2009 Dec.: Central America: A conceptual model for the link between Central American biomass burning aerosols and severe weather over the south central United States


2009 Dec.: Central America: A conceptual model for the link between Central American biomass burning aerosols and severe weather over the south central United States


JunWang1,2,3,6, Susan C van den Heever4 and Jeffrey S Reid5


Department of Geosciences, University of Nebraska-Lincoln, USA


Code 613.2, Laboratory for Atmospheres, NASA Goddard Space Flight Center, USA


Goddard Earth Sciences and Technology Center, University of Maryland in Baltimore

County, USA


Department of Atmospheric Sciences, Colorado State University, USA


Aerosol and Radiation Modeling Section, Marine Meteorology Division,

Naval Research Laboratory-Monterey, USA



Received 6 September 2008

Accepted for publication 8 December 2008

Published 13 January 2009

Online at



Each spring, smoke particles from fires over the Yucatan Peninsula and south Mexico cross over the Gulf of

Mexico into the United States (US) under the control of moist oceanic air flow from the southwestern branch of

the subtropical (Bermuda) high. Smoke can be transported deep into the south central US, where dry lines and

warm conveyor belts are frequently formed and cause deep convection and severe weather. Lyons

et al (1998


282 77–80) and Murray et al (2000 Geophys. Res. Lett. 27 2249–52) noticed a 50% increase of

lightning along the smoke transport path over the south central US during the May 1998 Central American

smoke episode. Here we present a conceptual model of coherent microphysical and meteorological mechanisms

through which smoke may impact convective clouds and subsequently result in more severe weather over the

south central US. The conceptual model depicts a chain of processes in which smoke particles are first activated

as cloud condensation nuclei when they are entrained into the warm conveyor belt, a convective zone formed

over the south central US as a result of the encounter between the mid-latitude trough and the subtropical

Bermuda high. As the convection continues with deepening of the mid-latitude trough, the greater concentration

of water cloud condensation nuclei delays the warm rain processes, enhances the development of ice clouds, and

invigorates the updrafts, all of which contribute to the formation of severe weather such as hail and lightning.

The conceptual model is based on the reasoning of physical mechanisms revealed in previous studies (over the

tropical biomass region), and is supported here through the analysis of satellite data, ground observations,

aerosol transport model results, and idealized cloud resolving simulations of a day in May 2003 when record

tornado events occurred over the south central US. Further assessment of this conceptual model is discussed for

future investigations.


Central American smoke, aerosol and severe weather in the United States, aerosol-cloud interaction

1. Introduction

Smoke particles modulate atmospheric radiative energy and

precipitation processes directly by absorbing and scattering


303 Bessey Hall, Lincoln, NE 68588-0340, USA. http://www.geosciences.



radiation, and indirectly by theirmicrophysical effects on cloud

formation. As reviewed in the

2007 IPCC report, the indirect

effect of aerosols on climate is the largest source of uncertainty

in global climate models (GCMs), partially because different

aerosol–cloud interaction mechanisms have been proposed

with qualitative but not quantitative understanding (Andreae

1748-9326/09/015003+09$30.00 © 2009 IOP

1 Publishing Ltd Printed in the UK

Environ. Res. Lett.

4 (2009) 015003 J Wang et al

and Rosenfeld

2008). The indirect aerosol mechanisms

pertinent to smoke particles include: (1) the first indirect effect,

where the size of water cloud droplets decreases as smoke

particles enhance the number of cloud condensation nuclei

available for activation (Twomey

1974); (2) the semi-direct

(choking) effect, where absorption of solar radiation by smoke

particles increases atmospheric stability and consequently

suppresses the low-level cloud formation (Koren

et al 2004);

(3) the second indirect effect, where the smaller cloud droplets

arising from the first indirect effect of smoke particles result

in longer cloud lifetimes and larger cloud fractions (Albrecht


, Kaufman et al 2005); and (4) the invigoration effect,

where for strong convection storms the warm rain process is

delayed by the smoke through its first and second indirect

effects, which in turn allows for more cloud water to be

transported vertically, a greater release of latent heat, and

a subsequent invigoration of the updrafts, thus supporting

the development of intense thunderstorms and large hail


1999, Andreae et al 2004, Lin et al 2006).

