LASE (Lidar Atmospheric Sensing Experiment) onboard the NASA DC-8 was used to measure high resolution profiles of water vapor and aerosols, and cloud distributions in 14 flights over the eastern Atlantic region during the NAMMA (NASA African Monsoon Multidisciplinary Analyses) field experiment, which was conducted from August 15 to September 12, 2006. These measurements were made in conjunction with flights designed to study African Easterly Waves (AEW), Tropical Disturbances (TD), and Saharan Aerosol Layers (SALs) as well as flights performed in clear air and convective regions. As a consequence of their unique radiative properties and dynamics, SAL layers have a significant influence in the development of organized convection associated with TD. Interactions of the SAL with tropical air during early stages of the development of TD were observed. These LASE measurements represent the first simultaneous water vapor and aerosol lidar measurements to study the SAL and its impact on TDs and hurricanes. Seven AEWs were studied and four of these evolved into tropical storms and three did not. Three out of the four tropical storms evolved into hurricanes

Anderson, Bruce

Browell, Edward

Burton, Sharon

Butler, Carolyn

Dunion, Jason

Fenn, Marta

Ferrare, Richard

Heymsfield, Gerry

Ismail, Syed

Kooi, Susan

Krishnamurti, T. N.

Notari, Anthony

NASA Technical Reports Server

 LASE MEASUREMENTS OF WATER VAPOR, AEROSOL, AND CLOUD 
DISTRIBUTIONS IN SAHARAN AIR LAYERS AND TROPICAL DISTURBANCES  
Syed Ismail1, Richard Ferrare1, Edward Browell2,, Susan Kooi3, Anthony Notari3, Carolyn Butler2, Sharon 
Burton3, Marta Fenn3, T. N. Krishnamurti4, Jason Dunion5, Gerry Heymsfield6, and Bruce Anderson1 
1NASA Langley Research Center, Hampton VA 23681, USA;  Syed.Ismail-1@nasa.gov 
2University of New Hampshire/NASA Langley Research Center, MS 401, Hampton Virginia, 23681, USA 
3SSAI, Hampton, VA 23666, USA 
4University of Southern Florida, Tallahassee, FL 32306, USA 
5NOAA/AOML/Hurricane Research Division, Miami, FL 33149, USA 
5NASA Goddard Space Flight Center, College Park, MD, USA 
 
ABSTRACT 
LASE (Lidar Atmospheric Sensing Experiment) on-
board the NASA DC-8 was used to measure high 
resolution profiles of water vapor and aerosols, and 
cloud distributions in 14 flights over the eastern 
Atlantic region during the NAMMA (NASA African 
Monsoon Multidisciplinary Analyses) field experiment,  
which was conducted from August 15 to September 12, 
2006. These measurements were made in conjunction 
with flights designed to study African Easterly Waves 
(AEW), Tropical Disturbances (TD), and Saharan 
Aerosol Layers (SALs) as well as flights performed in 
clear air and convective regions. As a consequence of 
their unique radiative properties and dynamics, SAL 
layers have a significant influence in the development 
of organized convection associated with TD.  
Interactions of the SAL with tropical air during early 
stages of the development of TD were observed. These 
LASE measurements represent the first simultaneous 
water vapor and aerosol lidar measurements to study 
the SAL and its impact on TDs and hurricanes. Seven 
AEWs were studied and four of these evolved into 
tropical storms and three did not.  Three out of the four 
tropical storms evolved into hurricanes. 
1.   INTRODUCTION 
The objective of the NAMMA mission was to examine: 
1) the formation and evolution of tropical hurricanes in 
the eastern and central Atlantic and their impact on the 
U.S. east coast, 2) the composition and structure of the 
Saharan Air Layer (SAL), and 3) whether aerosols 
affect cloud precipitation and influence cyclone 
development.  NAMMA was conducted from the Sal 
Island, Cape Verde, Africa. One of the focus areas of 
this field experiment was to study the interaction of the 
SAL with tropical disturbances that originate near the 
west coast of Africa.   
 
