Individual Convective Events Research Articles

Asymmetric Dipole Trends in Geodynamo Models and the Paleomagnetic Field

AbstractTemporal trends in the paleomagnetic dipole moment exhibit the property of positive skewness. On average, positive trends are larger and occur less frequently than negative trends over timescales of several tens of kyr. We explore the origin of this property using numerical geodynamo models. A suite of models reveals that skewness arises for a restricted set of boundary conditions. Models driven by heat flow at the top and bottom boundaries exhibit very little skewness, whereas models driven solely by heat flow on the lower boundary produce significant positive skewness. Further increases in skewness occur in the presence of thermal stratification at the top of the core. The level of skewness in the geodynamo models is correlated with estimates of upwelling near the core‐mantle boundary. Sustained upwelling is expected to increase magnetic‐flux expulsion, contributing to higher levels of skewness. Similar behavior is recovered from stochastic models in which the dipole is generated by a random series of cyclonic convection events. Skewness in the stochastic models is quantitatively similar to estimates from the geodynamo models when the average recurrence time of the convection events is 100 years. Extending the stochastic models to the paleomagnetic field implies a longer recurrence time of 1,000 years or more. We interpret this recurrence time in terms of the timing of flux‐expulsion events rather than individual convective events. Abrupt increases in the dipole moment from flux expulsion can produce skewed trends on timescales of tens of kyr.

Paired stable isotopologues in precipitation and vapor: A case study of the amount effect within western tropical Pacific storms

AbstractUnderstanding controls on the stable isotopic composition of precipitation and vapor in the West Pacific Warm Pool is vital for accurate representation of convective processes in models and correct interpretation of isotope‐based paleoclimate proxies, yet a lack of direct observational evidence precludes the utility of these isotopic tracers. Results from a measurement campaign at Manus Island, Papua New Guinea from 28 April to 8 May 2013 demonstrate variability in the stable isotopic composition (δD and δ18O) of precipitation and vapor in individual precipitation events and over a 10 day period. Isotope ratios in water vapor and precipitation progressively increased throughout the period of measurement, coincident with a transition from high to low regional convective activity. Vapor isotope ratios approached equilibrium with seawater during the quiescent period and likely reflected downwind advection of distilled vapor and re‐evaporation of rainfall during the period of regional convection. On a 5 min timescale across individual storms, isotope ratios in precipitation were strongly correlated with isotope ratios in surface vapor. However, individual precipitation isotope ratios were not strongly correlated with surface meteorological data, including precipitation rate, in all storms. Yet across all events, precipitation deuterium excess was negatively correlated with surface temperature, sea level pressure, and cloud base height and positively correlated with precipitation rate and relative humidity. Paired surface precipitation and vapor isotope ratios indicate condensation at boundary layer temperatures. The ratio of these paired values decreased with increasing precipitation rate during some precipitation events, suggesting rain re‐evaporation and precipitation in equilibrium with an isotopically distinct upper level moisture source. Results from the short campaign support the interpretation that isotope ratios in precipitation and vapor in the western tropical Pacific are indicators of regional convective intensity at the timescale of days to weeks. However, a nonstationary relationship between rain rate and stable isotope ratios in precipitation during individual convective events suggests that condensation, rain evaporation, moisture recycling, and regional moisture convergence do not always yield an amount effect relationship on intraevent timescales.

Assessment of a Satellite-Based Atmospheric Budget Analysis Method Using CINDY2011/DYNAMO/AMIE and TOGA COARE Sounding Array Data

A satellite-based method of moisture and thermal budget analysis is examined in comparison with sounding array observations from Cooperative Indian Ocean experiment on Intraseasonal variability in the Year of 2011 (CINDY2011)/Dynamics of the Madden-Julian Oscillation (MJO) (DYNAMO)/Atmospheric Radiation Measurements (ARM) MJO Investigation Experiment (AMIE) and from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). Overall, the satellite analysis is found to quantitatively reproduce the statistical behaviors of large-scale mean vertical motion, moisture convergence, and moist static energy (MSE) convergence as observed from the sounding arrays. However, individual convective events generally do not delineate a systematic evolutionary track but are heavily spread around the ensemble mean of moisture and MSE convergences in composite space. Next, the convective events are broken down into “developing”, “off-centered”, and “passing-by” classes using geostationary infrared measurements in an attempt to sort irrelevant samples that are not representative of convective dynamics. All the three composite classes show qualitatively similar evolutions except for the amplitude of variability, with genuine developing events being greatest in amplitude and passing-by disturbances being weakest. The spread among individual events is substantially reduced when the convective events immune to strong synoptic-scale influences are isolated and the contribution of horizontal advection is excluded from MSE convergence.

