Abstract

Relatively few studies have been carried out as to the conditions leading to the formation of monsoon depressions in the western North Pacific. Two monsoon depression formations during July 2007 were analyzed using ECMWF analyses and satellite observations. Wave-activity flux calculations indicated that cross-equatorial flow from the Southern Hemisphere played an important role in the formation of these monsoon depressions. A new conceptual model of monsoon depression formation in the western North Pacific is proposed that includes three southerly airstreams in the Southern Hemisphere that lead to cross-equatorial flows into the Northern Hemisphere. Examination of 44 monsoon depressions from April to December 2009 confirms the critical role of these cross-equatorial flows in monsoon depression formation. All of the monsoon depressions in the 2009 sample for which formation conditions could be established had at least one of three possible airstreams that interacted with a confluent region and, thus, may be a necessary condition for monsoon depression formation. This conceptual model of monsoon depression formation was further confirmed by means of wave-activity flux calculations and backward trajectory ensembles for the 2009 cases.

Abstract

Carbon monoxide is important in tropospheric chemsitry and useful in studies of cloud dynamics, yet measurements of this gas, especially from aircraft, are too few to characterize fully the atmospheric sources, sinks and distribution of CO. This article describes how a commercial infrared gas filter correlation analyzer (GFC) can be modified to provide sufficient sensitivity and response time for clean air CO measurements. Modifications include improved IR detection, a chemical zero, and sample gas preparation to eliminate interferences effectively, including a minor interference from ozone. The modified instrument demonstrates a detection limit of ∼24 ppb (signal-to-noise ratio 2:1 at the ±1σ noise level, with a 60 s time constant), a broad linear dynamic range, temperature and pressure independence, calibration stability, and a minimum response time of ∼30 s. We present examples of surface and airborne measurements including results from 12 flights over New Mexico showing rapid vertical transport of CO near convective clouds.

The Global Historical Climatology Network version 2 temperature database was released in May 1997. This century-scale dataset consists of monthly surface observations from ~7000 stations from around the world. This archive breaks considerable new ground in the field of global climate databases. The enhancements include 1) data for additional stations to improve regional-scale analyses, particularly in previously data-sparse areas; 2) the addition of maximum–minimum temperature data to provide climate information not available in mean temperature data alone; 3) detailed assessments of data quality to increase the confidence in research results; 4) rigorous and objective homogeneity adjustments to decrease the effect of nonclimatic factors on the time series; 5) detailed metadata (e.g., population, vegetation, topography) that allow more detailed analyses to be conducted; and 6) an infrastructure for updating the archive at regular intervals so that current climatic conditions can constantly be put into historical perspective. This paper describes these enhancements in detail.

Abstract

Convection-allowing model (CAM) ensemble forecasts provide quantitative probabilistic guidance of convective hazards that forecasters would otherwise qualitatively assess. Various initial condition (IC) strategies can be used to generate CAM probabilistic forecasts, but it is still unclear how different configurations perform. Schwartz et al. verified five 10-member IC CAM ensembles over one month of 0000 UTC initializations with a focus on precipitation. Here, we initialize four 42-member IC CAM ensembles every 12 h over 6 weeks and verify forecasts of precipitation, column maximum reflectivity, and hourly maximum updraft helicity. The Texas Tech University real-time EnKF ensemble and three additional IC ensemble modeling systems are verified. Holding the model configuration constant, additional ICs are generated by downscaling time-lagged Global Ensemble Forecast System (GEFS) members, applying correlated random noise to Global Forecast System (GFS) analyses, and recentering EnKF perturbations about GFS analyses. We found that ensemble ICs constructed with correlated random noise and EnKF perturbations about GFS analyses both produced higher-quality precipitation forecasts than downscaled GEFS and EnKF strategies. However, downscaled GEFS and EnKF perturbations about GFS analyses frequently initialized more skillful forecasts of reflectivity than ICs with random perturbations, suggesting that flow-dependent perturbations are important for forecasting deep convection. Even with a suboptimal EnKF configuration, our findings still echo those of Schwartz et al. We extend their work by exploring 1) verification of additional convective hazards and 2) empirical scaling of IC perturbations as a computationally inexpensive method for improving CAM ensemble forecasts.

