Using a time-scale analysis based upon ship observations, large-eddy simulations, and brightness temperature measurements, it was found that if midtropospheric moisture availability is assumed to be a limiting factor in the development of convection, congestus moistening of the midtroposphere proceeds too slowly to explain the rapid transition to deep convection that is observed and simulated in the tropics. (2) of Bretherton et al. Gray stands for all points, dashed black for oceanic points, and black for land points. Flight and Ground Instructor (CFI) and FOI. As compared with the situation over land, isolated congestus clouds already populate the region of future deep convection 4 h before triggering (see the region enclosed by the orange ellipse in Figs. The GATE domain covers the area 4°–14°N, 19°–28°W. Indubitably, cumulus congestus is an important cloud category per se. Assuming a dominant effect of congestus moistening and given, for instance, Fig.

The values are 52.9 and 58.9 mm for the two Polarstern soundings and 59.4 and 61.4 mm in W0. Centrifugal Stop Pin / Anti-Feathering Lock | Picture or Diagram? The estimation of τ* involves a number of subjective choices, as noted in section 2c. (2009) simulated one episode from the GATE period, which includes prescribed large-scale forcing. Parameterizations include a Smagorinsky-type scheme to represent subgrid-scale turbulence. Khairoutdinov et al.

Similar conclusions can be deduced for the Atlantic region; see Figs. It follows that, for congestus moistening to become important, observed transitions should be slow and the atmosphere relatively quiescent over a day at least. The induced cooling amounts to 2.6 K given a lapse rate of 4 K km−1.

Two views were contrasted. 2009). The study was supported by the Max Planck Society for the advancement of science. If the top of Cumulus clouds is similar to cauliflower, it´s called Cumulus Congestus or “towering cumulus”.

The observation that cumuli congestus either rapidly or never grow into cumulonimbus does not support the congestus moistening hypothesis.

Each of the towers is numbered 1 through 3 with comments on each tower. Second, some dynamical forcings (e.g., gravity waves) can generate much more intense vertical motions on smaller scales. The lower troposphere moistens by 2.3 g kg−1 in this case. The situation is even more dramatic over the main region of deep convective activity, as visible in Fig. (2004)], entrainment of dry air effectively limits cloud depth. Initial conditions are based on one representative sounding from the Polarstern (see section 4 for the choice of the sounding). Figure 10 is analogous to Fig. Use of GATE data and of the supercomputer facilities at the Deutsches Klimarechenzentrum (DKRZ) is acknowledged. Hence, the bulk of our conclusions remains unchanged.

Besides w, the specific humidity measured at t1 (see Fig. If cumuli congestus are present for 4 h over a region, they have a 29% chance to transition to deep convection in the next 2 h and a 71% chance to disappear. 6b. The larger the box, the larger the minimum scale of the captured system and the fewer events recorded. Cumuli congestus transition almost instantaneously to cumulonimbi (see their Fig. Larger deep convective disturbances are preceded by a longer congestus phase. 2002; Derbyshire et al. Username * The latter time scales are shorter than any of the estimates of the time needed by congestus clouds to bring the atmosphere into a moist enough state to support deep convection (see Table 1). (2011)—using Global Precipitation Climatology Project (GPCP), 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), and TRMM data—indicated that positive low- and midlevel moisture anomalies precede deep convection. Culmulonimbus (CB) are associated with more unstability of the air than Fair weather culmulus. 6a or Fig. Password *. In contrast, shallow cumuli are in essence viewed as warm clouds. (1c) transforms to. In the second view, deep convection is triggered because of some form of imposed ascent. Whereas Masunaga (2012) considered the whole tropics, Holloway and Neelin (2009) focused on Naru. 1). Haertel et al. Masunaga et al. A majority of cumulus congestus (60%), which never transitions to deep convection. Profiles of (a) potential temperature (K) and (b) specific humidity (g kg−1) at time of deep convection initiation as well as (c) condensate (precipitating and nonprecipitating) evolution in simulations W0 (solid black), W1 (solid blue), W1q (dashed blue), and W5 (gray). It bears mentioning that several studies have documented the sensitivity of simulated deep convection to aspects of the experimental setup (e.g., Bryan et al. Figure 1 shows the relative frequency of the large-scale vertical velocities derived from GATE data, vertically averaged below 10 km.

6b (see black curve). A comment/reply has been published regarding this article and can be found at http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-13-0165.1 and http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-13-0216.1. Not surprisingly, Fig. But they do not enhance the probability for transition, because the transition is more efficient under dynamical forcing.

