Abstract

A near-real-time integrated temperature and water vapor sounding system has been designed and in operation since June 1993. It combines hourly data from the ground-based radio acoustic sounding system (RASS), a two-channel microwave radiometer, standard surface meteorological instruments, a lidar ceilometer, and the Aerodynamic Research Incorporated Communication, Addressing and Reporting System aboard commercial airlines with space-based data from the TIROS-N Operational Vertical Sounder (TOVS). The physical retrieval algorithm provided by the International TOVS Processing Package is used for combining the ground- and space-based temperature and humidity profiles. The first-guess profiles of temperature and humidity required by the physical retrieval algorithm arc obtained by using a statistical inversion technique and the ground-based remote sensors measurements.

Statistical error estimates are presented for the hourly. near-real-time, ground-, and space-based retrieved temperature and humidity profiles based on 119 soundings collected during a two-month-long experiment conducted at Platteville, Colorado, during February and March 1994. Radiosonde data collected by the Environmental Technology Laboratory and the Winter Icing and Storms Program in Platteville and the National Weather Service in Denver, Colorado, are used for comparison. The comparison showed excellent agreement between retrieved and radiosonde soundings. Retrieved temperature profiles show better performance than the retrieved humidity profiles because of the high vertical resolution of the RASS measurements. It is suggested that adding more information from the new individual remote sensors as they develop, through the technique used here, would lead to further profiling improvements.

A new method, Multisensor Retrieval of Atmospheric Properties (MRAP), is presented for deriving vertical profiles of atmospheric parameters throughout the troposphere. MRAP integrates measurements from multiple, diverse, remote sensing, and in situ instruments, the combination of which provides better capabilities than any instrument alone. Since remote sensors can deliver measurements automatically and continuously with high time resolution, MRAP provides better coverage than traditional rawinsondes. MRAP's design is flexible, being capable of incorporating measurements from different instruments in order to take advantage of new or developing advanced sensor technology. Furthermore, new or alternative atmospheric parameters for a variety of applications may be easily added as products of MRAP.

A combination of passive radiometric, active radar, and in situ observations provide the best temperature and humidity profile measurements. Therefore, MRAP starts with a traditional, radiometer-based, physical retrieval algorithm provided by the International TOVS (TIROS-N Operational Vertical Sounder) Processing Package (ITPP) that constrains the retrieved profiles to agree with brightness temperature measurements. The first-guess profiles required by the ITPP's iterative retrieval algorithm are obtained by using a statistical inversion technique and ground-based remote sensing measurements. Because the individual ground-based remote sensing measurements are usually of sufficiently high quality, the first-guess profiles by themselves provide a satisfactory solution to establish the atmospheric water vapor and temperature state, and the TOVS data are included to provide profiles with better accuracy at higher levels, MRAP provides a physically consistent mechanism for combining the ground- and space-based humidity and temperature profiles.

Data that have been used successfully to retrieve humidity and temperature profiles with MRAP are the following: temperature profiles in the lower troposphere from the ground-based Radio Acoustic Sounding System (RASS); total water vapor measurements from the Global Positioning System; specific humidity gradient profiles from the wind-profiling radar/RASS system; surface meteorological observations from standard instruments; cloud-base heights from a lidar ceilometer; temperature from the Aeronautical Radio, Incorporated Communication, Addressing and Reporting System aboard commercial airlines; and brightness temperature observations from TOVS.

Data from the experiment conducted in the late summer of 1995 at Point Loma, California, were used for comparisons of MRAP results and 20 nearby rawinsonde releases to assess the statistical error estimates of MRAP. The temperature profiles had a bias of −0.27°C and a standard deviation of 1.56°C for the entire troposphere. Dewpoint profile retrievals did not have an overall accuracy as high as that of the temperature profiles but they exhibited a markedly improved standard deviation and bias in the lower atmosphere when the wind profiler/RASS specific humidity gradient information was available as a further constraint on the process. The European Centre for Medium-Range Weather Forecasts (ECMWF) model profiles of humidity and temperature for the grid point nearest to the Point Loma site were also used for comparison with the rawinsonde soundings to establish the usefulness of MRAP profiles to the weather forecasting community. The comparison showed that the vertical resolution of the ECMWF model profiles within the planetary boundary layer is not capable of detecting sharp gradients.

