Enhanced atmospheric motion vectors (AMVs) produced from the geostationary Multifunctional Transport Satellite (MTSAT) are assimilated into the U.S. Navy Operational Global Atmospheric Prediction ...System (NOGAPS) to evaluate the impact of these observations on tropical cyclone track forecasts during the simultaneous western North Pacific Ocean Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (TPARC) and the Tropical Cyclone Structure—2008 (TCS-08) field experiments. Four-dimensional data assimilation is employed to take advantage of experimental high-resolution (space and time) AMVs produced for the field campaigns by the Cooperative Institute for Meteorological Satellite Studies. Two enhanced AMV datasets are considered: 1) extended periods produced at hourly intervals over a large western North Pacific domain using routinely available MTSAT imagery and 2) limited periods over a smaller storm-centered domain produced using special MTSAT rapid-scan imagery. Most of the locally impacted forecast cases involve Typhoons Sinlaku and Hagupit, although other storms are also examined. On average, the continuous assimilation of the hourly AMVs reduces the NOGAPS tropical cyclone track forecast errors—in particular, for forecasts longer than 72 h. It is shown that the AMVs can improve the environmental flow analyses thatmay be influencing the tropical cyclone tracks. Adding rapid-scan AMV observations further reduces the NOGAPS forecast errors. In addition to their benefit in traditional data assimilation, the enhanced AMVs show promise as a potential resource for advanced objective data-targeting methods.
Abstract
The impacts of special Geostationary Operational Environmental Satellite (GOES) rapid-scan (RS) wind observations on numerical model 24–120-h track forecasts of Hurricane Katrina are ...examined in a series of data assimilation and forecast experiments. The RS wind vectors are derived from geostationary satellites by tracking cloud motions through successive 5-min images. In these experiments, RS wind observations are added over the area 15°–60°N, 60°–110°W, and they supplement the observations used in operational forecasts. The inclusion of RS wind observations reduces errors in numerical forecasts of the Katrina landfall position at 1200 UTC 29 August 2005 by an average of 12% compared to control cases that include “targeted” dropsonde observations in the Katrina environment. The largest average improvements are made to the 84- to 120-h Katrina track forecasts, rather than to the short-range track forecasts. These results suggest that RS wind observations can potentially be used in future cases to improve track forecasts of tropical cyclones.
The impacts of assimilating dropwindsonde data and enhanced atmospheric motion vectors (AMVs) on tropical cyclone track forecasts are examined using the Navy global data assimilation and forecasting ...systems. Enhanced AMVs have the largest impact on eastern Pacific storms, while dropwindsonde data have the largest impact on Atlantic storms. Results in the western Pacific are mixed. Two western Pacific storms, Nuri and Jangmi, are examined in detail. For Nuri, dropwindsonde data and enhanced AMVs are at least as likely to degrade as to improve forecasts. For Jangmi, additional data improve track forecasts in most cases. An erroneous weakening of the forecasted subtropical high appears to contribute to the track errors for Nuri and Jangmi. Assimilation of enhanced AMVs systematically increases the analyzed heights in this region, counteracting this model bias. However, the impact of enhanced AMVs decreases rapidly as the model biases saturate at similar levels for experiments with and without the enhanced AMVs after the first few forecast days. Experiments are also conducted in which the errors assigned to synthetic tropical cyclone observations are increased. Moderate increases in the assigned errors improve track forecasts on average, but larger increases in the assigned errors produce mixed results. Both experiments allow for reductions in innovations and residuals when compared to dropwindsonde observations. These experiments suggest that a reformulation of the synthetic tropical cyclone observation scheme may lead to improved forecasts as more in situ and remote observations become available.
Some real-time radiosonde reports are now available with higher vertical resolution and higher precision than the alphanumeric TEMP code. There are also extra metadata; for example, the software ...version may indicate whether humidity corrections have been applied at the station. Numerical weather prediction (NWP) centers and other users need to start using the new Binary Universal Form for Representation of Meteorological Data (BUFR) reports because the alphanumeric codes are being withdrawn. TEMP code has various restrictions and complexities introduced when telecommunication speed and costs were overriding concerns; one consequence is minor temperature rounding errors. In some ways BUFR reports are simpler: the whole ascent should be contained in a single report. BUFR reports can also include the time and location of each level; an ascent takes about 2 h and the balloon can drift 100 km or more laterally. This modernization is the largest and most complex change to the worldwide reporting of radiosonde observations for many years; international implementation is taking longer than planned and is very uneven. The change brings both opportunities and challenges. The biggest challenge is that the number and quality of the data from radiosonde ascents may suffer if the assessment of the BUFR reports and two-way communication between data producers and data users are not given the priority they require. It is possible that some countries will only attempt to replicate the old reports in the new format, not taking advantage of the benefits, which include easier treatment of radiosonde drift and a better understanding of instrument and processing details, as well as higher resolution.
