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M. Carrier
,
X. Zou
, and
William M. Lapenta

Abstract

An effort is made to increase the number of Advanced Infrared Sounder (AIRS) cloud-uncontaminated infrared data for regional mesoscale data assimilation and short-term quantitative precipitation forecast (QPF) applications. The cloud-top pressure from Moderate Resolution Imaging Spectroradiometer (MODIS) is utilized in combination with weighting functions (WFs) to develop a channel-based cloudy-data-removal algorithm. This algorithm identifies “clear channels” for which the brightness temperature (BT) values are not cloud contaminated. A channel-dependent cutoff pressure (COP) level is first determined based on the structure of the WF of each channel. It is usually below the maximum WF level. If the cloud top (as identified by a MODIS cloud mask) is above (below) the COP level of a channel, this channel is then deemed cloudy (clear) and removed (retained). Using this algorithm, a sizable increase of cloud-uncontaminated AIRS data can be obtained. There are more usable domain points for those channels with higher COP levels. A case study is conducted. It is shown that instead of having less than 20% AIRS clear-sky observations, the algorithm finds 80% (58%) of the AIRS pixels on which there are channels whose COP levels are at or above 300 hPa (500 hPa) and the BT data in these channels at these pixels are cloud uncontaminated. Such a significant increase of the usable AIRS cloud-uncontaminated data points is especially useful for regional mesoscale data assimilation and short-term QPF applications.

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William M. Lapenta
and
Nelson L. Seaman

Abstract

On 27–28 February 1982 cyclogenesis occurred along a Carolina coastal front. Despite the relatively weak low pressure center typical of many coastal storms, this case produced widespread hazardous conditions—within 12 h up to 30 cm of snow fell in the mountains of western Virginia and moderate icing persisted throughout 27 February in the Carolinas. The event contained many mesoscale and synoptic-scale phenomena such as cold-air damming, coastal frontogenesis, upper- and lower-tropospheric jet streaks, a thermally direct vertical-transverse ageostrophic circulation, and heavy mixed precipitation.

A nested version of the PSU–NCAR three-dimensional mesoscale model with 35-km resolution successfully reproduced most principal synoptic and mesoscale feature associated with the event. This study presents a series of numerical experiments designed to examine the role of several physical processes on the evolution of and interaction between atmospheric phenomena having dithering scales, each of which contributed to the development of the storm. In particular, the physical processes studied include: 1) the role of diabatic heating associated with convective and grid-scale precipitation, 2) the role of a thermally direct transverse circulation about the entrance region of a strong polar jet streak and 3) modification of the marine planetary boundary layer by fluxes of heat and moisture over the Gulf Stream.

Of the three mechanisms investigated, the diabatic heating associated with precipitation is found to have the most significant impact on storm development. Without latent heating, cyclogenesis does not occur along the Carolina coastal front despite the presence of strong low-level baroclinicity and cyclonic vorticity. A less dramatic but still important relationship is found between storm formation and the other two physical mechanisms. The experiments indicate that the timing of storm development is delayed and the intensity weakened by reducing the strength of both the polar jet streak and fluxes over the Gulf Stream. In particular, weakening these processes disrupts the positive phase relationship between upper- and lower-tropospheric forcing in the last 12 h of the study. The three basic mechanisms are shown to affect the cyclogenesis by altering many of the important mesoscale features and processes that contribute to storm development, including the intensity of the vertical-transverse circulation around the jet streak, the location of the upward branch of the circulation, precipitation intensity, buoyancy of parcels advected over the coastal front, low-level and upper-level height falls associated with latent heating, and the southeasterly low-level jet over the coastal front.

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William M. Lapenta
and
Nelson L. Seaman

Abstract

On 27–28 February 1982 cyclogenesis occurred along a Carolina coastal front. As the relatively weak low pressure center developed and moved northeastward along the front, up to 30 cm of snow fell in 12 hours in the mountains of western Virginia and moderate icing persisted throughout 27 February in the Carolinas. On 28 February the mesoscale cyclone intensified more rapidly, turned east-northeast after passing Cape Hatteras, and gradually became a typical synoptic scale oceanic storm.

