In Fig. 1, we plot composite radiosondes for our events. Overall, the behaviors are similar to previous studies drawn from 12:00 UTC GoAmazon2014/5 radiosondes (e.g., Giangrande et al., 2020). The main shift between seasonal profiles is associated with the drier middle to upper levels observed for the dry season. Each composite indicates a low-level capping or remnant nocturnal temperature inversion that may act to inhibit daytime shallow cumulus and/or promote deeper cloud modes when convection initiates.

A breakdown of event convective parameters (see also Tables 1 and 2) is as follows: dry-season low-level (0–6 km) MLCAPE values are larger than during wet-season events, with a mean MLCAPE value of 239 J kg-1 compared with a mean wet-season MLCAPE of 152 J kg-1. This difference is statistically significant at the 95th confidence level based on a student's t test (herein “significant”). The dry-season profile MLCAPE values are also larger, MLCAPE of 1506 J kg-1 (dry) versus MLCAPE of 1337 J kg-1 (wet); however, this difference is not statistically significant. Insignificant seasonal differences are found in low-level wind shear (not shown) and MLCIN. As expected, mean relative humidity (RH) values in the lower free troposphere are significantly larger in the wet season (78 %) than in the dry season (56 %).

In Fig. 2, we plot composite diurnal cloud and boundary layer properties to inform on pre-DCC onset differences between wet and dry events. In Fig. 2a and b, we plot the ARSCL cloud frequency of occurrence for the event hours around radiosonde launch through convective initiation (typically prior to 14:00 LT). In Fig. 2c and d, we plot T3 soil surface temperature (ARM Surface Energy Balance System – SEBS) and the ceilometer-estimated PBL heights, respectively. Morning shallow to mid-level clouds are more common for our typical wet-season event when compared with composite dry-season cases. This observation is not surprising and is consistent with previous studies that infer higher humidity as a control for increased cloudiness. The reduction in dry-season cloud cover is also consistent with a more rapid PBL height increase that follows sunrise than in the wet season (Fig. 2d), which (presumably) results from greater insolation in the dry season. The largest PBL height discrepancies build prior to 12:00 LT, preceding the transition to congestus or deeper cloud modes. This transition is also suggested by cloud radar profiling in that more frequent cloud radar echoes to higher altitudes are observed (an initial congestus transition occurs prior to 12:00 LT for wet and slightly later for dry). In short, both seasonal composites indicate similar tendencies for the daytime shallower cloud mode (echo-top heights <2 km) shortly after 10:00 LT, with a transition towards deeper clouds (echo-top height ≥4 km) after 12:00 LT. However, we observe an earlier presence of upper-level (anvil) cloud signatures coupled with an absence of mid-level clouds (Fig. 2b) in the dry season (by approx. 13:00 LT), which suggests that a more rapid transition to deeper convection occurs in the dry season.

The PBL evolution in the dry season also suggests a more rapid onset of deeper convection. The physical arguments that support this include the higher morning MLCAPE (at similar or reduced MLCIN) coupled with building PBL instability during the pre-convective hours from an increase in incoming solar radiation (reduced cloud frequency and slightly higher surface temperature). While complete surface flux measurements were unavailable, the authors speculate that dry-season conditions may favor a higher Bowen ratio (i.e., reduced soil moisture and humidity) and stronger generation of turbulent boundary layer growth (leading to the observed higher PBL height). Note that, while our subset of radar-tracked cells exhibited similar onset timing, wet-season cells were longer-lived (using the Z>25 dBZ threshold). As we plot in Fig. 3, echo statistics drawn from the larger SIPAM domain cell-tracking populations from these events suggests that our wet (solid lines) and dry (dashed lines) events show a similar frequency of occurrence and diurnal timing for Z>25 dBZ and Z>35 dBZ convective echoes. However, dry events exhibited more frequent occurrence of intense convective echoes Z>45 dBZ, consistent with arguments for stronger dry-season cells overall, and a more rapid deep transition and/or increased anvil cloud presence. In contrast, wet-season events suggested slightly earlier Z>25 dBZ and Z>35 dBZ populations, which may be associated with additional precipitating congestus, or extended congestus-to-deep cloud transitioning.