The SAL originates over continental Africa and 
propagates over the Atlantic during the summer.  Dust 
storms associated with the SAL can be as large as the 
size of continental U.S. and recur every 3-5 days.    The 
SAL contains warm and dry air and is associated with 
midlevel easterly jet [1].  There is a temperature 
inversion at the base of the SAL. Higher temperature in 
these layers is maintained by absorption of solar 
radiation by suspended dust.  Dunion et al. [1] point out 
that the presence and enhancement of a trade wind 
inversion associated with SAL can limit and inhibit 
vertical motions and thus development of convection in 
weak AEW.   However, the SAL can also influence 
cloud microphysical properties and act as a source of 
atmospheric aerosols. Aged SAL mineral dust particles 
get coated by soluble sulfate and sea salt.  This coating 
permits moistening of dust-based particles and allows 
them to act as cloud condensation nuclei leading to 
cloud formation and to the release of latent heat. 
Consequently the SAL can influence precipitation 
processes and dynamical properties of the tropical 
atmosphere.  The characterization of the SAL is also 
important for developing and evaluating passive 
satellite retrieval techniques. 
 
Water vapor is a key element for the understanding of 
the processes of precipitation, evaporation, and the 
release of latent heat.  The lack of adequate and 
accurate moisture measurements with sufficient vertical 
and horizontal resolutions limits the ability of most 
numerical models to represent these processes.  
Krishnamurti et al, [2] have found that deficiencies in 
the modeling of moisture and diabatic processes are due 
in part to the lack of knowledge of the tropical humidity 
fields.  Model forecasts have been shown to be very 
sensitive to the surface layer moisture.  Krishnamurti et 
al. [3] have shown that models which incorporate an 
explicitly resolved surface layer have been able to more 
accurately compute the strong moisture flux between 
the ocean and atmosphere resulting in more accurate 
prediction of the formation of hurricanes. Earlier NASA 
CAMEX missions have shown positive impacts of 
accurate, high resolution measurements of water vapor 
obtained by LASE on forecasts of hurricane track and 
intensity [4,5]. NAMMA measurements provide an 
opportunity to study the influence of accurate high 
resolution moisture profiles in the eastern Atlantic 
https://ntrs.nasa.gov/search.jsp?R=20080023467 2019-08-30T04:36:10+00:00Z
 region associated with genesis of hurricanes. LASE 
measurements of water vapor and aerosols during 
NAMMA have provided a unique opportunity to 
characterize the SAL and the impact of the SAL on the 
development of organized convection associated with 
TDs.  In this paper we present a brief description of 
LASE followed by a characterization of the SAL 
observations during NAMMA.  An example of a 
tropical wave and its interaction with the SAL is 
presented.  Examples of water vapor measurements 
over a moist tropical region and those associated with a 
dry air intrusion are given.  Comparison of LASE 
measurements with GPS dropsondes are also presented.  
 
2.  LASE SYSTEM 
The LASE DIAL system was developed at the NASA 
Langley Research Center in 1995 [6] and has 
subsequently participated in over 12 field experiments 
while deployed onboard the NASA ER-2, P-3, and DC-
8 aircraft.  LASE was operated onboard the DC-8 
during NAMMA. The laser system consists of a 
double-pulsed Ti:sapphire laser that operates in the 
815-nm absorption band of water vapor and is pumped 
by a frequency-doubled Nd:YAG laser. During 
NAMMA, LASE operated locked to a strong water 
vapor absorption line at 817.223 nm and electronically 
tuned to other spectral positions on the side of the 
absorption line.  In this mode, LASE transmitted up to 
three (on- and off-line) wavelength pairs that together 
permitted profiling of water vapor across the entire 
troposphere. Total laser output pulse energy was about 
90 mJ in each of the on- and off-line laser pulses 
transmitted at 5 Hz.  This energy was nominally split in 
a 7:3 ratio for transmission in nadir and zenith 
orientations, respectively.  The nadir detector system 
used two silicon avalanche photo diodes and three 
digitizers to cover a signal dynamic range of 106. 
3.  SAL CHARACTERIZATION 
An example of an intense SAL in the region of eastern 
Atlantic near the coast of NW Africa is shown in 
Figure 1.  These data have been derived from 
METEOSAT IR observations on September 5, 2006.  A 
NASA DC-8 flight was dedicated to characterize the 
SAL and conduct cloud microphysics measurements. 
This flight was conducted along a line at 19˚N latitude 
from Sal Island, Cape Verde into continental Africa and 
back.  LASE measurements of aerosol scattering ratio 
and water vapor distributions over the ocean and 
continental Africa are shown in Figure 2.  Aerosol 
scattering ratios were derived from the off-line 
wavelength that was positioned at about -35 pm away 
from the strongly absorbing H2O line at 817.223 nm.  
Corrections for absorption by water vapor, air, and 
aerosol extinction) were applied to retrieve the aerosol 
scattering profiles [7].  The layer extended from near 
the surface to about 6 km in altitude. The most intense 
region of scattering was located at higher altitudes over 
the ocean compared that over the land.   
 