Convection–Climate Feedbacks in the ECHAM5 General Circulation Model: Evaluation of Cirrus Cloud Life Cycles with ISCCP Satellite Data from a Lagrangian Trajectory Perspective

Abstract A process-oriented climate model evaluation is presented, applying the International Satellite Cloud Climatology Project (ISCCP) simulator to pinpoint deficiencies related to the cloud processes in the ECHAM5 general circulation model. A Lagrangian trajectory analysis is performed to track the transitions of anvil cirrus originating from deep convective detrainment to cirrostratus and thin cirrus, comparing ISCCP observations and the ECHAM5 model. Trajectories of cloudy air parcels originating from deep convection are computed for both, the ISCCP observations and the model, over which the ISCCP joint histograms are used for analyzing the cirrus life cycle over 5 days. The cirrostratus and cirrus clouds originate from detrainment from deep convection decay and gradually thin out after the convective event over 3–4 days. The effect of the convection–cirrus transitions in a warmer climate is analyzed in order to understand the climate feedbacks due to deep convective cloud transitions. An idealized climate change simulation is performed using a +2-K sea surface temperature (SST) perturbation. The Lagrangian trajectory analysis over perturbed climate suggests that more and thicker cirrostratus and cirrus clouds occur in the warmer climate compared to the present-day climate. Stronger convection is noticed in the perturbed climate, which leads to an increased precipitation, especially on day−2 and −3 after the individual convective events. The shortwave and the longwave cloud forcings both increase in the warmer climate, with an increase of net cloud radiative forcing (NCRF), leading to an overall positive feedback of the increased cirrostratus and cirrus clouds from a Lagrangian transition perspective.

The Double-ITCZ Syndrome in Coupled General Circulation Models: The Role of Large-Scale Vertical Circulation Regimes

Abstract The double–intertropical convergence zone (DI) systematic error, affecting state-of-the-art coupled general circulation models (CGCMs), is examined in the multimodel Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) ensemble of simulations of the twentieth-century climate. The aim of this study is to quantify the DI error on precipitation in the tropical Pacific, with a specific focus on the relationship between the DI error and the representation of large-scale vertical circulation regimes in climate models. The DI rainfall signal is analyzed using a regime-sorting approach for the vertical circulation regimes. Through the use of this compositing technique, precipitation events are regime sorted based on the large-scale vertical motions, as represented by the midtropospheric Lagrangian pressure tendency ω500 dynamical proxy. This methodology allows partition of the precipitation signal into deep and shallow convective components. Following the regime-sorting diagnosis, the total DI bias is split into an error affecting the magnitude of precipitation associated with individual convective events and an error affecting the frequency of occurrence of single convective regimes. It is shown that, despite the existing large intramodel differences, CGCMs can be ultimately grouped into a few homogenous clusters, each featuring a well-defined rainfall–vertical circulation relationship in the DI region. Three major behavioral clusters are identified within the AR4 models ensemble: two unimodal distributions, featuring maximum precipitation under subsidence and deep convection regimes, respectively, and one bimodal distribution, displaying both components. Extending this analysis to both coupled and uncoupled (atmosphere only) AR4 simulations reveals that the DI bias in CGCMs is mainly due to the overly frequent occurrence of deep convection regimes, whereas the error on rainfall magnitude associated with individual convective events is overall consistent with errors already present in the corresponding atmosphere stand-alone simulations. A critical parameter controlling the strength of the DI systematic error is identified in the model-dependent sea surface temperature (SST) threshold leading to the onset of deep convection (THR), combined with the average SST in the southeastern Pacific. The models featuring a THR that is systematically colder (warmer) than their mean surface temperature are more (less) prone to exhibit a spurious southern intertropical convergence zone.

