Variability of the Boundary Layer Depth over Certain Regions of the Subtropical Ocean from 3 Years of COSMIC Data S. Sokolovskiy, D. Lenschow, C. Rocken, W. Schreiner, D. Hunt, Y.-H. Kuo and R. Anthes University Corporation for Atmospheric Research National Center for Atmospheric Research AMS 90th Annual Meeting, Atlanta GA, 17-21 January, 2010
Abstract Global Positioning System (GPS) Radio Occultation (RO) remote sensing with its high vertical resolution and sensitivity to moisture gradients is an efficient method for monitoring Atmospheric Boundary Layer (ABL) depth, particularly over the ocean. In previous studies by different authors the ABL top was determined mainly from vertical gradients of scalars retrieved from the GPS RO signals, such as bending angle, refractivity, moisture, etc. In this study we apply the approach based on the bending angle for analysis of the spatial and temporal variability of the ABL depth over the subtropical oceanic regions west of the coasts of North America, South America and South Africa. These regions are characterized by a relatively shallow stratocumulus-topped ABL with a strong capping inversion. The study is based on 3 years of COSMIC data. We found very clear seasonal variation with magnitude ~200-300m and weaker diurnal variation with magnitude ~20-100m. The diurnal variation over the oceans shows afternoon minimum which is somewhat opposite to the variation over land (with the afternoon maximum).
The Atmospheric Boundary Layer The Atmospheric Boundary Layer (ABL) is the turbulent layer that couples the surface to the overlying intermittently turbulent free atmosphere (troposphere) where the wind becomes geostrophic. Interactions with the surface generate turbulence in the ABL. Turbulence causes the ABL to be well-mixed with an adiabatic lapse rate, in contrast to the stably-stratified free atmosphere. Typically the ABL is capped by a thin transition layer characterized by large gradients in temperature and humidity. This is especially pronounced in the subtropics to the west of continents due to atmospheric subsidence and cold oceanic upwelling. The ABL depth is determined by a finely-tuned balance between buoyancy- and shear-generated turbulence that deepens the ABL, and large- scale subsidence that suppresses ABL growth. Therefore, it is a sensitive variable for testing global and mesoscale atmospheric models.
An example of strong inversion layer on top of ABL Radiosonde data RO observables modeled 23 January 2002 from the radiosonde data. 15.97S, 5.70W The “step-like” structures in bending angle and refractivity
Radio occultation – active limb sounding (remote sensing of refractivity) Refraction in the atmosphere causes slowing of radiowaves and bending of their trajectories c = = + − 6 N - refractivity 1 10 * n N Refractive index: v LEO receiver GPS transmitter Earth Main observable: phase delay – precisely calibrated by GPS transmitter clock. The phase allows accurate determination of ray bending angles (amplitude is used in radio-holographic signal processing, but its precise calibration is not required). The vertical profile of the bending angle allows determination of the vertical profile of refractivity (Abel inversion)
P P = + ⋅ 5 w 77 . 6 3 . 73 10 Refractivity: N 2 T T dry term wet term (dominant in the moist troposphere) Significant decrease of refractivity on top of the ABL (due to decrease of humidity) causes significant increase of the bending angle of radio waves, that can be accurately measured from the phase and amplitude of radio occultation signals. z refracted waves receiver equivalent phase screen stably incident wave stratified atmosphere inversion layer turbulence N Earth
Determining the height of ABL (and other inversion layers) from radio occultation signals (two methods) max. bending angle lapse max. lapse of N gradient (BAL) in a sliding window obtained by linear regressions ∆ α ∆ α ∆ = in two adjacent sliding windows ( / ) max z
Comparison of the ABL depths obtained from bending angle and refractivity Large differences are mainly related to existence of multiple layers. Either approach can be used to study variability of the ABL depth. In this study we use the approach based on BA which is not affected by super-refraction (N is affected).
Global distribution of the ABL depth over the oceans from COSMIC - sharpest ABL tops in sub-tropics – few pronounced ABL tops in ITCZ - decrease of ABL depth toward west coasts of continents January April October July height of the strongest (BAL>1E-2rad, z<3km) inversion layer (km)
Distribution of occultations with sharp ABL top over the oceans (and some land areas) in 2008 The sharpest ABL tops are observed over the subtropical oceans (red boxes show the regions selected for the study of variations)
Distribution of COSMIC data in time and local time for the South Atlantic Region At the beginning of the mission all satellites were in one orbit plane, that resulted in sampling at two local times. With the separation of the orbit planes the sampling became close to uniform in local time.
Data processing (determination of the variations of the ABL depth by re-sampling & boxcar filtering) 1 2 1) Sampling in longitude. Detection and removal 3 of the longitudinal variation. 2) Re-sampling in day (time). Detection and removal of the seasonal variation 3) Re-sampling in local time. Detection of the diurnal variation.
Longitudinal variations of the ABL depth North Pacific South Atlantic South Pacific (I) South Pacific (II)
Seasonal variations of the ABL depth North Pacific South Atlantic South Pacific (I) South Pacific (II)
Diurnal variations of the ABL depth (thin lines – standard deviations of the mean) North Pacific South Atlantic South Pacific (I) South Pacific (II)
Validation of the diurnal variations of the ABL depth with atmospheric models is difficult because the models: (i) do not provide sufficient vertical resolution at the ABL top (ii) do not provide uniform sampling in local time Internal validation: estimation of the diurnal variation over desert produces expected results (deeper ABL in the afternoon). Sahara Desert
Summary Radio occultation signals, sensitive to vertical refractivity (moisture) gradients, provide a useful tool for global monitoring of the ABL depth. 3 years of COSMIC radio occultation data allow global monitoring of the geographical, seasonal, and diurnal variabilities of the ABL depth. Over the subtropical oceans, the seasonal variation of the ABL depth has a magnitude of several hundred meters with the maxima shifted by 1-3 months from mid-summer. The diurnal variation of the ABL depth over the subtropical oceans is different in different regions, its magnitude (several tens of meters) is much smaller than over land (up to several hundred meters) and it commonly has an afternoon minimum (contrary to the afternoon maximum over land).
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