Commercial aircraft now provide over 130,000 meteorological observations per day, including temperature,winds, and in some cases, humidity, vertical wind gust, or eddy dissipation rate (turbulence). ...The temperature and wind data are used in most operational numerical weather prediction models at NCEP and at other centers worldwide. At nonsynoptic times, these data are often the primary source of upper-air information over the United States. Even atsynoptic times, these data are critical in depicting the atmosphere along oceanic air routes.
A Web site >http://acweb.fsl.noaa.gov/< has been developed that gives selected users access to these data. Because the data are proprietary to the airlines, realtime access is restricted to entities such as government agencies and nonprofit research institutions (although sample past data are available to all). Data can be displayed in a variety of ways and can be downloaded for local processing.
These data are described here, and examples of how they have been useful in weather forecasting and numerical weather prediction are shown.
The origin of two separate Southern High Plains dust storms, which occurred over a 2 day period in February 2007, is traced to an interaction between the subtropical jet (STJ) and the polar jet (PJ). ...A large‐scale thermal wind imbalance resulting from the confluence of these two jets led to a series of mesoscale circulations that ultimately produced the dust storms. Understanding the connectivity between the dust storms with differing geometries is central to the present investigation. The study rests on the interpretation of analyses from upper air and surface observations complemented by imagery from satellites, the 32 km gridded data set from the North American Regional Reanalysis, and a fine‐resolution (6 km grid) simulation from the Weather Research and Forecasting model. Principal assertions from the present study are (1) scale interaction is fundamental to the creation of an environment conducive to dust storm development, (2) low to middle tropospheric mass adjustment is the primary response to a large‐scale imbalance, (3) the mesoscale mass adjustment is associated with circulations about a highly accelerative jet streak resulting from the merger of the PJ and STJ, (4) the structure of the jet streak resulting from this merger governs the evolution of the geometry of the dust plumes, with plumes that initially had a straight‐line orientation developing a semicircular geometry, and (5) it is concluded that improvements in dust storm prediction will depend on an augmentation to the upper air network in concert with a flow‐dependent data assimilation strategy.
Key Points
Understanding the connectivity between the dust storms
Merger of polar and subtropical jet streams
Mesoscale response to large‐scale imbalance
Abstract
Distance-dependent weighted averaging (DDWA) is a process that is fundamental to most of the objective analysis schemes that are used in meteorology. Despite its ubiquity, aspects of its ...effects are still poorly understood. This is especially true for the most typical situation of observations that are discrete, bounded, and irregularly distributed.
To facilitate understanding of the effects of DDWA schemes, a framework that enables the determination of response functions for arbitrary weight functions and data distributions is developed. An essential element of this approach is the equivalent analysis, which is a hypothetical analysis that is produced by using, throughout the analysis domain, the same weight function and data distribution that apply at the point where the response function is desired. This artifice enables the derivation of the response function by way of the convolution theorem. Although this approach requires a bit more effort than an alternative one, the reward is additional insight into the impacts of DDWA analyses.
An important insight gained through this approach is the exact nature of the DDWA response function. For DDWA schemes the response function is the complex conjugate of the normalized Fourier transform of the effective weight function. In facilitating this result, this approach affords a better understanding of which elements (weight functions, data distributions, normalization factors, etc.) affect response functions and how they interact to do so.
Tests of the response function for continuous, bounded data and discrete, irregularly distributed data verify the validity of the response functions obtained herein. They also reinforce previous findings regarding the dependence of response functions on analysis location and the impacts of data boundaries and irregular data spacing.
Interpretation of the response function in terms of amplitude and phase modulations is illustrated using examples. Inclusion of phase shift information is important in the evaluation of DDWA schemes when they are applied to situations that may produce significant phase shifts. These situations include those where data boundaries influence the analysis value and where data are irregularly distributed. By illustrating the attendant movement, or shift, of data, phase shift information also provides an elegant interpretation of extrapolation.
Although wind and temperature observations from commercial aircraft have been shown to improve operational numerical weather prediction (NWP) on global and regional scales, the quality and potential ...importance of newly available moisture observations are less well recognized. Because moisture changes often occur at much smaller scales than wind and temperature variations, these temporally and spatially frequent moisture observations can have exceptionally large impacts on forecasts of disruptive weather events and could help offset the dwindling number of global moisture observations. Currently, more than 148 aircraft-based Water Vapor Sensing Systems (WVSS; 139 operating in the US and 9 in Europe) provide specific humidity observations en route and in 1200 profiles made daily during takeoff/landing. Results of a series of assessments comparing data from initial WVSS sensors installed on 25 United Parcel Service (UPS) Boeing 757 aircraft with collocated raobs show agreement to within 0.5 g kg−1, with minimal biases. Intercomparisons of observations made among nearby aircraft agree to better than 0.2 g kg−1. The combined results suggest that the WVSS measurements are at least as accurate as water vapor observations from high-quality raobs. Information regarding observed spatial and temporal moisture variability could be important in optimizing the use of these observations in future mesoscale assimilation systems. Forecasts of disruptive weather events made by NWS and airline forecasters demonstrate the benefits obtained from combined temperature/moisture/wind profiles acquired during aircraft ascents and descents. Finally, initial NWP impact studies show that WVSS reports that include moisture obtained throughout the day have greater influence than twice-daily raob humidity data on contiguous U.S. (CONUS) forecasts for 24 h and beyond.
