1). 2009). The height of the cloud is at least twice its width. Although taken at different locations, both soundings were launched in the equatorial region, at 0°, 11.65°W and 3.27°N, 14.3°W for the first and second soundings, respectively. Sections 6 and 7 discuss and summarize the obtained results. The observed values increase to about 7 h over land and 10 h over ocean. 2007; Takayabu et al. The other criteria used in the detection algorithm stay as previously. The existence of four conjoint grid points with brightness temperature below 240 K and no other deep activity in a radius of 50 km is requested. Brightness temperature is provided each 15 min on a grid with a 3-km resolution at nadir. Figures 8a–g emphasize the poor spatial correlation that exists between areas of cumulus congestus and of cumulonimbus. Large-scale convergence may promote deep convection, which, in turn, will reinforce convergence in the lower layers and divergence in the upper ones. The simulation results are averaged horizontally and over 30-min intervals. Insightful comments of the reviewers, which helped improve this manuscript and the presented arguments, are acknowledged. Some of these effects are hard to disentangle from each other. Cumulonimbus clouds fully developed are much different than your typical fair-weather cumulus cloud, but a cumulus congestus cloud is the precursor to a cumulonimbus cloud. The profiles are seen as representative for different convective phases. (1a). This is much faster than the time needed (10 h and longer) by congestus clouds to sufficiently moisten the atmosphere. Similar conclusions can be deduced for the Atlantic region; see Figs. Figure 7b thus indicates that convective disturbances of scale smaller than about 200 km are unlikely to be triggered by congestus moistening. Even if the congestus clouds may have disappeared many hours before the start of a new deep convection cycle, the deposed moisture anomalies can survive them for many hours. 4b). Several studies have also tried to relate relative humidity and precipitation strength (e.g., Bretherton et al. 7b across scales, τ* should be scale invariant if congestus moistening were the main driver of the transition. Ascent (or convergence) may be achieved through various mechanisms, as noted in section 2c. (3a)–(3c), w is assumed constant in space and time and set to 0.01 and 0.05 m s−1, respectively (see section 2a). The fact that the presence of cumulus congestus over a region does not enhance the likelihood for future deep convection initiation for lead times longer than 4 h, as compared to clear-sky conditions. Characteristic time scales associated with these two processes, as well as the actual transition time, will be derived throughout this study. The data are available on a 3-hourly basis for the time period 30 August–18 September 1974, and on a 1° × 1° resolution grid with 19 vertical levels. Even in that case, the transition time does not drop below 6 h. In the simulation of Kuang and Bretherton (2006) congestus clouds need 3 days to sufficiently moisten the atmosphere. (2)] Pm = exp[11.4(r − 0.522)]. 6b. (1c), whereby the moistening follows from the imposed vertical motion. The white line encloses the main region of deep convective activity. The values scatter around 2 h over northern South America and central Africa versus 4 h over the Atlantic Ocean. But they do not enhance the probability for transition, because the transition is more efficient under dynamical forcing. The following three sections deal with the time-scale analysis: section 3 estimates the time scales resulting from moistening by congestus clouds and from some form of imposed ascent using a bulk approach, section 4 repeats the analysis based on LES, and section 5 deals with observed transition times as derived from brightness temperature measurements. Although the effects of congestus clouds are hard to disentangle in observations, numerical experiments with cloud-resolving models can help guide our thinking. The logical consequence is that, if a process can moisten the atmosphere, it will force the development of deeper clouds. The daily rhythm imposes strong constraints on the timing of convection with a short window for deep convection initiation. 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. In the second view, deep convection is triggered because of some form of imposed ascent. There is also a hint toward shorter τ* over the Atlantic ITCZ (see the region enclosed in white in Fig. This comparison suggests that deep convection primarily responds to dynamical forcing over the tropics. Very low probabilities, with P(4) = 0.36, are obtained for the land region in Fig. The column water vapor peaks at 52 mm at time of congestus clouds and at 54.5 mm at time of cumulonimbi in Masunaga (2012). To investigate this, ratios of conditionally sampled transient congestus to conditionally sampled terminal congestus are computed. 1). The analysis combines ship observations, large-eddy simulations, and 1 month of brightness temperature measurements with a focus on the tropical Atlantic and adjacent land areas. In contrast to shallow (trade wind) cumuli, they populate the midlevels of the atmosphere. The situation is even more dramatic over the main region of deep convective activity, as visible in Fig. The time of deep convection initiation is diagnosed, as alluded to in section 2c, by downgrading the resolution of the LES output to the MSG resolution and requiring the presence of at least four points in the domain with brightness temperature below 240 K for 1 h. Except for the lowest 1 km of the atmosphere, the simulated values remain close to the sounding curves. Gray stands for all points, dashed black for oceanic points, and black for land points. 2006; Schumacher et al. Moistening by cumulus congestus constitutes such a process. (2004)], entrainment of dry air effectively limits cloud depth. Other effects—for example, the need to make clouds unstable relative to their environment (Wu et al. 2009). This yields stronger cold pools, larger clouds, smaller entrainment rates, and possibly deeper clouds (Khairoutdinov and Randall 2006). This implies a faster transition over land than over ocean. where denotes the cumulus moisture flux, σ the cloud cover, and w the vertical velocity. Hence, the latter two values will be employed to get a rough idea of possible vertical motions that the atmosphere may experience. 1). This yields Δqυ = 1.6 g kg−1, (dqυ/dt)c = 1.1 g kg−1 day−1, and therefore τc = 35 h, as listed in Table 1. In observations, congestus clouds can be identified by a peak in reflectivity or by a layer of enhanced detrainment/divergence typically situated around 500 hPa in the tropics (e.g., Johnson et al. The question of the representativeness of the profiles driving the LES simulations and the convective development thus arises. This makes cumulus congestus an important cloud category per se. 2002; Siebesma et al. Figures 5 and 6 consider neither the horizontal scale of the convective systems nor the possible relationship between transition time and horizontal extent. This has an impact on the calculated τ* but not on the underlying conclusion that even weak ascent more efficiently initiates deep convection than congestus clouds. Figures 8a–g also confirm the rapid nature of the convective development over land. The analysis was, however, inconclusive concerning the origin of this moisture but did indicate that the amount of extra cloud water from shallow and congestus clouds available for evaporation prior to deep events fell short to explain the observed increase in moisture. Strictly speaking, this represents a dynamical effect and thus belongs to the forced ascent category (i.e., to our second hypothesis). Interestingly, the required moistening is smallest in W0. Averaged values are sufficient for the type of computation and analysis that will be performed.