1. Introduction  [Table of Contents]

For over twelve years the National Lightning Detection Network (NLDN) has been in operation collecting cloud-to-ground (CG) lightning data for the continental United States.  From this data set, areas of enhanced lightning flashes or ‘hot spots’ have been observed.  One such observed hot spot is near the city of Houston, Texas (Fig. 1.1 and Orville et al., 2001).  The phenomenon has been studied with the available data and hypotheses made as to the reason for the lightning hot spot.  However, more comprehensive data sets are needed in order to further examine this occurrence.  The Houston Environmental Aerosol Thunderstorm (HEAT) project will obtain the data sets necessary for further study of the hot spot near Houston.
 

1989-2000 Mean Annual Flash Density (Flashes km^-2 yr^-1)  

Figure 1.1. This map depicts the mean annual flash density (flashes/km²/year) for the years of 1989-2000. The Houston metro area is outlined in white, while the white box represents the urban enhancement region (coordinates of its lower left and upper right corners: 29.5° N, 95.7° W; 30.2° N, 94.85° W).

 
The HEAT project is proposed to be in operation during the summer of 2005.  During the summer months, the lightning enhancement is most pronounced during July and August (Fig. 1.2).  Optimal time for data acquisition is during the afternoon hours of these months.  HEAT is proposed to be a six-week campaign beginning in July that will sample both scattered and organized convection in the Houston area. The HEAT study domain will include the area enclosed by the following coordinates: 28.0° to 32.0° N, 97.0° to 93.0° W.  The urban region will be defined as the box with the following coordinates of its lower left and upper right corners: 29.5° N, 95.7° W; 30.2° N, 94.85° W (see Fig. 1.1).  Storms outside of the urban box, but within the study domain will be defined as ‘environmental,’ while storms within the urban box will be classified as ‘urban.’
 

(b)

 
Figure 1.2. These maps depict the mean monthly flash density during the months of July (a) and August (b) for the years of 1989-2003.  The Houston urban area is outlined in white.  Locations on the map include: Victoria (VCT), College Station (CLL), Intercontinental Airport of Houston (IAH), and League City (HGX), Texas, and Lake Charles (LCH), Louisiana.

1.1 Primary goals of HEAT  [Table of Contents]

The primary goals of HEAT are to examine the effects of pollution, the urban heat island, and the complex coastline on storms and lightning characteristics in the Houston area.  Also, the study of thunderstorms as efficient transporters of air pollutants and sources of nitrogen oxides (via lightning) will be a key goal.  The detailed discussions of the scientific objectives and procedures can be found later in section 3; however, a brief overview is provided here.

Pollution Effects  [Table of Contents]

In the recent study by Rosenfeld (2000), it was found that increased concentrations of small aerosols suppress precipitation in clouds.  It has been proposed that the increased concentration of aerosol, such as in polluted urban areas, results in a narrower cloud droplet spectra, deeper mixed phase region in the cloud, additional charge separation in this region, and enhanced lightning downwind of the aerosol sources.  The HEAT project will examine several aspects of this hypothesis including the effects of background aerosol concentrations on storm development (cloud droplet spectra, amount of supercooled water in the cloud) and lightning enhancement.

Throughout the HEAT project, electrical data sets will be collected using the National Lightning Detection Network (NLDN) and the lightning mapping system.  The electrical data will be compared with aerosol and microphysical data collected through various airborne and surface based methods.  Microphysical data will be collected with in-situ aircraft measurements and polarimetric radar.  Aerosol concentrations will be gathered by a combination of airborne, balloon-borne, and surface observations.  Cloud models will be used in combination with collected data to create idealized, complete data sets.

Urban Heat Island Dynamics  [Table of Contents]

For many years, a phenomenon known as the Urban Heat Island (UHI) has been observed over metropolitan areas (Huff and Changnon 1972).  The UHI has been the focus of several studies throughout the years, and it is believed to enhance convection over cities.  Several aspects of UHI dynamics and their contributions to storm and lightning intensification will be examined during the HEAT project.  Three hypotheses to be examined: 1) UHI thermodynamic effects on thunderstorm initiation/intensification and lightning intensification, 2) UHI convergence in association with initiation and/or intensification of electrically active thunderstorms in the metropolitan area, and 3) UHI enhancement of convective updraft strength in relation to intensity of lightning.  These hypotheses will be examined to determine the role of the UHI in the existence of the lightning enhancement over Houston.

