Convective Air Masses Research Articles

Updraft dynamics and microphysics: on the added value of the cumulus thermal reference frame in simulations of aerosol–deep convection interactions

Abstract. One fundamental question about atmospheric moist convection processes that remains debated is whether, or under which conditions, a relevant variability in background aerosol concentrations may have a significant dynamical impact on convective clouds and their associated precipitation. Furthermore, current climate models must parameterize both the microphysical and the cumulus convection processes, but this is usually implemented separately, whereas in nature there is a strong coupling between them. As a first step to improve our understanding of these two problems, we investigate how aerosol concentrations modify key properties of updrafts in eight large-eddy-permitting regional simulations of a case study of scattered convection over Houston, Texas, in which convection is explicitly simulated and microphysical processes are parameterized. Dynamical and liquid-phase microphysical responses are investigated using the following two different reference frames: static cloudy updraft grid cells versus tracked cumulus thermals. In both frameworks, we observe the expected microphysical responses to higher aerosol concentrations, such as higher cloud number concentrations and lower rain number concentrations. In terms of the dynamical responses, both frameworks indicate weak impacts of varying aerosol concentrations relative to the noise between simulations over the observationally derived range of aerosol variability for this case study. On the other hand, results suggest that thermals are more selective than cloudy updraft grid cells in terms of sampling the most active convective air masses. For instance, vertical velocity from thermals is significantly higher at upper levels than when sampled from cloudy updraft grid points, and several microphysical variables have higher average values in the cumulus thermal framework than in the cloudy updraft framework. In addition, the thermal analysis is seen to add rich quantitative information about the rates and covariability of microphysical processes spatially and throughout tracked thermal lifecycles, which can serve as a stronger foundation for improving subgrid-scale parameterizations.

Structure Function Analysis of Water Vapor Simulated with a Convection-Permitting Model and Comparison to Airborne Lidar Observations

Abstract The spatial scale dependence of midlatitude water vapor variability in the high-resolution limited-area model COSMO is evaluated using diagnostics of scaling behavior. Past analysis of airborne lidar measurements showed that structure function scaling exponents depend on the corresponding airmass characteristics, and that a classification of the troposphere into convective and nonconvective layers led to significantly different power-law behaviors for each of these two regimes. In particular, scaling properties in the convective air mass were characterized by rough and highly intermittent data series, whereas the nonconvective regime was dominated by smoother structures with weaker small-scale variability. This study finds similar results in a model simulation with an even more pronounced distinction between the two air masses. Quantitative scaling diagnostics agree well with measurements in the nonconvective air mass, whereas in the convective air mass the simulation shows a much higher intermittency. Sensitivity analyses were performed using the model data to assess the impact of limitations of the observational dataset, which indicate that analyses of lidar data most likely underestimated the intermittency in convective air masses due to the small samples from single flight tracks, which led to a bias when data with poor fits were rejected. Though the quantitative estimation of intermittency remains uncertain for convective air masses, the ability of the model to capture the dominant weather regime dependence of water vapor scaling properties is encouraging.

Horizontal structure function and vertical correlation analysis of mesoscale water vapor variability observed by airborne lidar

Analysis is presented of airborne lidar measurements of water vapor, covering a height range from 1.5 to 10.4 km, from three field campaigns (midlatitude summer, polar winter, and subtropical summer). The lidar instrument provides two‐dimensional cross sections of absolute humidity, with high accuracy (errors less than 5–7%) and high vertical (∼ 200 m) and horizontal (∼ 2 km) resolution. Structure functions, i.e., statistical moments up to the fifth‐order of absolute increments over a range of scales, are investigated, and power law scaling or statistical‐scale invariance was found over horizontal distances from 5 to 100 km. The scaling exponents are found to take different values, depending on whether or not the observations were taken in an air mass where convective clouds were present. The exponent of the first‐order structure function in nonconvective regions, H=0.63±0.10, is large indicating a smooth series with long‐range correlations, in contrast to the lower value H=0.35±0.11 found in convective air masses. Correspondingly, the moisture field in the convective regime was found to be more intermittent than for the nonconvective regime, i.e., water vapor structures in convectively influenced air mass show more jump discontinuities, which could be explained by the moistening and drying effects of updrafts and downdrafts in convective air mass. Within each regime (convective or nonconvective), the values appear to be universal, with no significant dependence on the season, latitude, or height where the observations were made. Furthermore, some evidence is found that vertical correlation lengths are longer in convective air masses.