In Fig. 4, we plot seasonal life cycle composites for precipitation properties of our tracked storms. These depictions apply a normalized cell lifetime for compositing purposes, where 0 represents the echo onset time and 1 represents the final time a qualifying echo was observed. Most tracked cells (19 for wet and 12 for dry) within the lower-threshold set (Z>25 dBZ; Fig. 4a) are contained within the higher-threshold tracking set (Z>35 dBZ; Fig. 4b) (i.e., exceeding 10 pixels for multiple scans exceeding 50 min). Each event recorded a maximum Z>35 dBZ at multiple points during its evolution. In Fig. 4c and d, we plot the mean Z associated with the tracked cells, and we plot the corresponding maximum Z composite properties for those cells in Fig. 4e. The distribution of the cell overpass times relative to the normalized life cycle stage is found in Fig. 4f.

The plot of seasonal cell properties for precipitation area coverage is found in Fig. 4a and b. Initially, composite cell properties with Z>25 dBZ (Fig. 4a) display similar increases in coverage throughout earlier stages (normalized time <0.4). However, dry-season cells typically remain a similar size in light rain/periphery area coverage (e.g., Z∼25 dBZ) for times >0.4, whereas wet-season cells continue to increase in such coverage until a peak at normalized time ≅0.7. Adopting a more stringent Z>35 dBZ convective echo threshold (Fig. 4b), composite area properties are more consistent across the seasons, albeit representing a shorter-lived subset of the previous tracking. For the wet season, there are hints that Z>35 dBZ echo regions eventually outgrow those of the dry season, although most cells quickly dissipate at later relative stages (times >0.7). Overall, composites suggest that dry-season cells are relatively compact and intense, occupied by higher Z>35 dBZ echoes and retaining modest precipitation intensity for much of their lifetimes. These findings are consistent with previous GoAmazon2014/5 studies by Giangrande et al. (2020) that proposed that drier mid-levels in the drier seasons may limit periphery precipitation (i.e., enhanced evaporation), whereas wet-season cells may exhibit more resilient periphery precipitation.

Seasonal differences are also revealed when considering radar properties that are more directly associated with Z magnitude (Fig. 4c, d, e). Dry-season composites skew their strongest Z signatures to earlier life cycle stages, often with maximum behaviors found prior to normalized life cycle time <0.2 (i.e., within the first few qualifying radar volumes). An early storm intensification is consistent with arguments from the previous section indicating increased PBL instability during the dry season, reduced MLCIN, and higher low-level MLCAPE. Nevertheless, composite dry-season cell areas remain relatively unvarying after an initial intensification period throughout a lengthy portion of their normalized lifetime. In contrast, wet-season composites indicate a gradual increase in Z and areal precipitation coverage, with peak Z (normalized time ≅0.4) preceding an associated peak in precipitation coverage (i.e., normalized time ≅ 0.7). Composite wet-season storms appear to achieve similarly intense Z cores to our sampled dry counterparts at later moments in the cell life cycle. This result may not be surprising because our events share statistically similar CAPE values and these comparisons target longer-lived cells that conditionally may favor the more intense behaviors from the wet season.

In Fig. 5, we plot cumulative overpass vertical air velocity retrievals contingent on season (panels a and d present “wet”, panels b and e present “dry”, and panels c and f present the “dry–wet difference”) and according to multiple RWP retrieval thresholds (panels a–c show >25 dBZ and panels d–f show >35 dBZ). Median vertical air velocity (solid lines) and 5th/95th percentiles (dashed lines) are overlaid on the CFADs as reference for the extreme instantaneous observations from these events. Overall, composite velocity CFADs indicate that downdrafts are common at low levels, but CFADs transition towards more prominent updraft observations aloft (peak updrafts ≅10 m s-1; Fig. 5a, b, c).

With respect to updraft observations, the Z>25 dBZ CFADs suggest that dry-season maximal updrafts are more intense, although the relative enhancement is modest O[2–3 m s-1] and primarily observed at low levels below the melting level (to approx. 6 km). The dry-season samples in these Z>25 dBZ CFADs are favoring more frequent and modest downdrafts aloft (to be discussed). However, updraft extremes aloft indicate that dry-season observations are recording updrafts of comparable intensity to our wet-season samples; the most intense (95th percentile) updraft retrievals for both seasons are O[10 m s-1]. Potential physical reasons for observed seasonal updraft profile characteristics, comparable magnitudes aloft, and shifts therein will be discussed in Sect. 4.3.