Re-examination of the I-5 dust storm Kaplan, Michael L.; Vellore, Ramesh K.; Lewis, John M. ...
Journal of geophysical research. Atmospheres,
01/2013, Letnik:
118, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The infamous dust storm over the thanksgiving holiday of 1991 that led to loss of life from numerous automobile accidents on Interstate 5 (I‐5) has been re‐examined. Pauley et al. (1996) conducted an ...earlier investigation of this dust storm following the tenets of Danielsen's paradigm—a paradigm that links the tropopause fold phenomenon and a balanced thermally indirect circulation about the upper level jet stream. However, a cursory examination of mesoscale structures in the storm from the North American Regional Reanalysis (NARR) indicated evidence of a low‐level unbalanced thermally direct circulation that demanded further investigation using a high‐resolution Weather Research and Forecasting (WRF) model simulation. Principal results from the present study follow: (1) Although the model simulation showed evidence of a weak indirect circulation in the upper troposphere in support of the Danielsen's paradigm, the dynamic control of the storm stemmed from the lower tropospheric mesoscale response to geostrophic imbalance. (2) A lower tropospheric direct circulation led to mass/temperature adjustments that were confirmed by upper air observations at locations in proximity to the accident site, and (3) boundary layer deepening and destabilization due to these mesoscale processes pinpointed the timing and location of the dust storm. Although the present study does not underestimate the value of analyses that focus on the larger/synoptic scales of motion, it does bring to light the value of investigation that makes use of the mesoscale resources in order to clarify synoptic‐mesoscale interactions.
Key Points
Mesoscale direct circulation and associated mass/temperature adjustments are important dynamic controls for dust storms
On 29 November 1991 a series of collisions involving 164 vehicles occurred on Interstate 5 in the San Joaquin Valley in California in a dust storm that reduced the visibility to near zero. The ...accompanying high surface winds are hypothesized to result from intense upper-tropospheric downward motion that led to the formation of a strong upper front and tropopause fold and that transported high momentum air downward to midlevels where boundary layer processes could then mix it to the surface. The objectives of the research presented in this paper are to document the event, to provide support for the hypothesis that both upper-level and boundary layer processes were important, and to determine the structure of the mesoscale circulations in this case for future use in evaluating the navy's mesoscale data assimilation system. The strong upper-level descent present in this case is consistent with what one would expect for jet streak and frontal circulations in combination with quasigeostrophic processes. During the period examined, upper-level data and analyses portray a strong upper-tropospheric jet streak with maximum winds initially in excess of 85 m s−1 (≈170 kt) that weakened as it propagated southward around the base of a long-wave trough. The jet streak was accompanied by a strong upper front and tropopause fold, both of which imply intense downward motion. The vertical motion field near the time of the accidents had two maxima—one that was associated with a combination of quasigeostrophic forcing and terrain-induced descent in the lee of the Sierra and one that was associated with the descending branch of the secondary circulation in the jet streak exit region and the cold advection by both the geostrophic wind and the ageostrophic wind in the upper front. The 700-hPa wind speed maximum over and west of the San Joaquin Valley overlapped with the latter maximum, supporting the hypothesized role of downward momentum transport. Given the significant 700-hPa wind speeds over the San Joaquin Valley during daytime hours on the day of the collisions, boundary layer mixing associated with solar heating of the earth's surface was then able to generate high surface winds. Once the high surface winds began, a dust storm was inevitable, since winter rains had not yet started and soil conditions were drier than usual in this sixth consecutive drought year. Surface observations from a variety of sources depict blowing dust and high surface winds at numerous locations in the San Joaquin Valley, the Mojave and other desert sites, and in the Los Angeles Basin and other south coast sites. High surface winds and low visibilities began in the late morning at desert and valley sites and lasted until just after sunset, consistent with the hypothesized heating-induced mixing. The 0000 UTC soundings in California portrayed an adiabatic layer from the surface to at least 750 hPa, also supporting the existence of mixing. On the other hand, the high winds in the Los Angeles Basin began near sunset in the wake of a propagating mesoscale trough that appeared to have formed in the lee of the mountains that separate the Los Angeles Basin from the San Joaquin Valley.
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