During HEAT, observations of thermodynamic conditions will be made via balloon-borne soundings, and a network of surface observing stations, including Texas Natural Resource Conservation Commission (TNRCC) observation sites, HEAT surface sites, and mobile observation stations.  Low-level convergence conditions will be observed through balloon soundings, the surface observation network, and wind profilers. The convective updraft strength will be measured by dual polarimetric Doppler radar.  These observations will be made and compared to lightning data collected by the NLDN and the lightning mapping system.

The Effect of a Complex Coastline  [Table of Contents]

Due to the proximity of Houston to the coast, effects on thunderstorm development and lightning enhancement in association with the irregular shape of the coastline and the sea breeze interactions must be considered.  Irregularities in the shape of the coast lead to localized areas of enhanced convergence along the sea breeze front.  McPherson (1970) shows that a preferred location for convective initiation and enhancement due to these interactions is in the Houston area.  In order to examine this effect, the HEAT project will examine three aspects: 1) low level convergence associated with the complex coastline and the effect on convective initiation in the Houston area, 2) interaction of the sea breeze with the UHI and the effects on convective initiation and development, and 3) the effect of the sea breeze on convection intensity. 

Atmospheric Chemistry  [Table of Contents]

The convective transport of air contaminants has been recognized as integral in the study of atmospheric chemistry (Dickerson et al. 1987). Thunderstorms are efficient at transporting planetary boundary air to the upper troposphere, where the residence time of many chemical species is increased significantly. This may have vital impacts on such processes as ozone production and the oxidation capacity of the atmosphere. Nitrogen oxides are an interesting species in this respect. They are relatively abundant in urban environments like Houston, but are also significantly produced by lightning. Houston makes an interesting and unique case study because it is not only polluted, but also has a significant lightning enhancement associated with it.

Aircraft observations will be taken in different storm regions (inflow, downdraft, and anvil) of the concentration of certain atmospheric constituents (CO, CO2, O3, HC, NOx (NO, NO2) and aerosols). The flux of these species will be determined into and out of the storms. This data will also be compared to lightning data from the lightning mapping system and NLDN to determine how much NOx is produced by lightning. Cloud and chemical transport models will be employed for much of the analysis.

Lightning  [Table of Contents]

Cloud-to-ground lightning data have been analyzed for a twelve year period, 1989-2000, centered on Houston (Orville et al., 2001; Steiger et al., 2002). Our studies reveal a 58% enhancement over the urban area in the summer. Of the 1.6 million flashes in the area of study, approximately 752,000 flashes occurred in the summer months of June, July, and August and 119,000 flashes in the months of December, January, and February. The highest flash densities, greater than 4 flashes km-2 in the summer and 0.7 flashes km-2 in the winter, are near the urban areas of Houston. We hypothesize that the elevated flash densities relative to the surrounding areas may result from several factors, including, 1) the convergence due to the urban heat island effect and complex sea breeze, and 2) the increasing levels of air pollution from anthropogenic sources producing numerous small droplets and thereby suppressing mean droplet size. The latter effect would enable more cloud water to reach the mixed phase region where it is involved in the formation of precipitation and the separation of electric charge, leading to an enhancement of lightning. Our results so far are incomplete as they are only for cloud-to-ground lightning and say nothing about the variation of cloud lightning over an urban area. Houston provides an ideal location for the study of total lightning to determine the effects of a polluted urban area upon the frequency of lightning. Does the total lightning increase over Houston or is it just the fraction of cloud-to-ground lightning that increases?

We also observe a decrease (-12%) in the percentage of positive flashes over Houston (Steiger et al., 2002) suggesting that the polarity of the electrical structure in some thunderstorms over Houston may be affected by the urban atmosphere. Pollution effects are speculated to cause a change in a thunderstorm's charge distribution, which can affect the polarity of CG flashes. Consequently, electric field soundings through thunderstorms over Houston are a desired component of this experiment (Rust and MacGorman, 2002) to estimate the charge distribution in the urban storms.

We propose to make electric field soundings in storm regions to estimate the vertical electric field profile in the storm. Simultaneously, aircraft measurements of the electric field will be taken to determine the uniformity of the horizontal electric field, a common, but untested assumption in all previous measurements of the electric fields in thunderstorms.