Potential future changes in the characteristics of daily precipitation in Europe simulated by the HIRHAM regional climate model

In this study the potential future changes in various aspects of daily precipitation events over Europe as a consequence of the anticipated future increase in the atmospheric greenhouse gas concentrations are investigated. This is done by comparing two 3-member ensembles of simulations with the HIRHAM regional climate model for the period 1961–1990 and 2071–2100, respectively. Daily precipitation events are characterized by their frequency and intensity, and heavy precipitation events are described via 30-year return levels of daily precipitation. Further, extended periods with and without rainfall (wet and dry spells) are studied, considering their frequency and length as well as the average and extreme amounts of precipitation accumulated during wet spells, the latter again described via 30-year return levels. The simulations show marked changes in the characteristics of daily precipitation in Europe due to the anticipated greenhouse warming. In winter, for instance, the frequency of wet days is enhanced over most of the European continent except for the region on the Norwegian west coast and the Mediterranean region. The changes in the intensity and the 30-year return level of daily precipitation are characterized by a similar pattern except for central Europe with a tendency of decreased 30-year return levels and increased precipitation intensity. In summer, on the other hand, the frequency of wet days is decreased over most of Europe except for northern Scandinavia and the Baltic Sea region. In contrast, the precipitation intensity and the 30-year return level of daily precipitation are increased over entire Scandinavia, central and eastern Europe. The changes in the 30-year return level of daily precipitation are generally stronger than the corresponding changes in the precipitation intensity but can have opposite signs in some regions. Also the distribution of wet days is changed in the future. During summer, for instance, both the frequency and the length of dry spells are substantially increased over most of the European continent except for the Iberian Peninsula. The frequency and the length of wet spells, on the other hand, are generally reduced during summer and increased during winter, again, with the exception of the Iberian Peninsula. The future changes in the frequency of wet days in winter are related to a change in the large-scale flow over the North Atlantic and a corresponding shift of the North Atlantic storm track. The reduction in the frequency of wet days in summer is related to a northward extension of the dry subtropical region in the future, with a reduction of the convective activity because of the large-scale sinking motion in the downward branch of the Hadley cell. Because the atmosphere contains more moisture in the warmer future climate, the amount of precipitation associated with individual low-pressure systems or with individual convective events is increased, leading to a general increase in the intensity of individual precipitation events. Only in regions, where all the moisture evaporates from the ground already in spring, the intensity of precipitation events is reduced in summer.

A Statistical Overview of Convection During the First CPEA Campaign

Data from the Equatorial Atmosphere Radar (EAR) were analysed during the Coupling Processes in the Equatorial Atmosphere (CPEA) campaign of April-May 2004. Statistical averages of the daily perturbations of wind velocity, temperature and humidity were examined. In the lower stratosphere, horizontal wind variances of up to 1.5 m 2 s -2 were visible above the afternoon tropospheric convection. Vertical wind variances of up to 0.03 m 2 s -2 occurred at the same time. The average variances of temperature and humidity in the lower troposphere also increased in the afternoon. Estimates of average momentum flux were made. In the lower stratosphere (18.0-20.0 km), the average magnitudes were 0.7-0.9 m 2 s -2 for both |u'w'| and |u'w'|. These fluxes were observed above the convective cells and indicate the average momentum emitted by them. Residuals of the momentum flux components showed a small westward preference and very small northward directional preference for wave propagation during CPEA. Four individual convective events and three Super Cloud Clusters (SCCs) were examined in detail. Tropospheric horizontal and vertical wind variances increased by 5 to 10 times during intense convection. Directly above this convection, in the lower stratosphere, increases in variance were also recorded. Vertical wind velocity fluctuation amplitudes in the lower stratosphere increased from 0.05-0.1 ms -1 away from convection, to be 0.1-0.4 ms -1 during convection. The amplitudes decreased to their background levels as soon as the convection had passed the EAR site. In the lower troposphere, the virtual temperature perturbations increased to 1.0-2.0°C during convection, an increase from a background value of about 0.5°C. During two of the four individual events, the amplitude of the temperature perturbations stayed enhanced for several hours after the end of convection. This contrasts with the vertical velocities in the lower troposphere, which quickly decreased following the passage of the storms. Specific humidity profiles in most cases showed large increases in the amount of water vapour below 4 km during times of convection. At certain heights and times during intense convection, the specific humidity increased by up to 50% from its background level.

