Search Results

You are looking at 1 - 6 of 6 items for

  • Author or Editor: R. M. Hoff x
  • Refine by Access: All Content x
Clear All Modify Search
R. M. Hoff

Abstract

A Mie scattering lidar was operated at Alert, NWT, Canada, for 9 weeks for the winter of 1984/85 in order to determine vertical profiles of Arctic haze. During the study period, the strong sulphate aerosol concentration maximum, representative of previous years, was not found due to a low pressure system which remained in the Baffin Bay for much of the winter. The relatively clean air is believed to be typical of southerly air flow to Alert. The vast majority of lidar profiles, which did not contain scattering from hydrometeors, showed little change in scattering structure with height. Several events of increased light scattering due to aerosols were noted and allowed examination of three possible mechanisms for the vertical movement of haze: motions following potential temperature isentropes, foehn (chinook) development and ice crystal precipitation scavenging.

Full access
D. G. Steyn
,
M. Baldi
, and
R. M. Hoff

Abstract

A new method is presented for the extraction of mixed layer depth and entrainment zone thickness from lidar, backscatter ratio profiles. The method is based on fitting a four parameter, idealized profile to observed profiles. Optimization of the fit yields values for mixed layer depth and entrainment zone thickness. Since the fitting procedure is based on the entire measured profile, it has a robustness not found in methods based on critical backscatter or backscatter gradient. The method is tested by application to four measured profiles and three synthetic profiles. The sets of profiles include some that are very demanding because of small mixed layer to upper layer backscatter ratio contrasts, or have plumes of high backscatter imbedded in mixed and upper layers. It is shown that the method is robust and simple to implement, even for a sequence of independent profiles.

Full access
W. R. Leaitch
,
R. M. Hoff
,
S. Melnichuk
, and
A. W. Hogan

Abstract

Measurements spanning much of the particle size spectrum were made on the surface aerosol arriving at Igloolik, Northwest Territories, Canada during late February 1982. Vertical profiles of aerosol particle concentration were obtained during one day of the study period. Concentrations of Aitken nuclei and cloud condensation nuclei as well as the aerosol light-scattering coefficient were measured instrumentally several times a day. Inertial impaction systems were used to separate and collect particles for microscopic sizing and chemical analysis. Suspended and precipitating ice crystals were inertially collected on microscope slides. The aerosol observations were accompanied by observations of temperature, wind speed and direction, visibility and cloud type. An upper-air station at Hall Beach, <100 km from Igloolik, provided radiosonde data.

Diurnal variations in the Aitken nucleus concentrations were observed on several clear days. The concentrations were frequently seen to follow the diurnal temperature variation, reaching a maximum near midday. Vertical profiles of Aitken nucleus concentrations obtained prior to and during one of these diurnal peaks suggests that this pattern was limited to the near-surface layer. Throughout the study, virtually all of the aerosol particle mass lay between 0.2 and 4.0 μm diameter. There was little indication of any diurnal change in the particle concentration in this size range. A clear difference in the quality of the air reaching Igloolik was detected on 23 February. Associated with this was a doubling of the particle concentration while the apparent particulate mass increased from ∼6 to ∼11 μg m−3. Impacted aerosol particles were found to be composed of 15–50% water soluble compounds before 23 February and 40-100% after this date. Sulfate was the dominant ionic species in all cases. Vertical profiles of the large aerosol particles, obtained with an airborne nephelometer, suggested a slightly enhanced concentration at the surface and a uniform concentration in the lower troposphere. Profiles of Aitken nucleus concentrations pointed to a surface source of small nuclei which diffused vertically and independently of the larger particles.

Suspended ice crystals may have accounted for a significant portion of the degradation in visibility observed throughout the study.

