRAMS. Foram executados dois experimentos: um com convecção rasa e sem interação com a radiação, e o segundo com convecção rasa e interação com a radiação. Os resultados mostraram que a consideração da interação entre convecção rasa e radiação produz simulações mais realistas. Isso foi mais pronunciado por volta do meio-dia local e início da tarde, quando os cúmulos rasos estão com sua atividade máxima. Os fluxos e a cobertura foram bastante sensíveis às variações no forçamento radiativo. Apesar da melhoria da qualidade dos fluxos, do ponto de vista qualitativo, ficou evidente a deficiência do modelo em representar esses fluxos nas primeiras e últimas horas do ciclo diurno.

Seasonal variation of cloud activity and atmospheric profiles over the eastern part of the Tibetan Plateau

Cumulus activity over the Tibetan Plateau has a clear pattern of seasonal progression. Activity occurs with significant frequency in the eastern part of the plateau from April to mid‐May. From mid‐May to mid‐June the frequency of cumulus activity drastically decreases and then increases after mid‐June. Causes of such definite seasonal progression are revealed by in situ radiosonde observation data. In early spring the vertical profile of potential temperature is almost uniform from the surface to 7000 m above sea level around noon, and atmospheric stratification is vertically neutral for dry convection. Under such a condition, dry convection easily occurs and develops cumulus cloud that is frequently observed. From mid‐May to mid‐June the atmosphere has conditionally unstable stratification, but the atmosphere is relatively dry and hardly saturated and cumulus activity occurs less frequently. Though the atmosphere is also conditionally unstable after mid‐June, the increase in total precipitable water content allows easy atmospheric saturation and cumulus activity recommences. At the same time, satellite data and reanalysis data show, in the eastern part of the Tibetan Plateau, a consistent correspondence of the seasonal variation of cumulus activity, total precipitable water content, and vertical instability of the atmosphere. Results of synoptic‐scale analysis indicate the enhancement of cumulus convection in the northern part of India and the Tibetan High cause a moisture increase over the Tibetan Plateau and leads to the recommencement of frequent cumulus activity over the plateau.

Vertical wind observation in the tropical upper troposphere by VHF wind profiler: A case study

Features of upper tropospheric vertical wind (W) over Sumatra, Indonesia, are presented using data observed by a VHF wind profiler installed at West Sumatra (0.2°S, 100.32°E). During 5–9 May 2004, W from the middle to upper troposphere (8–14 km) changed in accordance with the cumulus activity over Sumatra. During 5–6 May, 3‐hourly averaged W continuously showed upward motions up to 0.09 m s−1. The upward motions were observed in the vicinity of deep convective events, which were continuously seen over Sumatra within a synoptic‐scale convectively active envelope. After 7 May, when cumulus activity was suppressed over Sumatra, 3‐hourly averaged upward motions of greater than 0.05 m s−1 almost disappeared. During 5–6 May, downward motions up to ∼0.11 m s−1 were observed above 14 km, while upward motions were observed below 14 km. Estimation of W by the European Centre for Medium‐Range Weather Forecasts operational analysis have revealed that a major part of observed downward motions above 14 km is explained by the leeward (southwestward) wind and leeward downward tilt of isentropes that existed over western Sumatra. The observed downward motions above 14 km during 5–6 May suggest that downward motions caused by leeward downward tilt of isentropes can be produced in the vicinity of the convectively active region, and leeward downward tilt of isentropes can suppress an upward transport of air mass into the tropical tropopause layer (TTL) by producing downward motions in the TTL.

Distributed Forcing of Forecast and Assimilation Error Systems

Abstract Temporally distributed deterministic and stochastic excitation of the tangent linear forecast system governing forecast error growth and the tangent linear observer system governing assimilation error growth is examined. The method used is to determine the optimal set of distributed deterministic and stochastic forcings of the forecast and observer systems over a chosen time interval. Distributed forcing of an unstable system addresses the effect of model error on forecast error in the presumably unstable forecast error system. Distributed forcing of a stable system addresses the effect on the assimilation of model error in the presumably stable data assimilation system viewed as a stable observer. In this study, model error refers both to extrinsic physical error forcing, such as that which arises from unresolved cumulus activity, and to intrinsic error sources arising from imperfections in the numerical model and in the physical parameterizations.