A nested version of the Penn State/NCAR mesoscale model with 35-km fine-mesh resolution is used to simulate the prestorm environment and subsequent cyclogenesis during a 36-h period. Evaluation of the numerical results indicates that the model successfully reproduced most principal synoptic and mesoscale features associated with this complex east coast cyclogenesis case, including storm path and intensification, coastal front structure, cold-air damming, circulations induced by a polar jet streak, low-level jets, and precipitation. In particular this study 1) provides an in-depth numerical examination of a case of east coast cyclogenesis in which entrance region jet streak dynamics provides the dominant upper-level support white only a weak baroclinic wave was approaching from the west, 2) reveals the existence of two moist airstreams fed by onshore flow from the marine boundary layer east of the coastal front (a southeasterly low-level jet and a rapidly rising “feeder” supporting the ascending branch of the polar jet streak's entrance circulation), 3) explores the origin, history, and significance of these moist airstreams to cyclogenesis (the southeasterly low-level jet supports inland precipitation while the rapidly ascending airstream contributes to heavy precipitation and failling pressure along the coast), and 4) demonstrates that both airstreams are well developed very early during the cyclogenesis before the midlevel baroclinic wave reaches the coast.

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Roy W. Spencer
,
William M. Lapenta
, and
Franklin R. Robertson

Abstract

Spatial fields of satellite-measured deep-layer temperatures are examined in the context of quasigeostrophic theory. It is found that midtropospheric geostrophic vorticity and quasigeostrophic vertical motions can be diagnosed from microwave temperature measurements of only two deep layers. The lower- (1000–400 hPa) and upper- (400–50 hPa) layer temperatures are estimated from limb-corrected TIROS-N Microwave Sounding Units (MSU) channel 2 and 3 data, spatial fields of which can be used to estimate the midtropospheric thermal wind and geostrophic vorticity fields. Together with Trenberth's simplification of the quasigeostrophic omega equation, these two quantities can be then used to estimate the geostrophic vorticity advection by the thermal wind, which is related to the quasigeostrophic vertical velocity in the midtroposphere.

Critical to the technique is the observation that geostrophic vorticity fields calculated from the channel 3 temperature features are very similar to those calculated from traditional, “bottom-up” integrated height fields from radiosonde data. This suggests a lack of cyclone-scale height features near the top of the channel 3 weighting function, making the channel 3 cyclone-scale “thickness” features approximately the same as height features near the bottom of the weighting function. Thus, the MSU data provide observational validation of the LID (level of insignificant dynamics) assumption of Hirshberg and Fritsch.

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Matthew J. Carrier
,
Xiaolei Zou
, and
William M. Lapenta

Abstract

An adjoint sensitivity analysis is conducted using the adjoint of the hyperspectral radiative transfer model (RTM) that simulates the radiance spectrum from the Advanced Infrared Sounder (AIRS). It is shown, both theoretically and numerically, that the height of the maximum sensitivity of radiance in a channel could be higher or lower than the height of the maximum weighting function of that channel. It is shown that the discrepancy between the two heights is determined by the vertical structures of the atmospheric thermodynamic state. The sensitivity finds the level at which changes in temperature and/or moisture will have the largest influence on the simulated brightness temperature (BT), and the maximum weighting function (WF) height indicates the level where the model atmosphere contributes most significantly to the emission at the top of the atmosphere. Based on the above findings, an adjoint method for forecast verification using AIRS radiances is presented. In this method, model forecasts are first mapped into radiance space by an RTM so that they can be compared directly with the observed radiance values. The adjoint sensitivity analysis results are then used to connect the deviations of the model forecasts from observed radiances to the changes of temperature and moisture variables in model space. This adjoint sensitivity based model verification provides useful information on forecast model performances based on indirect observations from satellites.

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Katherine M. LaCasse
,
Michael E. Splitt
,
Steven M. Lazarus
, and
William M. Lapenta

Abstract

High- and low-resolution sea surface temperature (SST) analysis products are used to initialize the Weather Research and Forecasting (WRF) Model for May 2004 for short-term forecasts over Florida and surrounding waters. Initial and boundary conditions for the simulations were provided by a combination of observations, large-scale model output, and analysis products. The impact of using a 1-km Moderate Resolution Imaging Spectroradiometer (MODIS) SST composite on subsequent evolution of the marine atmospheric boundary layer (MABL) is assessed through simulation comparisons and limited validation. Model results are presented for individual simulations, as well as for aggregates of easterly- and westerly-dominated low-level flows. The simulation comparisons show that the use of MODIS SST composites results in enhanced convergence zones, earlier and more intense horizontal convective rolls, and an increase in precipitation as well as a change in precipitation location. Validation of 10-m winds with buoys shows a slight improvement in wind speed. The most significant results of this study are that 1) vertical wind stress divergence and pressure gradient accelerations across the Florida Current region vary in importance as a function of flow direction and stability and 2) the warmer Florida Current in the MODIS product transports heat vertically and downwind of this heat source, modifying the thermal structure and the MABL wind field primarily through pressure gradient adjustments.

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