Abstract

We calculated integral scales for horizontal and vertical velocity components, temperature, humidity and ozone concentration, as well as for their variances and covariances from aircraft measurements in the convective atmospheric boundary layer over both ocean and land surfaces. We found that the integral scales of the second-order moment quantities are 0.67± 0.09 that of the variables themselves. Consequently, only the second-order moment integral scales are presented here. These results are used to calculate the averaging lengths necessary to measure second-order moment quantities to a given accuracy. We found that a measurement length of 10 to 100 times the boundary-layer height is required to measure variances to 10% accuracy, while scalar fluxes require a measurement length of 10² to 10⁴ and stress a measurement length of 10³ to 10⁵ times the boundary layer height. We also show that the ratio of the wavelength of the spectral peak to the integral scale can be used to estimate the sharpness of the spectral peak.

Abstract

A new method for deriving profiles of tropospheric water vapor and liquid water from a combination of ground-based remote sensors was applied and tested under winter conditions in Colorado. The method is an extension of physical retrieval techniques used to derive coarse profiles from passive microwave radiometer measurements. Unlike an earlier method, it does not depend on climatological data for first-guess profile inputs. Instead, information about current cloud conditions aloft, obtained with active remote sensors, is used to determine physically realistic, first-guess vertical distributions of the radiometer's integrated vapor and liquid measurements. In preliminary tests, the retrieved profiles were compared with in situ measurements by aircraft and radiosondes during the Winter Icing and Storms Project. The shape of the retrieved liquid profiles agreed well with the aircraft measurements, but heights, thicknesses, and amplitudes differed considerably in some cases. The derived vapor profiles agreed better with radiosonde measurements than the traditional climatological retrievals, but standard deviations of the dewpoint differences wore still quite large (5°C). In an integrated, unattended instrument design, the new method has the potential to provide continuous real-lime profiles of temperature, wind, humidity, liquid water, and pressure.

Abstract

Even slight terrain inhomogeneities can cause large horizontal variations in the clear, stably stratified, nocturnal boundary layer largely through cold air drainage. By early morning the valleys and depressions can be several degrees cooler than the adjacent slopes and plateaus. As surface heating begins in the morning, these horizontal variations can lead to abrupt changes in temperature and wind speed at valley observation sites, as the boundary layer warms and becomes unstably stratified. Temperature and wind speed changes of 12 K and 6 m s⁻¹ respectively, within a 30 min period are observed even in valleys as shallow as 50 m with slopes of only 0.007. These changes are too large to be accounted for by vertical convergence of turbulent beat flux. Rather, it appears that a well-mixed boundary layer is advected into the valley from the upstream slopes or plateaus. Data from the National Hail Research Experiment (NHRE) 1976 surface mesonet are used to show that, statistically, this abrupt change is a frequent occurrence, throughout the summer, even in broad shallow valleys, but almost never occurs on plateau observation sites.

A case study from the Haswell, Colorado, experiment of 1975 shows in detail, through a variety of observations, the sequence of events that occurs during this rapid morning transition. As surface heating begins, the valley air, which is about 4 K colder than the air over the upstream slope and plateau, becomes less stably stratified and increasingly turbulent. Eventually, the shear stress at the top of the boundary layer becomes large enough to pull the cold air out of the valley. The valley air is then replaced by warmer upstream air that is already well mixed. The criteria necessary for this transition to occur are evaluated and generalized for application to other situations. These criteria are then applied to several previous observational studies of the dissipation of cold air pools formed in valleys through nighttime radiational cooling.

The observed transition in temperature typically precedes the velocity transition by 20–40 min. This lag appears to be due to both the adverse pressure gradient developed during the temperature transition, and the difference in the shear and temperature gradient production terms in the equations for shear stress and heat flux.

Abstract

A method is presented to obtain a high-vertical-resolution humidity profile if the location and strength of only a few significant segments of the humidity gradient profile are known. The method is based on a previously developed statistical inversion technique coupled with moisture gradient information derived from wind profiler and the radiosonde temperature measurements. An existing retrieval algorithm uses an independent historical radiosonde-derived dataset and data from a two-channel microwave radiometer, standard surface meteorological instruments, and a lidar ceilometer. In this study, the possibility of constraining the statistical retrieval using measurements of significant moisture gradients derived from wind profiler signals and radiosonde temperature observations is investigated. An example is given to illustrate the method: on 26 May 1994 the 449-MHz wind profiler/RASS at Erie, Colorado, detected a strong humidity gradient at 4.9 km MSL. A statistical inversion algorithm constrained to the radar-measured gradient at 4.9 km was used to estimate the moisture profile. Results from this example show that an improvement in retrieved humidity profiles in particular, in the strength and location of a shallow layer, can be obtained if only significant radar-sensed humidity gradient information is added to other ground-based remote sensing measurements.