Abstract

Seasonal and regional variations in characteristics of the Arctic low-level temperature inversion are examined using up to 12 years of twice-daily rawinsonde data from 31 inland and coastal sites of the Eurasian Arctic and a total of nearly six station years of data from three Soviet drifting stations near the North Pole. The frequency of inversions, the median inversion depth, and the temperature difference across the inversion layer increase from the Norwegian Sea eastward toward the Laptev and East Siberian seas. This effect is most pronounced in winter and autumn, and reflects proximity to oceanic influences and synoptic activity, possibly enhanced by a gradient in cloud cover. East of Novaya Zemlya during winter, inversions are found in over 95% of all soundings and tend to be surface based. For all locations, however, inversions tend to he most intense during winter due to the large deficit in surface net radiation. The strongest inversions are found over eastern Siberia, and reflect the effects of local topography. The frequency of inversions is lowest during summer, but is still >50% at all locations. Most summer inversions are elevated, and are much weaker than their winter counterparts. Data from the drifting stations reveal an inversion in every sounding from December to April. The minimum frequency of 85% occurs during August. While the median inversion depth is over 1200 m during March, it decreases to approximately 400 m during August, with median temperature differences across the inversion layer of 12.6° and 2.8°C, respectively. The median depth of the summertime mixed layer below inversions at the drifting stations ranges from 300 to 400 m. Seasonal changes in these inversion characteristics show a strong relationship with seasonal changes in cloud cover.

Abstract

A physical basis is provided for representing the large-scale operationally analyzed wind fields around western North Pacific tropical cyclones by use of empirical orthogonal functions (EOFs). The synoptic differences in the environmental flow are demonstrated for cyclones having different initial directions and translation speeds. It is also shown that the wind-based EOF coefficients may be used to differentiate between future tracks of cyclones that have a similar initial direction of motion. Thus, the small sets of EOF coefficients that are used in statistical regression techniques for track prediction by Peak et al. have physical meaning and are not statistical artifacts.

A new application of the EOF coefficients is to post-process the track predictions from the One-way influence Tropical Cyclone Model (OTCM), which is presently the best objective aid at the Joint Typhoon Warning Center (Guam). A 30% reduction in the forecast error at 72 h is achieved in the dependent sample. Thus, the synoptic influences represented by the EOF coefficients can differentiate situations in which the dynamical model (OTCM) is likely to provide poor guidance.

Abstract

The effect of the Luers–Eskridge adjustments on the homogeneity of archived radiosonde temperature observations is evaluated. Using unadjusted and adjusted radiosonde data from the Comprehensive Aerological Reference Dataset (CARDS) as well as microwave sounding unit (MSU) version-d monthly temperature anomalies, the discontinuities in differences between radiosonde and MSU temperature anomalies across times of documented changes in radiosonde are computed for the lower to midtroposphere, mid- to upper troposphere, and lower stratosphere. For this purpose, a discontinuity is defined as a statistically significant difference between means of radiosonde–MSU differences for the 30-month periods immediately prior to and following a documented change in radiosonde type. The magnitude and number of discontinuities based on unadjusted and adjusted radiosonde data are then compared. Since the Luers–Eskridge adjustments have been designed to remove radiation and lag errors from radiosonde temperature measurements, the homogeneity of the data should improve whenever these types of errors dominate.

It is found that even though stratospheric radiosonde temperatures appear to be somewhat more homogeneous after the Luers–Eskridge adjustments have been applied, transition-related discontinuities in the troposphere are frequently amplified by the adjustments. Significant discontinuities remain in the adjusted data in all three atmospheric layers. Based on the findings of this study, it appears that the Luers–Eskridge adjustments do not render upper-air temperature records sufficiently homogeneous for climate change analyses. Given that the method was designed to adjust only for radiation and lag errors in radiosonde temperature measurements, its relative ineffectiveness at producing homogeneous time series is likely to be caused by 1) an inaccurate calculation of the radiation or lag errors and/or 2) the presence of other errors in the data that contribute significantly to observed discontinuities in the time series.