Downslope convection north of Elephant Island, Antarctica: Influence on deep waters and dependence on ENSO

We present a long time series (1992–2001) of bottom temperature from 1040m on the continental slope north of Elephant Island, Antarctic Peninsula. A strong annual signal is apparent, with minimum temperatures during or following the austral winter, and a range similar to that found in much shallower waters on the adjacent shelf. Individual convective events are observable as large‐amplitude cold spikes that are confined to the period of seasonally coldest temperatures. These plumes are dense enough to seasonally displace the Circumpolar Deep Water, and directly inject cold water into the deep open ocean. The coldest water occurred when anomalous sea ice conditions were prevalent, caused by the strong 1997/98 ENSO event. We thus expect repeats of such strong convective events on ENSO timescales. The observed long‐term warming and reduction in sea ice along the Peninsula may, however, act to suppress this mode of ventilating the deep Southern Ocean.

雲解像モデル、SSM/I、観測船レーダのデータを用いて求めたTOGA COARE対流システムの潜熱放出の鉛直分布とそのリトリーバル

Latent heating profiles associated with three (TOGA COARE) Tropical Ocean and Global Atmosphere Coupled Ocean Atmosphere Response Experiment active convective episodes (December 10-17 1992; December 19-27 1992; and February 9-13 1993) are examined using the Goddard Cumulus Ensemble (GCE) Model and retrieved by using the Goddard Convective and Stratiform Heating (CSH) algorithm . The following sources of rainfall information are input into the CSH algorithm: Special Sensor Microwave Imager (SSM/1), Radar and the GCE model. Diagnostically determined latent heating profiles calculated using 6 hourly soundings are used for validation. The GCE model simulated rainfall and latent heating profiles are in excellent agreement with those estimated by soundings. In addition, the typical convective and stratiform heating structures (or shapes) are well captured by the GCE model. Radar measured rainfall is smaller than that both estimated by the GCE model and SSM/I in all three different COARE IFA periods. SSM/I derived rainfall is more than the GCE model simulated for the December 19-27 and February 9-13 periods, but is in excellent agreement with the GCE model for the December 10-17 period. The GCE model estimated stratiform amount is about 50% for December 19-27, 42% for December 11-17 and 56% for the February 9-13 case. These results are consistent with large-scale analyses. The accurate estimates of stratiform amount is needed for good latent heating retrieval. A higher (lower) percentage of stratiform rain can imply a maximum heating rate at a higher (lower) altitude. The GCE model always simulates more stratiform rain (10 to 20%) than the radar for all three convective episodes. SSM/I derived stratiform amount is about 37% for December 19-27, 48% for December 11-17 and 41% for the February 9-13 case. Temporal variability of CSH algorithm retrieved latent heating profiles using either GCE model simulated or radar estimated rainfall and stratiform amount is in good agreement with that diagnostically determined for all three periods. However, less rainfall and a smaller stratiform percentage estimated by radar resulted in a weaker (underestimated) latent heating profile and a lower maximum latent heating level compared to those determined diagnostically. Rainfall information from SSM/I can not retrieve individual convective events due to poor temporal sampling. Nevertheless, this study suggests that a good 4r, rainfall retrieval from SSM/I for a convective event always leads to a good latent heating retrieval. Sensitivity testing has been performed and the results indicate that the SSM/I derived time averaged stratiform amount may be underestimated for December 19-27. Time averaged heating profiles derived from SSM/I, however, are not in bad agreement with those derived by soundings for the December 10-17 convective period. The heating retrievals may be more accurate for longer time scales provided there is no bias in the sampling.

Convection-Evaporation Feedback in the Equatorial pacific

The coupling between convection and surface evaporation is investigated to assess the importance of evaporative cooling in regulating the tropical sea surface temperature. It is found that such a coupling is scale dependent. On timescales of several days, convective activity enhances surface evaporation, which together with the decrease of surface solar radiation, acts to cool the sea surface. However, on scales of climatic interest, convection acts to reduce surface evaporation. High sea surface temperature gives rise to more convective activity, which through interaction with the large-scale circulation, increases the low-level large-scale convergence and decreases the surface wind, leading to low evaporation in spite of the increased surface-air humidity difference. Therefore, although individual convective events can significantly enhance surface evaporation on short timescales, the long-term average effect of convection is to suppress surface evaporation at high SST due to its interaction with the large-scale circulation. One potential implication of this result is that evaporative cooling on climate timescales may not provide a negative feedback on the sea surface temperature of warm oceans with convectively disturbed tropospheres.