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Baranowski, DB, Flatau PJ, Chen S, Black PG.  2014.  Upper ocean response to the passage of two sequential typhoons. Ocean Science. 10:559-570.   10.5194/os-10-559-2014   AbstractWebsite

The atmospheric wind stress forcing and the oceanic response are examined for the period between 15 September 2008 and 6 October 2008, during which two typhoons - Hagupit and Jangmi - passed through the same region of the western Pacific at Saffir-Simpson intensity categories one and three, respectively. A three-dimensional oceanic mixed layer model is compared against the remote sensing observations as well as high-repetition Argo float data. Numerical model simulations suggested that magnitude of the cooling caused by the second typhoon, Jangmi, would have been significantly larger if the ocean had not already been influenced by the first typhoon, Hagupit. It is estimated that the temperature anomaly behind Jangmi would have been about 0.4 degrees C larger in both cold wake and left side of the track. The numerical simulations suggest that the magnitude and position of Jangmi's cold wake depends on the precursor state of the ocean as well as lag between typhoons. Based on sensitivity experiments we show that temperature anomaly difference between "single typhoon" and "two typhoons" as well as magnitude of the cooling strongly depends on the distance between them. The amount of kinetic energy and coupling with inertial oscillations are important factors for determining magnitude of the temperature anomaly behind moving typhoons. This paper indicates that studies of ocean-atmosphere tropical cyclone interaction will benefit from denser, high-repetition Argo float measurements.

Markowicz, KM, Flatau PJ, Kardas AE, Remiszewska J, Stelmaszczyk K, Woeste L.  2008.  Ceilometer retrieval of the boundary layer vertical aerosol extinction structure. Journal of Atmospheric and Oceanic Technology. 25:928-944.   10.1175/2007jtecha1016.1   AbstractWebsite

The CT25K ceilometer is a general-purpose cloud height sensor employing lidar technology for detection of clouds. In this paper it is shown that it can also be used to retrieve aerosol optical properties in the boundary layer. The authors present a comparison of the CT25K instrument with the aerosol lidar system and discuss its good overall agreement for both the range-corrected signals and the retrieved extinction coefficient profiles. The CT25K aerosol profiling is mostly limited to the boundary layer, but it is capable of detecting events in the lower atmosphere such as mineral dust events between 1 and 3 km. Assumptions needed for the estimation of the aerosol extinction profiles are discussed. It is shown that, when a significant part of the aerosol layer is in the boundary layer, knowledge of the aerosol optical depth from a sun photometer allows inversion of the lidar signal. In other cases, surface observations of the aerosol optical properties are used. It is demonstrated that additional information from a nephelometer and aethalometer allows definition of the lidar ratio. Extinction retrievals based on spherical and randomly oriented spheroid assumptions are performed. It is shown, by comparison with the field measurements during the United Arab Emirates Unified Aerosol Experiment, that an assumption about specific particle shape is important for the extinction profile inversions. The authors indicate that this limitation of detection is a result of the relatively small sensitivity of this instrument in comparison to more sophisticated aerosol lidars. However, in many cases this does not play a significant role because globally only about 20% of the aerosol optical depth is above the boundary layer.

Markowicz, KM, Flatau PJ, Quinn PK, Carrico CM, Flatau MK, Vogelmann AM, Bates D, Liu M, Rood MJ.  2003.  Influence of relative humidity on aerosol radiative forcing: An ACE-Asia experiment perspective. Journal of Geophysical Research-Atmospheres. 108   10.1029/2002jd003066   AbstractWebsite

We present direct radiometric observations of aerosol radiative forcing during the ACE-Asia experiment (March and April of 2001). The observational analysis is based on radiometer data obtained from the NOAA ship Ronald H. Brown, and shipboard measurements of the aerosol chemical and scattering properties are used to construct a model of the aerosol optical properties for use in radiative transfer calculations. The model is validated against the radiometric observations and is used to diagnose the aerosol and environmental factors that contribute to the observed forcings. The mean value of aerosol optical thickness observed during the ACE-Asia cruise over the Sea of Japan was 0.43 (+/-0.25) at 500 nm, while the single-scattering albedo was 0.95 (+/-0.03) at ambient relative humidity. We find a large correlation (r(2) = 0.69) between single-scattering albedo and relative humidity. Aerosols caused a mean decrease in the diurnally averaged solar radiation of 26.1 W m(-2) at the surface, while increasing the atmospheric solar absorption and top of atmosphere reflected solar radiation by 13.4 W m(-2) and 12.7 W m(-2), respectively. The mean surface aerosol forcing efficiency (forcing per unit optical depth) over the Sea of Japan was -60 W m(-2) and is influenced by high values of relative humidity. We show that decreasing the relative humidity to 55% enhances the aerosol forcing efficiency by as much as 6-10 W m(-2). This dependency on relative humidity has implications for comparisons of aerosol forcing efficiencies between different geographical locations.