Comment on "No Evidence for Iris"

SEPTEMBER 2002 AMERICAN METEOROLOGICAL SOCIETY | number of points put forth by Hartmann and Michelsen (2002; hereafter HM02) are, in fact, irrelevant to the iris effect (i.e., their discussion of the meteorological origin of various systems). We shall discuss this in greater detail later. Some are quantitatively insignificant (the effect of spreading cloud cover near the equator); we shall also return to this later. Others are simply wrong (such as the assertion that we assume cloud cover over latitudes around 20° originates from cumulus convection over 1000 km away). Digging through such items that basically amount to “red herrings,” we find that their primary assertion is that cloud cover variations in the region between about 15° and 30° latitude are not primarily convective in origin, but rather occur because of the penetration of extratropical systems into the Tropics. Allegedly, the appearance of such clouds reduces the cloud-weighted sea surface temperature (CWT) as well as the ratio of cloud cover to cumulus activity, and spuriously produces the iris effect in Lindzen et al. (2001, hereafter LCH). It is on this assertion that we will focus. Although the Bulletin chose to refer to HM02 as “a more careful analysis,” HM02’s arguments are fundamentally dependent on ignoring important caveats in LCH.

Convective Precursors and Predictability in the Tropical Western Pacific

Conditions leading to convective outbreak in the Tropics are investigated by multivariate analysis of sounding and satellite data from the tropical western Pacific area. Circumstances that make the prediction problem difficult are discussed and addressed by applying linear “error-in-variables” and nonlinear statistical simulation techniques to a large dataset. Low- to midtropospheric moisture is identified as the dominant factor regulating convective outbreak in this region. Based on the results it is argued that such moisture is particularly important in regulating spontaneous convective outbreak, but instability and near-surface wind speed probably play some role in allowing previous shallow or midtopped cumulus activity to deepen. Mesoscale-mean convective available potential energy sufficient for convection is found to exist almost 90% of the time. Quantitative estimates of noise in the data are obtained and accounted for in reaching these conclusions. The results imply that large-scale mean fields alone may not contain enough information to determine the behavior of convection except probabilistically. Both types of statistical model predict that even under favorable mesoscale-mean conditions, convection is typically only 20%–30% likely to break out during a given 3-h period.

THE CLIMATE OF THE KALAHARI TRANSECT

SUMMARY The climate of the Kalahari Transect is defined by the nature of the general circulation of the atmosphere over Africa south of the equator and the regional circulation variations that determine local weather. Individual synoptic systems and the transport of moisture into the region by converging and diverging airstreams in both summer and winter govern conditions favouring or inhibiting the major rainfall-producing process of convective precipitation. The climate is illustrated by the spatial variation of moisture, rainfall, temperature, evaporation and other fields. It is shown that the climate of the transect varies from the subtropical in the south to the tropical in the north with characteristic gradients of most parameters between the two regions in both summer and winter. The Kalahari Transect is a multi-disciplinary regional project having as its aim the advancement of the understanding of the effects of human and climatically induced global change over a large area of southern Africa. It has a near-unique advantage of being a study where the underlying regional geology, comprising Kalahari sand, is uniform over a considerable range of latitude. This offers the prospect of reducing the uncertainties associated with regional manifestations of global change. Climate and climatic change remain major determinants of all facets of the Kalahari Transect study. In this paper, the nature of the regional climate of the transect and how it may change in the future are discussed. The Kalahari Transect has a latitudinal extent from 14°S to 28°S and a longitudinal equivalent from 21°E to 28°E. The delimited area covers much of central southern Africa, ranging from shrubland and grassy desert in the south to evergreen dense woodland in the north. The whole region is dominated by the southern hemisphere descending limb of the Hadley cell of the general atmospheric circulation, the southern part of the transect more so than the northern part. This means that anticyclonic circulation patterns are the dominant circulation type for most of the year. The attendant atmospheric conditions are characterised by subsiding, stable air masses, hazy lower tropospheric conditions and generally cloud-free skies. South of the Tropic of Capricorn disturbances in the mid-latitude westerlies assume an increasingly important role in determining rainfall, whereas to the north seasonal migrations of the Inter-Tropical Convergence Zone (ITCZ) become the major determinant of the annual rainfall cycle. Over the whole region cumulus convective activity, enhanced by atmospheric disturbances associated with negative pressure departures and suppressed by those with positive departures, is the major determinant of rainfall, which everywhere exhibits a strong seasonal cycle with a summer maximum. In general, it is difficult to acquire a dense coverage of continuous, long-period, modern meteorological records for Africa south of the Sahara. The same is true of the region covered by the Kalahari transect. Good general descriptions of the climate of the region are provided in the atlases of Jackson (1961) and Thompson (1965). The reviews of Torrance (1972), Schulze (1972, 1984). Hastenrath (1984) and Tyson (1986) provide additional and more up-to-date material. These sources and others have been used to compile a climatology of the Kalahari transect region. After a brief summary of the regional atmospheric circulation, the main parameters to be considered are rainfall, cloud cover, energy budget components, temperature, atmospheric moisture, evaporation and potential evapotranspiration. The summary of the transect climate concludes with estimates of possible future regional conditions that may follow from global greenhouse warming.