Figure 1.  Satellite imagery of the distribution of SAL 
over the eastern Atlantic observed on September 5, 
2006.  
Aerosol scattering values ranged up to 20 and intense 
scattering associated with localized cloud formations 
were observed from regions near 4.5 km as seen in 
Figure 2a. Higher levels of moisture were observed in 
the layer over the oceanic region (Figure 2b) compared 
to that over the land indicating interactions with tropical 
moist air. The localized cloud regions showed nearly 
100% relative humidity (RH) values; these RH values 
were computed using LASE water vapor profiles and 
GPS dropsonde temperature profiles.  This SAL was  
 Figure 2. Aerosol scattering ratio over SAL (a) top, and  
Water vapor mixing ratio measurements (b) bottom from 
LASE. 
 
 sampled by direct in situ sampling instruments onboard 
the DC-8 during the return portion of the flight.  LASE 
observations were used to guide the in situ sampling.  
In situ sampling confirmed regions of high RH as 
obtained from LASE data in regions of localized clouds 
embedded in SAL. 
3.1 Aerosol extinction and optical thickness 
Aerosol extinction profiles and optical thickness values 
(at 817 nm) were derived from the LASE measurements 
during NAMMA.  An example of the retrieval aerosol 
extinction profile in SAL is shown in Figure 3a. In 
order to derive these profiles, the ratio of aerosol 
extinction/backscatter (“lidar ratio”) was derived 
directly from the LASE data. LASE measurements both 
above and below an elevated dust layer were used to 
derive an estimate of transmission through the layer 
from which a value of the lidar ratio = 36 sr was 
obtained [7,8]. Also shown in Figure 3a are the 
extinction profiles at three wavelengths derived from 
the DC-8 in situ nephelometer measurements of aerosol 
scattering and PSAP measurements of absorption when 
the DC-8 descended through the dust layer.  The LASE 
and in situ retrievals of aerosol extinction for the 
elevated dust layer are in excellent agreement.  Aerosol 
optical thickness profiles derived from the LASE 
aerosol extinction data for this flight are shown in 
Figure 3b. These values represent the aerosol optical 
thickness above each altitude.  The SAL optical 
thickness ranged from 0.05 to 0.5.  
 
 
Figure 3.  Aerosol extinction profiles from LASE compared 
with in situ measurements (a) top, and aerosol optical 
thickness of SAL (b) bottom. 
4.  SAL INTERACTION WITH AEW 
Seven tropical waves were studied during NAMMA. 
Three of these systems developed into hurricanes 
(Ernesto, Debby, and Helene), one into tropical 
disturbance (Gordon), and three waves did not develop. 
SALs appear to act as a deterrent to the development of 
storms. An example of LASE data associated with a 
weak wave that interacted with SAL on August 26, 
2006 is given in Figure 4.  On this case, the SAL was 
located below 4 km altitude over most of the DC-8 
flight region.  High levels of moisture (>6 g/kg up to 6 
km altitude) and convection were observed through the 
SAL and the DC-8 flight sampled cirrus clouds 
associated with this convection.  An intrusion of low 
humidity air into the convective region of the tropical 
wave was associated with a ridge located to the 
southwest of the wave as seen in Figure 4.  Interaction 
with the SAL and the dry layer air appear to have 
dissipated this wave.  Two other waves observed during 
September 3-4, and 8-9, 2006 that interacted with 
strong SALs did not develop into storms. On the other 
hand two strong waves with well organized convection 
continued to develop while interacting with SALs and 
later developed into Hurricanes Debby and Helene.   
Although Debby did indeed form but other observations 
indicate that it appeared to be inhibited by the SAL.  
SALs were present in all quadrants of these two 
developing storms and various degrees of moistening 
within the dust layers was observed.   
 