Full access
M. M. Millán
,
M. J. Estrela
,
M. J. Sanz
,
E. Mantilla
,
M. Martín
,
F. Pastor
,
R. Salvador
,
R. Vallejo
,
L. Alonso
,
G. Gangoiti
,
J. L. Ilardia
,
M. Navazo
,
A. Albizuri
,
B. Artíñano
,
P. Ciccioli
,
G. Kallos
,
R. A. Carvalho
,
D. Andrés
,
A. Hoff
,
J. Werhahn
,
G. Seufert
, and
B. Versino

Abstract

Mesometeorological information obtained in several research projects in southern Europe has been used to analyze perceived changes in the western Mediterranean summer storm regime. A procedure was developed to disaggregate daily precipitation data into three main components: frontal precipitation, summer storms, and Mediterranean cyclogenesis. Working hypotheses were derived on the likely processes involved. The results indicate that the precipitation regime in this Mediterranean region is very sensitive to variations in surface airmass temperature and moisture. Land-use perturbations that accumulated over historical time and greatly accelerated in the last 30 yr may have induced changes from an open, monsoon-type regime with frequent summer storms over the mountains inland to one dominated by closed vertical recirculations where feedback mechanisms favor the loss of storms over the coastal mountains and additional heating of the sea surface temperature during summer. This, in turn, favors Mediterranean cyclogenesis and torrential rains in autumn–winter. Because these intense rains and floods can occur anywhere in the basin, perturbations to the hydrological cycle in any part of the basin can propagate to the whole basin and adjacent regions. Furthermore, present levels of air pollutants can produce greenhouse heating, amplifying the perturbations and pushing the system over critical threshold levels. The questions raised are relevant for the new European Union (EU) water policies in southern Europe and for other regions dominated by monsoon-type weather systems.

Full access
M. P. McCormick
,
D. M. Winker
,
E. V. Browell
,
J. A. Coakley
,
C. S. Gardner
,
R. M. Hoff
,
G. S. Kent
,
S. H. Melfi
,
R. T. Menzies
,
C. M. R. Piatt
,
D. A. Randall
, and
J. A. Reagan

The Lidar In-Space Technology Experiment (LITE) is being developed by NASA/Langley Research Center for a series of flights on the space shuttle beginning in 1994. Employing a three-wavelength Nd:YAG laser and a 1-m-diameter telescope, the system is a test-bed for the development of technology required for future operational spaceborne lidars. The system has been designed to observe clouds, tropospheric and stratospheric aerosols, characteristics of the planetary boundary layer, and stratospheric density and temperature perturbations with much greater resolution than is available from current orbiting sensors. In addition to providing unique datasets on these phenomena, the data obtained will be useful in improving retrieval algorithms currently in use. Observations of clouds and the planetary boundary layer will aid in the development of global climate model (GCM) parameterizations. This article briefly describes the LITE program and discusses the types of scientific investigations planned for the first flight.

Full access

The CALIPSO Mission

A Global 3D View of Aerosols and Clouds

D. M. Winker
,
J. Pelon
,
J. A. Coakley Jr.
,
S. A. Ackerman
,
R. J. Charlson
,
P. R. Colarco
,
P. Flamant
,
Q. Fu
,
R. M. Hoff
,
C. Kittaka
,
T. L. Kubar
,
H. Le Treut
,
M. P. Mccormick
,
G. Mégie
,
L. Poole
,
K. Powell
,
C. Trepte
,
M. A. Vaughan
, and
B. A. Wielicki

Aerosols and clouds have important effects on Earth's climate through their effects on the radiation budget and the cycling of water between the atmosphere and Earth's surface. Limitations in our understanding of the global distribution and properties of aerosols and clouds are partly responsible for the current uncertainties in modeling the global climate system and predicting climate change. The CALIPSO satellite was developed as a joint project between NASA and the French space agency CNES to provide needed capabilities to observe aerosols and clouds from space. CALIPSO carries CALIOP, a two-wavelength, polarization-sensitive lidar, along with two passive sensors operating in the visible and thermal infrared spectral regions. CALIOP is the first lidar to provide long-term atmospheric measurements from Earth's orbit. Its profiling and polarization capabilities offer unique measurement capabilities. Launched together with the CloudSat satellite in April 2006 and now flying in formation with the A-train satellite constellation, CALIPSO is now providing information on the distribution and properties of aerosols and clouds, which is fundamental to advancing our understanding and prediction of climate. This paper provides an overview of the CALIPSO mission and instruments, the data produced, and early results.

Full access