Abstract

This study of a 5-yr continuous record of midtropospheric horizontal wind components from a radar wind profiler operating at Fleming, Colorado, shows a broad spectral peak centered around a period of 1 week and a minimum at about 4 months, in addition to the expected 1-yr peak. However, when the records are separated according to seasons, the pattern becomes more complicated, with several distinct peaks and clear differences between the summer and winter behavior emerging. In this paper the different spectral patterns observed are presented and the synoptic-scale features in the weather that could produce them are speculated on.

Abstract

Methods by which attitude ranges of supercooled cloud liquid water in the atmosphere may be estimated are explored using measurements from a combination of ground-based remote sensors. The tests were conducted as part of the Winter Icing and Storms Project that took place in eastern Colorado during the winters of 1990, 1991, and 1993. The basic method augments microwave radiometer measurements of path-integrated liquid water with observations from additional remote sensors to establish height limits for the supercooled liquid. One variation uses a simple adiabatic parcel lifting model initiated at a cloud-base height determined from a ecilometer, temperature and pressure from a radio acoustic sounding system or rawinsonde, and combines these with the radiometers total liquid measurement to obtain an estimate of the liquid cloud-top height. Since it does not account for liquid loss by entrainment or ice-liquid interaction processes this method tends to underestimate the true liquid cloud top; for two cases examined in detail, 54% of icing pilot reports in the area were from above this estimated height. Some error is introduced due to differences in sampling locations and from horizontal variability in liquid water content. Vertical cloud boundaries from a K_a-band radar were also used in the study; these often indicated thicker clouds than the liquid-layer depths observed from research aircraft, possibly due to the ambiguity of the ice-liquid phase distinction.

Comparisons of liquid vertical profiles are presented, using normalized profile shapes based an uniform, adiabatic, and aircraft-derived composite assumptions. The adiabatic and climatological profile shapes generally agreed well with measurements from a research aircraft and were more realistic than the uniform profile. Suggestions for applications of these results toward a red-time aviation hazard identification system are presented.

Abstract

Bragg backscatter of radar waves from elevated turbulent layers is very highly correlated with the height profile of the gradient of radio refractive index through elevated turbulent layers, as has often been documented in past research. However, many users need profiles of radio refractive index or the associated humidity rather than profiles of their gradients. Simple integration of the gradients is usually not feasible because clutter and various noise sources often severely contaminate the lower-range gates. The authors show that if the total integrated humidity is independently available [for example, from the Global Positioning System (GPS)] and if the surface value of humidity is known, the profiles of humidity are retrievable with good accuracy. This method is demonstrated with data collected in Southern California, where 7 h of 449-MHz data were recorded along with GPS data. Three radiosonde balloons were launched during that period, and the profiles of humidity from the two sources are compared. Simulations are used to assess errors that result from factors such as lack of the sign of the humidity gradients. In conclusion, a humidity profile found by statistical retrieval is compared with one found by the technique proposed in this paper.

Abstract

A radio acoustic sounding system (RASS), coupled with the NOAA/Wave Propagation Laboratory 915-MHz wind profiler, observed an arctic front and arctic air mass that passed over Denver, Colorado, between 1 and 5 February 1989. The RASS temperature measurements extended to approximately 1.5 km above ground level and were taken at 15-min intervals during the frontal passage and at 1-h intervals thereafter. During the frontal passage on 1 February, the RASS documented a temperature decrease of >15°C. The succeeding cold air (∼−20° to −40°C) over Denver never exceeded 1.3 km in depth. The frontal inversion at the top of the cold air mass was 300 m in depth and possessed large static stability [−∂θ/∂p ∼ 80 K (100 mb)⁻¹] and vertical wind shear [∂V/∂p ∼ 30 m s⁻¹ (100 mb)⁻¹]. Temporal fluctuations (∼3 h) in the depth of the cold air were observed by the RASS between the operational 12-h rawinsonde observing periods. Simultaneous RASS and rawinsonde measurements showed good agreement with regard to key thermal features.