Welton, EJ, Voss KJ, Quinn PK, Flatau PJ, Markowicz K, Campbell JR, Spinhirne JD, Gordon HR, Johnson JE.  2002.  Measurements of aerosol vertical profiles and optical properties during INDOEX 1999 using micropulse lidars. Journal of Geophysical Research-Atmospheres. 107   10.1029/2000jd000038   AbstractWebsite

[1] Micropulse lidar (MPL) systems were used to measure aerosol properties during the Indian Ocean Experiment (INDOEX) 1999 field phase. Measurements were made from two platforms: the NOAA ship R/V Ronald H. Brown, and the Kaashidhoo Climate Observatory (KCO) in the Maldives. Sun photometers were used to provide aerosol optical depths (AOD) needed to calibrate the MPL. This study focuses on the height distribution and optical properties (at 523 nm) of aerosols observed during the campaign. The height of the highest aerosols (top height) was calculated and found to be below 4 km for most of the cruise. The marine boundary layer (MBL) top was calculated and found to be less than 1 km. MPL results were combined with air mass trajectories, radiosonde profiles of temperature and humidity, and aerosol concentration and optical measurements. Humidity varied from approximately 80% near the surface to 50% near the top height during the entire cruise. The average value and standard deviation of aerosol optical parameters were determined for characteristic air mass regimes. Marine aerosols in the absence of any continental influence were found to have an AOD of 0.05+/-0.03, an extinction-to-backscatter ratio (S ratio) of 33+/-6 sr, and peak extinction values around 0.05 km(-1) (near the MBL top). The marine results are shown to be in agreement with previously measured and expected values. Polluted marine areas over the Indian Ocean, influenced by continental aerosols, had AOD values in excess of 0.2, S ratios well above 40 sr, and peak extinction values approximately 0.20 km(-1) (near the MBL top). The polluted marine results are shown to be similar to previously published values for continental aerosols. Comparisons between MPL derived extinction near the ship (75 m) and extinction calculated at ship level using scattering measured by a nephelometer and absorption using a particle soot absorption photometer were conducted. The comparisons indicated that the MPL algorithm (using a constant S ratio throughout the lower troposphere) calculates extinction near the surface in agreement with the ship-level measurements only when the MBL aerosols are well mixed with aerosols above. Finally, a review of the MPL extinction profiles showed that the model of aerosol vertical extinction developed during an earlier INDOEX field campaign (at the Maldives) did not correctly describe the true vertical distribution over the greater Indian Ocean region. Using the average extinction profile and AOD obtained during marine conditions, a new model of aerosol vertical extinction was determined for marine atmospheres over the Indian Ocean. A new model of aerosol vertical extinction for polluted marine atmospheres was also developed using the average extinction profile and AOD obtained during marine conditions influenced by continental aerosols.

Collins, WD, Bucholtz A, Flatau P, Lubin D, Valero FPJ, Weaver CP, Pilewski P.  2000.  Determination of surface heating by convective cloud systems in the central equatorial Pacific from surface and satellite measurements. Journal of Geophysical Research-Atmospheres. 105:14807-14821.   10.1029/2000jd900109   AbstractWebsite

The heating of the ocean surface by longwave radiation from convective clouds has been estimated using measurements from the Central Equatorial Pacific Experiment (CEPEX). The ratio of the surface longwave cloud forcing to the cloud radiative forcing on the total atmospheric column is parameterized by the f factor. The f factor is a measure of the partitioning of the cloud radiative effect between the surface and the troposphere. Estimates of the f factor have been obtained by combining simultaneous observations from ship, aircraft, and satellite instruments. The cloud forcing near the ocean surface is determined from radiometers on board the National Oceanic and Atmospheric Administration P-3 aircraft and the R/V John Vickers. The longwave cloud forcing at the top of the atmosphere has been estimated from data obtained from the Japanese Geostationary Meteorological Satellite GMS 4. A new method for estimating longwave fluxes from satellite narrowband radiances is described. The method is based upon calibrating the satellite radiances against narrowband and broadband infrared measurements from the high-altitude NASA ER-2 aircraft. The average value of f derived from the surface and satellite observations of convective clouds is 0.15 +/- 0.02. The area-mean top-of-atmosphere longwave forcing by convective clouds in the region 10 degrees S-10 degrees N, 160 degrees E-160 degrees W is 40 W/m(2) during CEPEX. Those results indicate that the surface longwave forcing by convective clouds was approximately 5 W/m(2) in the central equatorial Pacific and that this forcing is the smallest radiative component of the surface energy budget.