While the aforementioned studies have improved our

understanding of smoke–cloud interaction, they (except


1974) are primarily built upon the analysis of isolated

data collections or observations over the smoke source regions

of South America, southern Africa, Indonesia, and their

downwind oceans. Extending these studies and their proposed

mechanisms of smoke indirect effects to other biomass

burning regions is essential for an improved characterization of

aerosol–cloud interaction in GCMs. This is a challenging task,

as the mechanisms proposed for the aerosol indirect effects

range from small-scale microphysical processes in clouds to

the meteorological and thermodynamic environment regulated

by mesoscale to synoptic-scale systems that vary with region

and season. It has been argued that some aerosol–cloud

interaction mechanisms may be facilitated in one region while

being suppressed in another, depending on meteorological

regimes and particle concentration (Feingold

et al 2001).

Despite there being much smoke-precipitation research in

tropical and southern hemisphere regions, considerably less

has been conducted in northern hemispheric mid-latitudes such

as the continental US. Based upon the analysis of data from

the tropical rainfall measuring mission (TRMM), Bell

et al

(2008) recently showed the urban (local) air pollution effect

on the weekly cycle of precipitation in the US. Here, we

want to point out that the US actually hosts a natural and

persistent laboratory for smoke–weather interaction, namely

the typical springtime transport of Yucatan Peninsula smoke

into Texas and the American Southeast. This feature of smoke

transport presents a number of intriguing scientific questions

which require exploration. Notably with limited observations

in 1998 in this region, Lyons

et al (1998) and Murray et al

(2000) hypothesized that there is a link between biomass

burning in Mexico and the occurrence of severe weather (hail

and lightning) over the downwind US region, and attributed

(with speculation) the cause to the microphysical effects of

smoke particles on cloud. They did not, however, consider

any meteorological or synoptic factors specific to this region

that could potentially facilitate the cloud invigoration processes

and minimize the smoke choking effect on cloud. The focus

Figure 1.

Total Ozone Mapping Spectrometer (TOMS) aerosol index

(filled colors) and wind vector (white arrows) at 700 hPa in May

averaged from 1978 to 2003. STH and ITCZ, respectively, denote the

subtropical high pressure system (e.g., Bermuda high) and

intertropical convergence zone. A larger TOMS aerosol index

generally indicates high concentration of absorbing aerosols such as

smoke particles. A similar figure but for shorter-time averages of

TOMS index and wind vector is shown in Rogers and Bowman


2001). Wind data are adopted from the National Centers for

Environmental Prediction (NCEP) and National Center for

Atmospheric Research (NCAR) reanalysis; TOMS aerosol index

data are obtained from National Aeronautics and Space

Administration (NASA) Goddard Space Flight Center (GSFC).

of this paper is to propose a conceptual model that provides a

more detailed, coherent, and physical explanation for such a

hypothesis. This conceptual model is described in section

2. In

section 3, we present the physical reasoning and basis of this

conceptual model through limited data and modeling analysis

of a 2003 case that has features similar to the 1998 event

studied by Lyons

et al (1998) andMurray et al (2000). Finally,

we summarize our analysis in section

4 and discuss the future

quantitative assessments of this conceptual model.

2. The conceptual model

Biomass burning in Central America is mainly used to

clear land for agricultural practices (Kauffman

et al 2003).

