B. Lynn

Possible Aerosol Effects on Lightning Activity and Structure of Hurricanes

According to observations of hurricanes located relatively close to the land, intense and persistent lightning takes place within a 250–300-km radius ring around the hurricane center, whereas the lightning activity in the eyewall takes place only during comparatively short periods usually attributed to eyewall replacement. The mechanism responsible for the formation of the maximum flash density at the tropical cyclone (TC) periphery is not well understood as yet. In this study it is hypothesized that lightning at the TC periphery arises under the influence of small continental aerosol particles (APs), which affect the microphysics and the dynamics of clouds at the TC periphery. To show that aerosols change the cloud microstructure and the dynamics to foster lightning formation, the authors use a 2D mixed-phase cloud model with spectral microphysics. It is shown that aerosols that penetrate the cloud base of maritime clouds dramatically increase the amount of supercooled water, as well as the ice contents and vertical velocities. As a result, in clouds developing in the air with high AP concentration, ice crystals, graupel, frozen drops and/or hail, and supercooled water can coexist within a single cloud zone, which allows collisions and charge separation. The simulation of possible aerosol effects on the landfalling tropical cyclone has been carried out using a 3-km-resolution Weather Research and Forecast (WRF) mesoscale model. It is shown that aerosols change the cloud microstructure in a way that permits the attribution of the observed lightning structure to the effects of continental aerosols. It is also shown that aerosols, which invigorate clouds at 250–300 km from the TC center, decrease the convection intensity in the TC center, leading to some TC weakening. The results suggest that aerosols change the intensity and the spatial distribution of precipitation in landfalling TCs and can possibly contribute to the weekly cycle of the intensity and precipitation of landfalling TCs. More detailed investigations of the TC–aerosol interaction are required.

Aerosol Effects on Intensity of Landfalling Hurricanes as Seen from Simulations with the WRF Model with Spectral Bin Microphysics

Abstract The evolution of a superhurricane (Katrina, August 2005) was simulated using the Weather Research and Forecasting Model (WRF; version 3.1) with explicit (nonparameterized) spectral bin microphysics (SBM). The new computationally efficient spectral bin microphysical scheme (FAST-SBM) implemented to the WRF calculates at each time step and in each grid point the size distributions of atmospheric aerosols, water drops, cloud ice (ice crystals and aggregates), and graupel/hail. The tropical cyclone (TC) evolution was simulated during 72 h, beginning with its bypassing the Florida coast (27 August 2005) to its landfall just east of New Orleans, Louisiana (near the end of 29 August). The WRF/SBM was used to investigate the potential impact of aerosols ingested into Katrina’s circulation during its passage through the Gulf of Mexico on Katrina’s structure and intensity. It is shown that continental aerosols invigorated convection largely at TC periphery, which led to its weakening prior to landfall. Max...

Simulation of a supercell storm in clean and dirty atmosphere using weather research and forecast model with spectral bin microphysics

[1]xa0The development of supercell storms was simulated using a 2-km-resolution weather research and forecast (WRF) model with spectral (bin) microphysics (WRF-SBM) and a recent version of the Thompson bulk-parameterization scheme. The simulations were performed in clean, semipolluted, and dirty air under two values of relative humidity, conditionally referred to as low and high humidity. Both SBM and the Thompson scheme simulated the development of supercell storm with storm splitting. Both SBM and the Thompson scheme demonstrated that an increase in relative humidity by ∼10% invigorates convection and increases precipitation by factor of 2, i.e., to much larger extent than can be achieved by variations of the aerosol concentration. At the same time the storms simulated by the schemes are quite different. The maximum updrafts in the Thompson scheme are about 65 m/s, and the left-moving storm prevails. The SBM predicts 35 m/s maximum updrafts, and the right-moving storm prevails in the SBM simulations. While the bulk scheme predicts decrease in precipitation in clean air at both low and high humidity, the SBM indicates decrease precipitation in polluted air under low humidity and increase in precipitation under high humidity. The SBM scheme shows a substantial effect of aerosols on spatial distribution of precipitation, especially in the low-humidity case. The sensitivity of the Thompson scheme to aerosols turns out to be much less than that of SBM. The difference in the results (vertical velocities, microphysical cloud structure, and precipitation) obtained by different schemes is much larger than the changes caused by variation of the aerosol concentration within each scheme. However, the average amount of precipitation in the Thompson scheme in each simulation was about twice that of the corresponding SBM simulation. The possible reasons for such difference are discussed. A scheme for classifying aerosol effects on precipitation from clouds and cloud systems is also discussed.