Flatau, M, Flatau PJ, Phoebus P, Niller PP.  1997.  The feedback between equatorial convection and local radiative and evaporative processes: The implications for intraseasonal oscillations. Journal of the Atmospheric Sciences. 54:2373-2386.   10.1175/1520-0469(1997)054<2373:tfbeca>;2   AbstractWebsite

Existing theories of the Madden-Julian oscillation neglect the feedback between the modification of sea surface temperature by the convection and development of a convective cluster itself. The authors show that the convection-generated SST gradient plays an important role in cluster propagation and development. The relative importance of radiative and evaporative fluxes in SST regulation is also discussed. Various Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment and Central Equatorial Pacific Experiment observation platforms are used to estimate the effects of equatorial convection on SST changes during March 1993. The data include drifting buoys and TAO-buoy array measurements, combined with the Navy Operational Global Atmospheric Prediction System analyzed surface wind fields and Geostationary Meteorological Satellite cloud-top temperatures. It is shown that during the equatorial convection episode SST is decreasing under and to the west of the convective heat source due to the large wind velocities and solar flux reduction. To the east of the source, in the convergence region of a Kelvin wave, low wind speeds and high insolation cause the SST to increase. The data are used to formulate an empirical relationship between wind speed and the 24-h SST change on the equator. Although formulated in terms of wind speed, this relationship implicitly includes radiative effects. This equation is then used in a global circulation model to examine the effect of SST feedback on the behavior of equatorial convection. A series of experiments is performed using an R15 general circulation model of the ''aquaplanet'' with a zonally symmetric SST distribution. In the case with fixed SSTs, equatorial wind fluctuations have the character of waves propagating around the globe with a phase speed of about 20 m s(-1). When the effect of SST modification is included, the fluctuations slow down and become more organized. In addition, a 40-60-day peak appears in the spectral analysis of equatorial precipitation.

Flatau, M, Schubert WH, Stevens DE.  1994.  The Role of Baroclinic Processes in Tropical Cyclone Motion - the Influence of Vertical Tilt. Journal of the Atmospheric Sciences. 51:2589-2601.   10.1175/1520-0469(1994)051<2589:trobpi>;2   AbstractWebsite

The numerical study presented here focuses on baroclinic processes that contribute to tropical cyclone (TC) propagation. A three-dimensional, semispectral, primitive equation model of baroclinic vortex was developed to study TC motion. In a tiled vortex, interaction between upper- and lower-level vorticity anomalies leads to vortex propagation relative to the steering flow. On a beta plane, with no environmental flow, the vortex is tilted toward the south and the interaction between the layers reduces the westward movement of the vortex. The vortex tilting can also occur due to the vertical shear in the environmental wind. On an f plane, the interaction between the layers causes the northward movement of the vortex in westerly linear shear, and southward movement in easterly linear shear, with a meridional velocity of about 1 ms-1. This velocity increases with increasing vortex intensity and vertical motion.

Flatau, M, Stevens DE.  1993.  The Role of Outflow-Layer Instabilities in Tropical Cyclone Motion. Journal of the Atmospheric Sciences. 50:1721-1733.   10.1175/1520-0469(1993)050<1721:trooli>;2   AbstractWebsite

The paper examines the role of the development of outflow-layer instabilities on the motion of tropical cyclones. The influence of barotropic instability is examined by comparing the time changes in the storm tracks with the frequencies of free, unstable barotropic modes. For intense vortices barotropic instability is shown to contribute to the slow (periods of a few days) trochoidal motion of a cyclone. The development of instability depends on the horizontal distribution and frequency of environmental forcing. The strongest response occurs when the frequency of the forcing matches the frequency of an unstable mode.

Verlinde, J, Flatau PJ, Cotton WR.  1990.  Analytical Solutions to the Collection Growth Equation - Comparison with Approximate Methods and Application to Cloud Microphysics Parameterization Schemes. Journal of the Atmospheric Sciences. 47:2871-2880.   10.1175/1520-0469(1990)047<2871:asttcg>;2   AbstractWebsite

A closed form solution for the collection growth equation as used in bulk microphysical parameterizations is derived. Although the general form is mathematically complex, it can serve as a benchmark for testing a variety of approximations. Two special cases that can immediately be implemented in existing cloud models are also presented. This solution is used to evaluate two commonly used approximations. The effect of the selection of different basis functions is also investigated.