Figure 4. Moisture distribution from LASE in and around a 
tropical wave observed on august 26, 2006. 
5.   COMPARISON WITH DROPSONDES 
Seventeen GPS dropsondes were deployed during the 
flight on August 26 2006 (Figure 4).  While dropsondes 
provide ancillary temperature and wind measurements, 
LASE provided continuous curtain profiles of moisture 
and aerosol distributions.  Two example comparisons of 
LASE water vapor measurements with dropsondes are 
given in Figures 5a,b that were made at points A and B 
identified in Figure 4.  High moisture and cloudy 
 regions were associated with measurements at A 
(Figure 5a), and dry regions at B (Figure 5b).  Figure 5a 
shows good agreement between LASE and dropsondes. 
However, LASE data are noisy due to extinction by 
cirrus clouds underneath the DC-8. Figure 5b shows 
excellent agreement over the entire altitude range 
including the dry regions. While LASE and dropsondes 
measurements agreed well in general, it was found that 
water vapor profiles measured by  dropsondes 
manufactured before 2001 were dryer than LASE water 
vapor profiles by about 10%; water vapor profiles from 
dropsondes manufactured after 2004 were in better 
agreement with the LASE water vapor profiles.    
 
Figure 5.  Comparison of LASE and dropsonde measurements 
over cloudy regions (a) left, and dry regions (b) right 
observed on August 26 2006. 
6.  CURRENT ACTIVITIES 
Images of LASE measurements of water vapor and 
aerosol scattering ratio profiles from 14 NAMMA flight 
are available at NASA Langley lidar website: 
http://asd-www.larc.nasa.gov/lidar/. These LASE 
moisture measurements are being assimilated in 
numerical weather prediction models at Florida State 
University to study their impact on hurricane genesis 
and development.  Plans are also underway to use 
LASE SAL measurements along with GOES and 
METEOSAT split window and SSM/I TPW imagery 
data to study 4-D evolution of the SAL.  A synopsis of 
these activities will be included in this presentation. 
7.  SUMMARY 
LASE measurements have been used to derive water 
vapor mixing ratios and aerosol scattering ratios.  
Relative humidity profiles have been derived for 
selected data sets, using temperature profiles from 
dropsondes, to study transport of air masses and the 
humidification of dust aerosols. The dust layers over 
the ocean and the continent were found to range in 
altitude from near surface to 6 km. Aerosol scattering 
ratios at 817 nm ranged from 0 to >20, and their layer 
optical thickness ranged from 0 to >0.5.  Dust layers 
were generally anti-correlated with the water vapor 
distributions; however, highly attenuating (cloud) 
regions with high (~100%) RH were occasionally 
observed within the dust layers. Moistening of dust 
layers in regions of convection was observed with 
largest increases in moisture observed in the lower 
altitude regions. Convection was generally suppressed 
in the vicinity of the dust layers.  Dust layers were 
situated generally north of the TD; however, dust layers 
were also observed in and around the TD.  LASE water 
vapor, aerosol and cloud measurements during 
NAMMA provided an opportunity to compare with in 
situ aircraft, dropsonde, and satellite observations.  
Examples of LASE measurements along with their 
relationship with the AEW, at various stages of their 
development will be presented in this paper. 
 
Acknolodgements: We thank Paul McClung for his 
support during the integration of LASE onto the DC-8.  
Dr. Ramesh Kakar provided financial support for the 
LASE deployment and data analysis.  
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[1] Dunion, J.P., and C.S. Velden, 2004: The impact of 
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Bedi, 1994: On the improvement of precipitation 
forecast skill from physical initialization.  Tellus.  46A, 
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[3] Krishnamurti, T. N. and D. Oosterhof, 1989: 
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[6] Browell, E. V.,  S. Ismail, W. M. Hall et al., 1997: 
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LASE Measurements of Water Vapor, Aerosol, and Cloud Distributions in Saharan Air Layers and Tropical Disturbances

LASE Measurements of Water Vapor, Aerosol, and Cloud Distributions in Saharan Air Layers and Tropical Disturbances

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