For downdraft observations, the most consistent downdrafts that we observed were associated with regions below the melting level (precipitation driven). Interestingly, downdrafts are observed to higher altitudes, but the most frequent and vigorous (>5 m s-1) are found within the dry-season events for the altitudes below 10 km. As will be confirmed with our subsequent Z>35 dBZ CFAD discussions, the majority of these dry-season downdraft retrievals aloft are associated with weaker Z<35 dBZ echoes and are, thus, found towards the peripheries of the more intense cores. Previously, Giangrande et al. (2016) investigated the GoAmazon2014/5 RWP dataset and suggested that strong downdrafts aloft may provide indirect evidence for stronger updrafts (under higher-CAPE/CIN and/or dry-season conditions). Their interpretation was that compensating toroidal-like circulations associated with stronger updrafts that were not directly observed may promote those stronger downdrafts aloft. While not stated by those authors, greater precipitation/condensate loading associated with stronger updrafts may also contribute to stronger downdrafts using those arguments. As with their efforts, we did not observe significantly stronger updrafts aloft, but we found that the observed shift in downdraft likelihood and intensity was primarily a dry-season phenomenon at these altitudes. A discussion on possible physical reasons for observing enhanced dry-season downdraft signatures at these altitudes is found in Sect. 4.4 to complement updraft discussions in Sect. 4.3.

In Fig. 5d, e, and f, cumulative CFAD plots shift towards prominent updraft signatures when we emphasize only those observations from the more intense Z>35 dBZ “core” precipitation instances from these same overpasses. Moreover, higher-reflectivity regions aloft are consistently, and increasingly to higher altitude, associated with updrafts. Physically, one interpretation is that larger or more intense precipitation signatures (lofted, larger, or more dense hydrometeors) aloft are also those conditionally associated with stronger updrafts overall. For the wet season in particular, maximum updraft signatures consistently peak above the melting level, with extreme values O[10 m s-1] similar to those retrieved during the dry-season events (above 6–7 km). Both seasons record less frequent observations of intense downdrafts within these higher or core Z>35 dBZ regions aloft. Stronger dry-season downdrafts are observed below the melting level, similar to Amazon MCS studies by Wang et al. (2019).

When interpreting the cumulative CFAD results above, the cell maturity of the corresponding overpasses is not explicitly revealed by these displays. In Figs. 6 and 7, we plot CFADs contingent on cell overpass ETH, where retrievals before/after an ETH of 10 km are used as a proxy for relative storm maturity. Overall, low-level precipitation-driven downdraft signatures for both seasons are more prominent and extend further above the melting level for our higher-ETH observations. As in cumulative CFADs, dry-season overpasses indicate stronger updrafts, but these stronger updrafts are primarily found below 6 km, and we associate them with developing cloud life cycle stages for the ETH <10 km retrieval instances. In later ETH >10 km stages, dry-season retrievals are dominated by downdrafts (frequency), with strong downdraft motions observed O[5 m s-1] aloft. In contrast, wet-season CFADs suggest a strengthening and more prevalent updrafts aloft to the later ETH <10 km stage observations.

The ETH displays filtered by Z>35 dBZ regions display a much clearer association between the presence of larger Z values reaching higher altitudes and stronger updraft observations (both seasons). However, our CFADs suggest that similarly intense reflectivity factors aloft (i.e., sampling 35 dBZ to 6 km) may be associated with a fairly wide range of updraft intensity contingent on the season and/or where those observations fall within the storm life cycle. Physically, these complications follow from storms having updrafts that are, at times, less impeded by precipitation, but any transition to stronger updrafts may also be convolved within increasing precipitation (i.e., heavier rainfall, graupel formation, and/or larger Z). Nevertheless, the Z>35 dBZ CFADs reinforce that strong Z signatures to higher altitudes may be attributed to strong updrafts and/or to close proximity to updrafts above the melting level.