The Macroscopic Behavior of Cumulus Ensembles Simulated by a Cumulus Ensemble Model

A number of simulation experiments are performed with the 2D UCLA cumulus ensemble model to study the macroscopic behavior of cumulus convection under a variety of different large-scale and underlying surface conditions. Specifically, the modulation of cumulus activity by the imposed large-scale processes and the eddy kinetic energy (EKE) budget are investigated in detail. In all simulations, cumulus convection is rather strongly modulated by large-scale advective processes in spite of the existence of some nonmodulated high-frequency fluctuations. The modulation exhibits some phase delays, however, when the basic wind shear is strong. This is presumably associated with the existence of mesoscale convective organization. The EKE budget analysis shows that the net eddy buoyancy generation rate is nearly zero for a wind range of cumulus ensembles.

An Observational Study of the First-Order Vorticity Dynamics in a Tropical Easterly Wave

Abstract GATE Phase III A/B- and B-scale data were analyzed and used to diagnostically test the first-order vorticity equation proposed by Cho et al. The results are given in the form of a composite easterly wave and the analysis shows that the first-order vorticity equation is capable of describing the vorticity dynamics of the composite easterly wave. Cumulus heating produces significant large-scale vertical motion, and it is demonstrated that the effect of cumulus clouds on the large-scale vorticity field is through vortex-tube stretching by cloud-induced vertical motion. The composite analysis of the first-order vorticity equation also shows that the vorticity dynamics of tropical easterly waves are adequately described by a linearized first-order vorticity equation and dominated by the advection of the perturbed vorticity by the relative zonal mean wind, the advection of the zonal mean vorticity by the perturbed wind, and the vortex-tube stretching induced by cumulus activity. It is explained how, du...

「水惑星」モデルにおける熱帯域の積雲活動のふるまいと循環構造

As an extension of the 'aqua planet' experiments by Hayashi and Sumi (1986, HS86), GCM experiments with various SST distributions and cumulus parameterizations are performed in order to examine the structure of cumulus activity and its parameter dependence. In part I, the results of the analyses concentrating on the super clusters are described.The super clusters, which were named by HS86, are expressed in the model as continuously moving precipitating areas of the grid scale. The associated circulation structure can be explained in terms of the wave-CISK dynamics of Kelvin waves. The super clusters become more active when a warmer SST is used. On the other hand, the 30 day oscillation, that is an eastward-moving planetary scale structure, is more distinct when a colder SST is used.The behavior of cumulus activity is highly sensitive to the choice of cumulus parameterization. When the moist convective adjustment scheme is used instead of Kuo's (1974) scheme, the eastward-moving structure of the grid scale is no longer observed at the equator. However, the planetary scale structure still clearly appears in the experiment with the moist convective adjustment scheme.

「水惑星」モデルにおける熱帯域の積雲活動のふるまいと循環構造

「水惑星」モデルにおける熱帯域の積雲活動のふるまいと循環構造

Altered steroidogenic pattern of human granulosa-lutein cells in relation to cumulus cell culture morphology

It has been reported that human cumulus-oocyte complexes (COC) retrieved at a stimulated cycle manifest an asynchrony between oocyte meiotic maturation and cumulus mucification. However, when mature COC were subdivided into subtypes marked by the culture morphology of their cumulus cells following 3 days' culture, successful fertilization and cleavage were approximately 1.5-fold lower in mature COC yielding cumulus cells aggregated into clumps (type A and B COC) than in mature COC yielding homogeneously spread cells (type C-D COC). To determine whether the existing relationship between cumulus culture morphology and oocyte functionality in the various COC types (A-D) could be extended to another follicular compartment—the granulosa-lutein (G-L) cells—basal steroid secretion by the corresponding G-L cells was evaluated within 5 days of culture. Over the first 3 days of culture, secretion of progesterone was 3-fold lower and secretion of testosterone (T) was 2.5-fold higher in cultures of G-L cells from follicles yielding type A COC than in type C-D COC. During days 4 and 5 of culture, G-L cells were incubated with or without 10 −7M 3β-hydroxy-5-pregnen-20-one (pregnenolone), dehydroepiandrosterone (DHA), 4-androstene-3,17-dione (androstenedione), or T. The pattern of progesterone level noted over the first 3 days of culture was not altered in the presence of pregnanolone, DHA, androstenedione, or T. Addition of pregnenolone, DHA, androstenedione, or T increased T level 2.5-, 5.6-, 7.3-, and 17.7-fold, respectively, in cultures of G-L cells from follicles yielding type A COC, but did not significantly alter T level in cultures of G-L cells from follicles yielding type C-D COC. In cultures of G-L cells from follicles yielding type A COC, addition of androgens unsaturated at position 4 preferentially increased oestradiol-17β (E 2) level, whereas in cultures of G-L cells of type C-D COC, DHA and androstenedione preferentially increased E 2 level. Taken together, the asynchrony between oocyte and cumulus activity could be diminished when the activity of various follicular cell compartments is evaluated according to cumulus culture morphology rather than cumulus expansion and mucification. The present study suggests that follicles yielding mature COC represent a non-homogeneous population in which G-L cells from follicles yielding type A-B COC manifest a less luteinized state than those from follicles yielding type C-D COC.