Only the cold waters of the Eastern boundary current regions, or the dry areas of the Sahara, remain void of cumulus congestus and cumulonimbus.

Section 2 presents the method and describes the observational datasets, large-eddy simulations, and metrics (time scales). The latter channel facilitates a distinction between congestus and cumulonimbus clouds on the basis of a much colder cloud-top temperature for the cumulonimbi. Whereas the first case only relies on moisture supplied by the surface fluxes, the second case benefits from moisture advected by the flow in the region of interest. This tower is fuzzier and not as vertically developed.

Despite the different observational period, the GATE dataset is retained because it represents the only dataset of observed large-scale vertical velocity for the tropical Atlantic. To disentangle the effects of congestus moistening from forced ascent in the transition to deep convection, measurements collected over the tropics as well as LES are explored. They all start from the Polarstern sounding t1 (see Fig. Here, we make use of the ability of the UCLA-LES (see section 2b) to explicitly resolve convection to derive a second set of time scales. The short answer is because towering cumulus isn’t cumulonimbus.

Such layers are associated with marked changes in the environmental conditions, either in thermal stability (e.g., Johnson et al. Typical values of σ = 0.05, wu(z1) = 1 m s−1, and give (dqυ/dt)c = 1 g kg−1 day−1 and τc = 38 h. This is on the order of magnitude of the previous estimate, which is not surprising given that the cloud base moisture flux in a mass flux scheme will more or less track the surface flux. Finally, the very recent study by Rapp et al. Cumuli congestus are defined as clouds with tops between 4 and 9 km. Although the effects of congestus clouds are hard to disentangle in observations, numerical experiments with cloud-resolving models can help guide our thinking. Longer transition times, with values up to 10 h, can only be found outside the 10°S–10°N belt, where the occurrence of deep events is rare (see Fig. To investigate this, ratios of conditionally sampled transient congestus to conditionally sampled terminal congestus are computed. It is imposed on temperature and humidity in W1 and W5 but only on humidity in W1q.

2b) is used in Eq. Satellite observations are used to isolate congestus from cumulonimbus clouds and thus to estimate characteristic transition times. 2003; Waite and Khouider 2010). As a further test, τ* is recomputed using different box sizes. The macroscopic behavior of cumulus ensembles simulated by a cumulus ensemble model, Physical characterization of tropical oceanic convection observed in KWAJEX, This site uses cookies. It is itself a subdivision of the Cumulus genus. While CB should be avoided like the plague, aircraft do fly through the other culmulus, although I would strongly suggest you have your shoulder belt/harness on, slow to maneuvering speed and NOT make sudden or abrupt control movements while flying through a larger culmulus cloud. Cumulus Castellanus ; This variety of towering cumulus cloud is characterized by having multiple castle-like projections on its summit which is a strong indication of the vertical movement of air. The zonal bands in (b) are between 2.5° and 10°N for the oceanic curve and between 10°S and 15°N for the land curve. 8).

i want to know why CB is more dangerous than towering cumulus? Building on this, Waite and Khouider (2010) specifically explored the role of congestus clouds. As nouns the difference between cumulonimbus and cumulus is that cumulonimbus is a cloud, with a tall structure and a flat base, that is often associated with thunderstorms while cumulus is a large white puffy cloud that develops through convection on a hot, humid day, they can form towers and even become cumulonimbus clouds.

The same is true concerning the profiles at later times. The data are from GATE and have been vertically averaged below 10 km. Polarstern crosses the Atlantic Ocean 2 times per year, in late fall and spring, on its way from its home port in Bremerhaven to Antarctica and back. Generally, it is a matter of degree. As a recent addition, the two-moment ice microphysics scheme of Seifert and Beheng (2006) was implemented in the model to allow the simulation of deep convection. 8i, the purple ellipse would represent the most likely location for a subsequent triggering of deep convection. To ease the comparison with the satellite observations, the resolution of the output is downgraded to 0.1°.

Most importantly, a comparison of the values obtained in Fig.

7a. The model domain has 88 vertical levels with spacing between levels of 100 m in the lower layers and stretching up to 600 m. The domain top lies at 24 km. The percentage climbs up to 90% by 9 h both over land and ocean.

This yields stronger cold pools, larger clouds, smaller entrainment rates, and possibly deeper clouds (Khairoutdinov and Randall 2006). Contrasting Figs. Just that towering CU is at a … The results are consistent with the observation that even rather weak vertical motion field, which is at least present over the Atlantic, can moisten the midtroposphere much more efficiently than congestus clouds. 6b.

2002; Siebesma et al. The simulations are performed with the University of California, Los Angeles (UCLA) LES (Stevens et al.