Burning coincides with the March–May northern tropical

dry season, and terminates in early June when the rainy

season begins (Reid

et al 2004, Wang et al 2006). As

detailed below (in (a)–(b)), because of its synoptic systems

and geographic layout (figure

1), the Central American smoke

region (centered over the Yucatan Peninsula) provides a unique

natural laboratory to study the hypothesis of the smoke

invigoration effect on organized cloud systems. A number of

dynamic and microphysical effects observed within this region

(and described in the following (c)–(d)) also contribute to the

proposed conceptual model.


Synoptic systems favorable for deep convection. During

spring, the major synoptic systems in the regions over and


Environ. Res. Lett.

4 (2009) 015003 J Wang et al

Figure 2.

A conceptual model that illustrates the typical synoptic regimes for the interactions over the south central US between mid-latitude

clouds and long-range transported smoke particles from the Yucatan Peninsula. Smoke particles interact with clouds in the warm conveyor

belt (WCB), delay the onset of warm rain, and consequently invigorate the updrafts, causing intensive thunderstorms and large hail over the

US. See the text in section

2 for details.

to the north of the Gulf of Mexico are the subtropical

(Bermuda) high and the mid-latitude westerly waves.

Dry lines frequently occur over the southern and central

Great Plains (covering parts of Texas, Oklahoma, Kansas,

Arkansas, etc) when the moist, warm southerly airflow

(e.g., the low-level jet) from the Gulf of Mexico meets the

dry, cold northwesterly flow from the Rocky Mountains,

causing deep convection and severe weather (figure



Unique smoke transport path into the continental US.

Smoke production is at a maximum during the springtime

dry season in the Yucatan Peninsula (Reid

et al 2004).

As is apparent in figure

1, smoke crosses the subtropical

Gulf of Mexico and can extend far northward into midlatitude

synoptic systems over the southern US. Hence,

in contrast to the normal oceanic airflow, the southerly

airflow from the Gulf of Mexico during spring brings

larger concentrations of smoke particles to the south

central regions of the US. These smoke particles not only

affect air quality but may also influence cloud processes

associated with convection often initiated by the dry lines

over Texas and the Great Plains (Wang and Christopher




Microphysical properties. Smoke particles serve as

efficient cloud condensation nuclei (CCN) (Reid

et al


). Given that severe storms naturally have strong

updrafts and high supersaturations, the Twomey effect can

become significant even for moderate aerosol loadings.

For the cases of smoke interaction discussed here, clouds

are likely to be ‘CCN saturated’. Reid

et al (1998)

found for such storms that the impacts of smoke on

cloud droplet concentration vary little between moderately

polluted particle concentrations and massively polluted

conditions. It should be noted that the hygroscopic growth

of smoke particle in the moist air originating over the

Gulf of Mexico also decreases absorption by the particles

and thus minimizes the semi-direct (choking) effect on

clouds, thereby favoring the Twomey effect on the warm

rain process.


Smoke invigoration effect on clouds. After smoke reaches

the Great Plains, its continued transport to the northeast

depends on the mid-latitude synoptic systems. Most

often, the presence of a mid-latitude ridge (centered

over the central US) tends to suppress the transport of

smoke. In contrast, the presence of a trough associated

with southward movement of a cold front facilitates

the transport, as this trough together with the flow

around the Bermuda high can act to enhance the warm

conveyor belt, thus lifting the smoke particles from the

boundary layer to the free troposphere and transporting

them further downwind to the central and eastern US


2). In convective processes initiated either by the

dry line or the warm conveyor belt, smoke particles have

been hypothesized to invigorate clouds (Rosenfeld



et al 2004, Lin et al 2006), thereby supporting the

hypothesis of the link between biomass burning in Mexico

and severe weather (hail and lightning) in the downwind

regions of the US (Lyons

et al 1998, Murray et al 2000).


Environ. Res. Lett.

4 (2009) 015003 J Wang et al

Figure 3.

(a) Climatological mean in May of tornado number distribution computed at 2 × 2 grid resolution. The tornado data are obtained

from the National Climate Data Center (NCDC) of the National Oceanic and Atmospheric Administration (NOAA). (b) Tornado number

distribution in May 2003. (c) The anomaly of tornado numbers in 3–5 May and 9–11 May 2003 relative to the climatological mean in May.