Abstract

A technique is developed and tested for estimating objectively the location of a tropical cyclone from a variety of fixes. The western North Pacific climatology and persistence (WPCLPR) track forecast technique is used to generate a potential track from each fix. A tentative warning position is interpolated from a smooth curve that is fit to the future and past positions. When multiple fixes are available, weighting functions are applied to account for the expected accuracy and the timeliness of each fix. Several empirical factors are determined by sensitivity tests with a dependent sample of eight storms that includes 226 warning positions. An independent sample of 22 storms with 610 warning positions is used to demonstrate that the accuracy of the objective technique is not significantly different from the official Joint Typhoon Warning Center (JTWC) warning positions during 1981–83. The short-term track forecast accuracy with WPCLPR is essentially the same whether the JTWC or the objective warning positions are used. Thus, the objective technique provides an efficient tool for the forecaster to use in establishing the present location of the tropical cyclone.

Abstract

Studies which utilize a long-term temperature record in determining the possibility of a global warming have led to conflicting results. We suggest that a time-series evaluation of mean annual temperatures is not sufficiently robust to determine the existence of a long-term warming. We propose the utilization of an air mass-based synoptic climatological approach, as it is possible that local changes within particular air masses have been obscured by the gross scale of temperature time-series evaluations used in previous studies of this type. An automated synoptic index was constructed for the winter months in four western North American Arctic locations to determine if the frequency of occurrence of the coldest and mildest air masses has changed and if the physical character of these air masses has shown signs of modification over the past 40 years. It appears that the frequencies of the majority of the coldest air masses have tended to decrease, while those of the warmest air masses have increased. In addition, the very coldest air masses at each site have warmed between 1°C to almost 4°C over the same time interval. A technique is suggested to determine whether these changes are possibly attributable to anthropogenic influences.

Abstract

Data obtained during two aircraft observing periods (AOP) from the TCM-93 mini field experiment are used to describe the transformation between 5° and 10°N of a large depression in the western North Pacific monsoon trough into a tropical cyclone over a 36-h period. The transformation is defined to occur in three stages. Although a large mesoscale convective system (MCS) was present along the eastern periphery of the monsoon depression during the preorganization stage characterized by observations from the first AOP, the overall convective organization of the broad circulation is weak. The structure of the MCS provided a midlevel subsynoptic contribution to the vorticity of the monsoon depression and contributed to a shift in the center of the monsoon depression circulation between 800 and 600 mb toward the MCS location. However, the presence of unsaturated downdrafts associated with the MCS perturbed the low-level thermodynamic conditions and contributed to the rapid decay of the MCS. Slow intensification of the monsoon depression circulation during the preorganization stage is primarily due to favorable interactions with large-scale mean and eddy circulations at both upper and lower levels. The overall convective signature was observed in hourly satellite imagery to become more organized during a 24-h period between the two AOPs. This organization stage was characterized by the formation of a new MCS near the midlevel circulation of the decaying MCS from the preorganization stage. Satellite imagery indicates that the broad monsoon depression began to organize around the new MCS and the outer convection started to be oriented in large principle bands. During the transformation to a tropical storm during the second AOP, the outer principal bands appear to separate the inner circulation of the monsoon depression from the large-scale monsoon trough environment. Convection rapidly develops along the periphery of the inner circulation that now contains a vigorous central updraft and high values of equivalent potential temperature that extend to the middle troposphere. Although several episodes of MCS generation and decay occurred throughout the development of the monsoon depression, it is hypothesized that the subsynoptic processes in the MCS during the first AOP and the MCSs that formed immediately following the second AOP contributed to the concentration of the monsoon depression center and transformation to a tropical cyclone.