Quartile breakdowns of storm life cycle for RWP retrievals are plotted in Fig. 8. Dry-season vertical air velocity retrievals suggest that the strongest upwards air motions are confined to the earliest life cycle stages, consistent with surveillance radar signatures for dry-season storm intensification. Wet-season quartile breakdowns reveal a gradual shift in draft characteristics towards more intense air motions by the middle quartiles (or associated peaks in Z and ETH), also in alignment with previous surveillance properties. The evidence for more intense dry-season upwards air motions at the low levels does not appear confined to any particular life cycle stage. However, stronger updrafts aloft are found with increasing time for the wet season, and more prominent downdrafts aloft are found with increasing time for the dry season. Late-cell-phase samples (time >0.75) are unavailable for the dry season, but late-stage wet-season samples imply a higher frequency of observations associated with downdrafts below the melting level as well as a shift for the observations of updrafts further aloft (i.e., possible signatures for remnant anvil/dissipating cloud air motions).

We explore the physical reasons for the differences in draft behaviors evident in Figs. 4–8 by applying a simplified updraft model to the 12:00 UTC sounding from each case. Because the entrainment rates in the observed updrafts are not known, we aim to simulate ascending parcels with a range of entrainment rates that encompasses what we might reasonably expect in weakly sheared tropical convection. To generate this range of entrainment rates, we use a stochastic parcel model (SPM) that is formulated in a similar manner to the eddy diffusivity/mass flux shallow convective scheme described in Sušelj et al. (2013, 2019). A detailed technical description of the model is available in Appendix A. In short, we simulate 100 different parcels in each sounding. The choice of 100 parcels was a compromise for model speed versus performance; however, repeat analyses using 1000 parcels (not shown) provided little change to the offered results. We assume entrainment in each parcel occurs in a series of discrete stochastic mixing events that follow a Poisson distribution, with the peak of the distribution corresponding to a typical fractional entrainment rate in tropical deep convection of 2×10-4 m-1 (e.g., Romps and Kuang, 2010). Finally, we produce histograms at each height for the vertical air velocities among those parcels to compare this SPM output to observed CFAD characteristics from the previous sections. These histograms were generated by dividing the vertical velocity versus height parameter space into 1 m s-1 and 250 m bins, respectively, and summing all the points along SPM parcel paths that fell into each bin over a given season (i.e., wet or dry). We divided by the number of events in that season and applied a Gaussian filter with a radius of influence of 5 m s-1 and 1250 m with respect to vertical velocity and height, respectively. For the model outputs, those parcels that did not reach 5 km were excluded to maintain our focus on DCCs (Fig. 9).

In Fig. 9a and b, we plot summary velocity profile behaviors from the multiple realizations that start from the morning radiosondes for each wet (Fig. 9a) and dry (Fig. 9b) event. Maximum parcel heights for SPM parcels were in the 10–13 km range (Fig. 9a, b), which is generally consistent with observed echo-top heights (e.g., Wang et al., 2018). This suggests that the entrainment rates of SPM parcels were reasonably consistent with those in the observed storms, as entrainment strongly regulates cloud depth. Peak vertical velocities are 50 %–100 % larger than what was observed by the RWPs. This discrepancy between peak SPM vertical velocities and observations is at least partially explained by an expected underestimation of the extreme/peak updraft velocities by RWP sampling (as was previously noted).

We subtracted the wet-season histogram from the dry-season histogram in Fig. 9c to plot seasonal differences in parcel behavior, where blue (red) values indicate that the SPM outputs a higher incidence for more intense wet-season (dry-season) updrafts. More intense dry-season updrafts are prevalent in model realizations at the lower levels, which is attributed to the higher low-level MLCAPE. This result is consistent with RWP observations that also suggest more intense low-level updrafts for dry-season samples. Strong (i.e., 5–10 m s-1) updrafts become more prominent aloft (>6–7 km) in the wet season and more comparable to those in the dry season. These comparatively stronger updrafts aloft between the wet- and dry-season model realizations are also consistent with the shift in our RWP difference fields (i.e., Figs. 7f, 9c).