Shown only are the anomalies that are beyond one standard deviation of the climatological mean. In this figure, the climatological mean is

computed from data in 1979–2001, as defined by the NCEP North American regional analysis (

http://www.cdc.noaa.gov/). (d) Averaged mass

of smoke particles along the smoke transport path near the surface in May 2003. The smoke transport path is defined as the region where the

model simulated smoke concentration is larger than 1.0 μg m3 near the surface (Wang et al 2006). Also shown in pink lines are the averages

of 700 mbar geopotential height (in units of 10 m) during days of smoke transport. The rectangle in gray in (d) denotes the area corresponding

to panels (a)–(c). Ellipses in red in (b)–(d) highlight the region where larger numbers of tornadoes occurred in May 2003.

3. Observational and modeling support for the

conceptual model

The description in section

2 suggests that cloud processes

over the south central US in spring have a high likelihood

of being affected by the smoke particles transported from the

Yucatan Peninsula. Based upon previous studies of the smoke

effect on deep convection (Rosenfeld

1999, Andreae et al


), we hypothesize that the formation of ice or mixedphase

clouds over the south and central US should be enhanced

as a result of the smoke invigoration process, which in turn

facilitates the formation of hail and lightning (Pruppacher

and Klett

2003). Unfortunately, Central America (unlike

southern Africa and South America) has never had a dedicated

campaign for studying the climate effect of smoke aerosols.

The data available for the 1998 case presented by Lyons

et al

(1998) and Murray et al (2000) are also limited for the

study of cloud microphysical processes, although they revealed

a 50% increase of cloud-to-ground lightning (as compared

to the climatological mean) along the smoke transport path

over the south central US. Hence, we lack sufficient data

to quantitatively evaluate the conceptual model described in


Environ. Res. Lett.

4 (2009) 015003 J Wang et al


2. Rather, we qualitatively articulate this model based

upon the physical reasoning and limited case studies for 2003

which showed some similar features to the 1998 case.

3.1. Limited observations supporting the conceptual model

While the transport of smoke from Central America to the US

occurs every year, the strength of such transport varies. The

largest transport in the 1990s was in May 1998 (Peppler

et al


) and it was studied by Lyons et al (1998) andMurray et al


2000). The largest transport of Central American smoke in

the last decade (since 1998) occurred during May 2003 (Wang

et al

2006). This smoke event also coincided with anomalously

severe weather. Weather observations during the smoke events

in May 2003 were reported in ‘State of the Climate in 2003’

(Levinson and Waple

2004) as follows: ‘May 2003 had a total

of 546 tornadoes, the most reported in any month for the US,

exceeding the previous month/year record by 145 tornadoes.

Two outbreaks of severe weather, on 3–5 May and on 9–11

May, led to 25 F3–F5 tornadoes for the month’. Previous

studies mandated by the US Congress have tried to use the

weather research and forecasting (WRF) model to simulate

and understand the cause of these severe weather events in

May 2003, but the simulations were not successful (personal

communication with Dr Julian Wang at NOAA Air Resources


The null results from traditional climatological analyses

and modeling studies are certainly suggestive for several

unaccounted factors. Could smoke particles that are not

included in the WRF have played an important role in the

severe storms that caused so many tornadoes in a short

time period? Alternatively, is the correlation between

smoke transport and severe weather confounded by other

meteorological phenomena? The correlation between smoke

concentration and severe storms is robust, at least on a

regional scale (figure

3). Our analysis shows that the

distribution of numbers of tornadoes in May 2003 reaches its

maxima values in a southwest-to-northeast band centered at

92W (the ellipse in figure 3(b)), but in the climatological

mean (from 1979 to 2001) the maximum occurs in a

south–north band along

100W (figure 3(a)). While

the tornado climatology can be biased by the observational

systems including population density and road networks, this

uncertainty is difficult to quantify. Nevertheless, the regions

in which the tornado numbers in May 2003 increased by a

factor of 3 or greater than the climatological mean plus one

standard deviation (such as in the red ellipses of figure


and during the two tornado outbreaks in figure

3(c)) are found

(and can only be found) along the smoke transport path (shown

in figure 3(d)). Hence, the alignment between smoke transport

path and the distribution of tornado number anomalies in May

2003 supports the proposed conceptual model described in

figure 2.