Interaction of acetone, hydroxyacetone, acetaldehyde and benzaldehyde with the surface of water ice and HNO3·3H2O ice

Oxygenated volatile organic compounds (OVOCs) influence the oxidative properties of the atmosphere, and their transport from the ground may occur by scavenging by the HNO(3)-rich supercooled water droplets found in polluted convective air masses. With infrared spectroscopy, we have studied the interactions of four typical atmospheric OVOCs (acetone, hydroxyacetone, acetaldehyde and benzaldehyde) with model surfaces of water ice and of trihydrated nitric acid (NAT) ice. We show that these molecules weakly adsorb on water ice and NAT by hydrogen bonding. No chemical reaction occurs between the molecules and the NAT substrate, the OVOCs remaining intact when in contact with hydrated HNO(3) in atmospheric ice clouds.

Mechanisms initiating deep convection over complex terrain during COPS

Precipitating convection in a mountain region of moderate topography is investigated, with particular emphasis on its initiation in response to boundary-layer and mid- and upper-tropospheric forcing mechanisms. The data used in the study are from COPS (Convective and Orographically-induced Precipitation Study) that took place in southwestern Germany and eastern France in the summer of 2007. It is found that the initiation of precipitating convection can be roughly classified as being due to either: (i) surface heating and low-level flow convergence; (ii) surface heating and moisture supply overcoming convective inhibition during latent and/or potential instability; or (iii) mid-tropospheric dynamical processes due to mesoscale convergence lines and forced mean vertical motion. These phenomena have to be adequately represented in models in order to improve quantitative precipitation forecast. Selected COPS cases are analyzed and classified into these initiation categories. Although only a subset of COPS data (mainly radiosondes, surface weather stations, radar and satellite data) are used here, it is shown that convective systems are captured in considerable detail by sensor synergy. Convergence lines were observed by Doppler radar in the location where deep convection is triggered several hours later. The results suggest that in many situations, observations of the location and timingmore » of convergence lines will facilitate the nowcasting of convection. Further on, forecasting of the initiation of convection is significantly complicated if advection of potentially convective air masses over changing terrain features plays a major role. The passage of a frontal structure over the Vosges - Rhine valley - Black Forest orography was accompanied by an intermediate suppression of convection over the wide Rhine valley. Further downstream, an intensification of convection was observed over the Black Forest due to differential surface heating, a convergence line, and the flow generated by a gust front.« less

End‐of‐storm oscillation in tropical air mass thunderstorms

Characteristics of the end‐of‐storm oscillation (EOSO) in the atmospheric surface electric field observed beneath isolated air mass thunderstorms at a tropical station, Pune, are investigated. Typically, the value of the electric field at the ground during the EOSO periods has been observed to range between −3 and +4 kV m−1. Durations of the EOSO at Pune are 24% of those for the large quasi‐stationary thunderstorms occurring at Florida and 55% of the convective air mass thunderstorms at New Mexico. During the EOSO periods, frequency of lightning decreases very much but some flashes exhibit unique features of charge transportation associated with them. Observation of a new phenomenon, the inverted EOSO, is reported in a storm with inverted polarity of electrical dipole. Our observations are found to be consistent with the convective mechanism scenario of the EOSO in which the surface field meters are exposed to the upper positive charge of the cloud by the downdrafts.

Convective generation of cirrus near the tropopause

During the July 2002 CRYSTAL‐FACE field program based from Key West, Florida, aircraft measurements showed that long‐lived thin tropopause cirrus (TTC) layers were often present above thunderstorm anvils. This paper describes these clouds and explores their possible origins. Measurements showed that the horizontal dimensions of TTC layers were nearly identical to convectively formed anvils beneath, but that TTC did not appear to have originated from convective detrainment. Rather, they appeared to have formed in stably stratified air derived from high altitudes near the tropopause. TTC were separated vertically from the top of the anvil by ∼1 km, they lacked precipitation particles, and they were strongly depleted in HDO, all of which indicate in situ formation. However, compared to surrounding clear air near the tropopause, TTC were enriched in moisture and trace gases. The degree of enrichment was consistent with mixing of air near the tropopause with the same convective air mass that produced the anvil. Unlike surrounding air, TTC had embedded a monochromatic gravity wave with a wavelength of 2 km and an amplitude of several hundred meters. This evidence, supported by a photograph from CRYSTAL‐FACE, leads to the conjecture that TTC originate as stratiform pileus clouds that form near the tropopause ahead of vigorous convective uplift. We hypothesize that the pileus are penetrated by the convection, moistened through turbulent mixing, and once the convection subsides, they are sustained by radiative cooling due to the presence of the anvil layer beneath.