Observational Analysis of Tropical Cyclogenesis in the Western North Pacific. Part I: Structural Evolution of Cloud Clusters

Abstract This paper is the first of a two-part series summarizing recent attempts to advance omit understanding of the physical processes associated with tropical cyclone formation in the western North Pacific. A combination of rawinsonde composite and individual case analyses are shown for both pre-cyclone tropical cloud clusters (genesis) and those-prominent cloud cluster that do not develop into tropical cyclones (nongenesis). This research is an extension of previous works at Colorado State University by McBride, Zehr, and Love. The first part of the study concerns the structural evolution of the genesis cloud clusters and nongenesis cloud clusters. Results show that nongenesis cloud clusters have about the same mean vertical motion and cumulus activity as the genesis cloud clusters. However, genesis cloud clusters are found to have much stronger middle-to low-level cyclonic circulation extending over a radius 2°–8° outward from the center. The buildup of this strong precyclogenesis circulation is ass...

Observational Analysis of Tropical Cyclogenesis in the Western North Pacific. Part II: Budget Analysis

Abstract This is the second of a two-part study which uses a combination of composite rawinsonde and individual FGGE case analyses to identify the important physical processes associated with tropical cyclone formation. Part I examined the structural evolution and circulation patterns for genesis and nongenesis cloud clusters in the western North Pacific. This paper analyzes moisture, energy and angular (tangential) momentum budgets for both types of cloud clusters. Results show that both the genesis and nongenesis cloud clusters have similar cumulus activity and similar moisture and energy budgets. An enhanced surface energy flux is found only after the formation of a tropical cyclone with a well-defined center. The results of Part I showed that genesis cloud clusters have comparatively strong 2°–8° radius middle- to low-level cyclonic circulations in comparison with nondeveloping cloud clusters. Transports by the mean transverse circulation are not efficient to produce the observed buildup of the tangen...

Numerical Simulation of the Development of Mean Monsoon Circulation in July

Eight different experiments are carried out, ranging from 8 to 20 days, with a primitive equation model consisting of five atmospheric layers and one oceanic layer. The purpose is to investigate the relative importance of radiative heating and deep cumulus condensation, orography, and initial conditions for the development of the mean monsoon circulation from June to July in the 0-180 deg E, 25 deg S - 55 deg N region. Two alternative initial states are used, one based on the observed monthly mean June pressure distribution, the other on the mean June zonal average pressure distribution. Whereas large-scale condensation and actual land and sea distributions are included in every experiment, deep cumulus cone condensation and radiative heating are always taken together. It is found that the means sea level pressure distribution and low-level flow pattern produced by these simulations are in the main determined by the diabatic heating distribution and are influenced somewhat by orography; they are, however, almost independent of the initial state. The low pressure systems are found to develop faster when diurnal variation of solar radiation is allowed for; this is thought to be due mainly to the more vigorous cumulus activity during the day.

Cyclone Development in Polar Air Streams over the Wintertime Continent

Abstract A polar air stream cyclogenesis over the wintertime North American Continent is documented. Numerous similarities are found between this land case and oceanic polar lows studied Harrold and Browning (1969), Reed (1979) and Mullen (1979). The main difference between the land case and oceanic polar lows is that the land cyclone develops in an air mass with negligible water vapor content whereas oceanic ones clearly do not since they are characterized by enhanced cumulus activity during their early stage of development Hence, it is concluded that cumulus convection and its associated latent heat release are not essential for cyclone development in a polar air stream. It is also shown that the surface occlusion which forms from the merger of the polar trough with the main polar front does not follow the typical surface occlusion process as described by the Norwegian cyclone model.

Tropical Cumulus Interaction with and Modification of the Subcloud Region

Abstract Nineteen cases of tropical cumulus convective activity at and below cloud base are described using simultaneous measurements at as many as six levels between the surface and 1200 m. Updrafts are traced from just above cloud base to near the ocean surface. An inventory of updrafts and downdrafts indicates that about 80% are either warm/moist (up) or cold/dry (down) relative to their immediate environments. However, the remaining 20% of the drafts are found to make significant contributions to the total cloud transports. Various degrees and types of modification of the lower PBL structure associated with cumulus activity are illustrated with before, during and after profiles. Cloud transports of mass, heat, moisture and momentum are estimated using only those drafts with a vertical velocity greater than ± 0.5 m s−1.