4. Discussion and summary

A conceptual model is proposed to describe the following

three processes that link the transport of smoke from Central

America with the enhancement of severe weather events (hail,

lightning, and strong updrafts) over the south and central US.

(1) Smoke particles are transported to the southern US under

the control of moist oceanic air flow from the southwestern

branch of the subtropical Bermuda high. (2) The concentration

of background aerosols is relatively low over the Great Plains


et al 2006). The transported smoke particles thus

significantly enhance the concentrations of CCN and the

resultant Twomey effect, thereby leading to smaller-size cloud

droplets. (3) When a mid-latitude trough and the associated

cold front move over the Great Plains and form dry lines (and

the warm conveyor belt) with the airflow from Gulf of Mexico,

the smoke particles are entrained into the deep convection,

which can enhance updrafts, hail formation and lightning. We

also expect that the smoke semi-direct effect is minimized in

the process because the smoke particles are hydrated in the

transport and thus are less absorptive.

The physical reasoning of the conceptual model is

supported by the physics of the smoke invigoration effect on

cloud revealed in previous studies (notably in South America)

and by the observational and modeling analyses of the record

Figure 6.

Time–height contour plot of the ratio of the horizontally

averaged (over the whole model domain) ice mixing ratio between

smoky and non-smoky simulations for cloud development in box A

on figure

5(a). Note that the rapid variation of ratios around the cloud

edges (top, side, bottom) should be treated with caution bec

ause the

actual values of the ice mixing ratio are very small in cloud edges,

and thus their absolute differences between smoky and non-smoky

simulations are much smaller in the cloud edges than in the cloud

centers. See the text in section 3.2 for details.

tornado event in May 2003 that were conducted in this

study. One caveat in studying smoke–cloud interactions is the

covariance between the smoke transport and meteorological

factors. Overcoming this caveat with observations alone is

difficult because (a) it is nearly impossible that the exact same

meteorological regimes respectively with and without smoke

contamination will occur in the real world, and (b) the majority

of current satellite and ground observations lack the capability

to monitor the life cycle of microphysical development in a

cloud. In contrast, numerical modeling is an excellent tool for

studying the aerosol–cloud interaction because it allows for the

investigation of experiments in which the synoptic conditions

can be kept the same while the smoke concentration is varied.

But such model experiments need to be calibrated with

in situ

observations for deep convective clouds. With these caveats

in mind, this paper should be viewed as a starting point to

systematically describe the major processes likely to cause

the smoke–cloud interaction over the south central US. More

continuous observations with innovative modeling approaches

and statistical analysis are needed to quantitatively understand

each process proposed in this conceptual model, in particular

the relative roles played by the smoke microphysical effects

and meteorological factors.


This project is supported by the NASA Earth Science New

Investigator Program and Radiation Science Program as well

as the NASA GSFC Yoram Kaufman visiting fellowship under

the administration of Goddard Earth Sciences and Technology


Environ. Res. Lett.

4 (2009) 015003 J Wang et al

Center (GEST), University of Maryland in Baltimore (UMBC).

J Wang is also grateful to Dr Lorraine Remer for hosting his

visit to NASA/GSFC. Dr Reid’s participation in this work is

provided by the NASA Interdisciplinary Science Program. We

thank two anonymous reviewers for their suggestions and one

reviewer in particular for providing constructive comments that

improved this manuscript.

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