Analysis of mesoscale convective systems with hail precipitation

Severe thunderstorms hit the Ebro Valley (Northeastern Spain) during the summer months, and hail precipitation is frequently registered. In this area, the spatial scale of storm cells is usually of 10 to 40 km. Nevertheless, in conditions of deep convection there may be precipitation systems on spatial scales from 40 to 500 km or larger. These types are known as mesoscale convective systems (MCS). Occasionally, there are long-lived mesoscale convective systems, termed mesoscale convective complexes (MCC), that cause intense precipitation and may produce hail. There are middle latitude classification criteria used to identify MCS and MCC from enhanced IR images, which may be derived from METEOSAT. This paper presents an analysis based on a classification of METEOSAT images for hail events in the Ebro Valley. The database consists of 72 of these hail events including 28 cases of MCS and 5 of MCC. In the remaining 36 cases, the convective air masses produced hail precipitation, but did not fulfill the classification criteria to be considered MCs. In addition, the paper includes a characterization of preconvective conditions according to different sounding variables. A logistic regressive analysis has been applied and the results show the difficulties encountered in forecasting the formation of MCs on the basis of preconvective variables.

Lightning and thunderstorms, Part I: Observational data and model results

Lightning is an important source for nitrogen oxides (NO x ) in the atmosphere. To differentiate between the impact of lightning produced NO x and anthropogenic NO x emissions on the global atmospheric chemistry, we present a simulation of flash densities and the corresponding NO x emissions from the global climate chemistry model ECHAM4.L39(DLR)/CHEM (E39/C). The basis for a realistic simulation of flash densities is the realistic representation of deep convection in the model E39/C. This will be validated by observations from the International Satellite Cloud Climatology Project (ISCCP). We will discuss the difficulties that arise, when cloud parameters from these satellite measurements will be compared with the corresponding properties taken from a global model. The flash densities in E39/C are represented by a new lightning parameterization that connects the flash frequency of a thunderstorm with the convective air mass transport within the thunderstorm cell. This property is a reasonable and physically based indicator for lightning within a deep convective cloud. The number of convective events is simulated realistically by E39/C. The simulated heights of convective cloud tops are consistent with the observations. However, in midlatitudes over Europe, the simulated cloud top heights seem to be underestimated. The flash densities show a good agreement with the OTD observations, although the ratio of flash densities over land and over the oceans seems to be underestimated. In midlatitudes over Europe, the simulated flash densities are too low compared to the observations, this can be explained through the shortcomings of the convection parameterization. The simulated NO x emissions show maxima in the tropics and in the midlatitudes, being separated significantly from the aircraft emissions. The maximum values of lightning NO x emissions (in terms of ng(N) s -1 m -3 ), lying approximately 5 km below the tropopause, are higher than the corresponding emissions from aircrafts by a factor of three. Furthermore, the maximum aircraft emissions tend to be at higher altitude and farther north compared to the lightning NO x production.

Neural network classification of convective airmasses for a flood forecasting system

A multilayer perceptron neural network was trained in order to classify shallow and deep convective airmasses for trial operational use in the GANDOLF thunderstorm flood forecasting system, being developed by the U.K. Meteorological Office and the Environment Agency. The airmass classification determined whether to use a precipitation forecast model based on extrapolation techniques, or an object-oriented convective precipitation forecast model based on a conceptual model of convective cells. The network was trained on a subset of 2190 Meteosat image samples taken during 1994, each labelled by a meteorologist at the U.K. Meteorological Office with the aid of surface pressure analysis charts. The output classes represented by the network included clear land and sea, dynamic cloud, and shallow and deep convective cloud. A total of seven spectral and two temporal features were used to discriminate the classes, resulting in a network with a correct classification rate of 80.8 +/- 2.2 per cent.