Predicting the potential for lightning activity in Mediterranean storms based on the Weather Research and Forecasting (WRF) model dynamic and microphysical fields

[1]xa0A new parameter is introduced: the lightning potential index (LPI), which is a measure of the potential for charge generation and separation that leads to lightning flashes in convective thunderstorms. The LPI is calculated within the charge separation region of clouds between 0°C and −20°C, where the noninductive mechanism involving collisions of ice and graupel particles in the presence of supercooled water is most effective. As shown in several case studies using the Weather Research and Forecasting (WRF) model with explicit microphysics, the LPI is highly correlated with observed lightning. It is suggested that the LPI may be a useful parameter for predicting lightning as well as a tool for improving weather forecasting of convective storms and heavy rainfall.

Effects of aerosols on the dynamics and microphysics of squall lines simulated by spectral bin and bulk parameterization schemes

[1]xa0A new spectral bin microphysical scheme (SBM) was implemented into the Weather Research and Forecasting model referred to as Fast-SBM, which uses a smaller number of size distribution functions than the original version of the scheme referred to as Exact-SBM. It was shown that both schemes produced similar dynamical and microphysical structure of a squall line simulated. An excellent agreement in the simulated precipitation amounts between the schemes was found within a range of cloud condensation nuclei concentrations from 100 to 3000 cm−3. The Fast-SBM requires about 40% of the computing power of the Exact-SBM, allowing it to be used for “real-time” simulations over limited domains. The results obtained using the SBM simulations have been compared with those using a modified version of the Thompson bulk parameterization scheme. The main extension of the bulk scheme was the implementation of the process of drop nucleation, so that drop concentration is no longer prescribed a priori but rather calculated using the prescribed aerosol concentration. This scheme is referred to as the Drop scheme. A large set of sensitivity studies have been performed, in which microphysical parameters and precipitation, droplet nucleation above cloud base, etc., have been compared with those obtained from SBM. The SBM scheme produces more realistic dynamical and microphysical structure of the squall line. The Drop scheme did relatively little to change the cloud structures simulated by the bulk scheme. Unlike the SBM simulations that show different precipitation sensitivities to aerosol concentrations in relatively dry and humid environments, the Drop scheme indicates monotonic decrease in precipitation with increasing aerosol concentrations.

Predicting Cloud-to-Ground and Intracloud Lightning in Weather Forecast Models

A new prognostic, spatially and temporally dependent variable is introduced to the Weather Research and Forecasting Model (WRF). This variable is called the potential electrical energy (Ep). It was used to predict the dynamic contribution of the grid-scale-resolved microphysical and vertical velocity fields to the production of cloud-to-ground and intracloud lightning in convection-allowing forecasts. The source of Ep is assumed to be the noninductive charge separation process involving collisions of graupel and ice particles in the presence of supercooled liquid water. The Ep dissipates when it exceeds preassigned threshold values and lightning is generated. An analysis of four case studies is presented and analyzed. On the 4-km simulation grid, a single cloud-to-ground lightning event was forecast with about equal values of probability of detection (POD) and false alarm ratio (FAR). However, when lighting was integrated onto 12-km and then 36-km grid overlays, there was a large improvement in the forecast skill, and as many as 10 cloud-to-ground lighting events were well forecast on the 36-km grid. The impact of initial conditions on forecast accuracy is briefly discussed, including an evaluation of the scheme in wintertime, when lightning activity is weaker. The dynamic algorithm forecasts are also contrasted with statistical lightning forecasts and differences are noted. The scheme is being used operationally with the Rapid Refresh (13 km) data; the skill scores in these operational runs were very good in clearly defined convective situations.