In Fig. 9d, we plot SPM results in the form of dry–wet differences, as from Fig. 9c, but after rerunning the realizations for each case and replacing all of the RH profiles (above 2 km) with an average RH over all dry-season cases. The motivation for these tests was an attempt to remove RH considerations, thereby possibly highlighting residual differences resulting from the different CAPE profiles. Given the more prominent dry-season updraft realizations to higher levels, one implication of this test is that stronger updrafts dominate the dry-season realizations if seasonal RH considerations are removed. This may suggest that the lower RH mitigates the intensity of dry-season updrafts or, equivalently, that the larger RH of the wet season is essential to its larger incidence of deeper updrafts.

To conclude our analysis, we provide plausible explanations for why downdrafts were more intense aloft in the dry season than observed for the wet season. Recent Amazon MCS observations from Wang et al. (2019, 2020) indicate that drier dry-season low- to mid-level conditions favor stronger downdrafts and/or higher downdraft origin heights. One hypothesis for our isolated cell events is that mixtures between drafts and environmental parcels are more negatively buoyant in the dry season. Consequently, these parcels will experience more intense downwards accelerations. To evaluate this, we leveraged the parcel properties simulated by the SPM in the previous subsection.

For each case, we selected the SPM parcel at each height with the median moist static energy (MSE). This parcel was defined as the “updraft parcel”, for which we recorded the MSE, water vapor (q_v), and condensate (q_c) mixing ratios of this parcel. For these tests, we assumed that the MSE and q_c of the updraft parcel mix linearly with the environment (q_c is zero in the environment), and we consider mixtures with fractions of updraft air ranging from 0.1 to 0.9 at intervals of 0.1 (i.e., the environment composes the other fraction of this mixture). Using this range of mixtures and assuming saturation, we solved for the parcel temperature and buoyancy for each updraft air fraction. We then recorded the average buoyancy of all negatively buoyant mixtures at each height, which gives a vertical profile of negative buoyancy for each event. We expect that mixtures of the updraft and the environment are more negatively buoyant in dry-season events than in wet-season events, due to the smaller free-tropospheric relative humidity in the former. We further assume that these mixtures between an updraft and its environment are responsible for initiating downdrafts and that strongly negatively buoyant mixtures will initiate stronger downdrafts than their less negatively buoyant counterparts.

As we plot in Fig. 10, the resulting dry-season buoyancy profiles are more negative than wet-season buoyancy profiles between 2 and 8 km. The difference is statistically significant between 4 and 6 km, with the dry-season buoyancy being a factor of 1.5 to 2 more negative than the wet-season negative buoyancy. Note that the calculations in Fig. 10 apply to updraft mixtures. However, we speculate that different mixtures of the cloud's surrounding environmental air mixed with detrained updraft air or downdrafts would behave similarly, where mean dry-season drafts would exhibit greater negative buoyancy compared with wet-season drafts.

Alternative interpretations for the observational differences may be rooted in RWP sampling as related to the seasonal differences in cell areal precipitation characteristics, cell lifecycle timing and intensity. Recall that dry-season downdrafts aloft were most frequently observed in 25<Z<35 dBZ samples and at later life cycle stages; these observations may be those preferentially collected near the edges of dry-season cells that radar indicates as more compact than their wet-season counterparts. This compact nature was attributed to evaporation and/or mixing with the drier RH environment limiting cell growth, potentially prioritizing RWP observations to locations where these processes, stronger air motions, and/or greater precipitation loading was occurring (e.g., Giangrande et al., 2016). In particular, dry-season RWP characteristics are consistent with additional graupel formation earlier in dry-season storm lifecycles, which may contribute to additional condensate loading in those events. There was evidence (not shown) for stronger downdrafts aloft (>8–10 km) at later stages for wet-season events that may also support an evolving relationship with stronger updrafts leading to additional loading; however, such observations were limited by the RWP sampling choices adopted in the present study. Equivalently, wet-season observations in those similar 25<Z<35 dBZ ranges may also include additional samples embedded within resilient and/or wider-spread precipitation areas (i.e., periphery or slower-falling snow) and regions that are more insulated from the cloud edge; these locations are consequently less prone to being associated with downdrafts in RWP samples. A comprehensive exploration of all downdraft possibilities is beyond the scope of our study but will be examined in future research.