Utilization of spectral bin microphysics and bulk parameterization schemes to simulate the cloud structure and precipitation in a mesoscale rain event

[1]xa0Sea breeze convection in Florida on 27 July 1991, accompanied by squall line formation, was simulated using MM5 with various microphysical schemes, including the Hebrew University spectral (bin) microphysics (SBM) and three recently developed bulk model parameterizations. The bulk schemes are the Seifert full two-moment scheme (FTMS), the Reisner-Thompson two-moment ice scheme (TMIS), and the Thompson two-moment ice scheme. The results were evaluated using observed rainfall and radar reflectivity, including radar derived contour frequency with altitude diagrams (CFAD). The SBM simulated quite well the time evolution of average and maximum rainfall amounts. A comparison of a CFAD derived from observations and CFADs derived from model calculated radar reflectivity suggests that the SBM simulates the three-dimensional structure of squall line convection and stratiform mixed phase cloud more realistically than the bulk parameterization schemes. However, the Thompson scheme shows a qualitative improvement over the other bulk parameterization schemes in the simulation of the three-dimensional structure of the squall line as indicated by comparison of its CFAD with the observed. All of the new bulk models simulate precipitation better than the earlier bulk parameterization schemes, but each still produces too much precipitation during too short periods of time and underestimates the area covered by stratiform clouds.

A spatial shift of precipitation from the sea to the land caused by introducing submicron soluble aerosols: Numerical modeling

[1] Precipitation in the eastern Mediterranean takes place during the cold season, when sea surface temperature is higher than the land surface temperature by 5°C-10°C. This temperature difference leads to the formation of the land breeze-like circulation, which interacts with dominating westerlies and leads to an intense cloud formation over the sea ~10-20 km from the coastal line. As a result, most of the precipitation falls on the sea without reaching the land. At the same time the eastern Mediterranean region experiences a lack of freshwater. For investigating a possibility to shift the release of precipitation from sea to land, numerical simulations were performed using the Hebrew University 2-D cloud model and the 3-D Weather Research and Forecasting model, both operating with spectral bin microphysics, and the 3-D COSMO model of the German Weather Service applying a two-moment bulk parameterization for cloud physics. The respective results indicate that an increase in concentration of small aerosols leads to a delay in raindrop formation and fosters the formation of extra ice particles with low settling velocity. This ice is advected inland by the background wind. As a result, precipitation over land increases at the expense of precipitation over sea by 15%-20%. The spatial shift of precipitation from sea to land can be as large as 50-70 km depending on the wind speed of the background flow. These results suggest a new possibility to enhance precipitation in a particular region by cloud seeding with small aerosols.

Aerosol Impacts on the Structure, Intensity, and Precipitation of the Landfalling Typhoon Saomai (2006)

Typhoon Saomai (2006) was simulated using the Weather Research and Forecasting Model (WRF) with explicit spectral (bin) microphysics (SBM) to investigate the effects of aerosol from mainland China on the intensity, structure and precipitation of the landfalling storm. MAR (maritime), MIX (semi-continental) and CON (continental) experiments were conducted with different initial aerosol concentrations. Varying aerosol concentrations had little influence on the storm track but resulted in pronounced deviations in intensity and structures. The experiment with a high initial aerosol concentration showed invigorated convection at the periphery of the tropical cyclone (TC), which interfered with the reformation of the eyewall, leading to TC weakening. The minimum pressures in the CON and MIX experiments were increased by more than 30 hPa and 14.6 hPa, and their maximum wind speeds were 20 m s-1 and 13 m s-1 weaker than that in the MAR experiment, respectively. The rain rates in the MIX and CON experiments were 16.6% and 56.2% greater than that in the MAR run, with the differences mainly occurring in the outer rainbands. These results indicate that the aerosol concentration substantially affects the spatial distributions of cloud hydrometeors and rainfall. The increase of rainfall was triggered by an increase in the melting of graupel and cloud droplets collected by raindrops. Similarly, the graupel melting process also enhanced in the outer rainbands with increasing aerosol. Furthermore, a positive microphysics feedback associated with the rainwater in the outer rainbands played an important role in increasing the rain rate in more aerosol scenarios.