ICESat-2海冰产品算法理论基础文档

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ICESat-2 ATLAS 产品说明 ICESat-2 是美国航空航天局（NASA）的一颗地球观测卫星，旨在测量地球的表面高度和冰川厚度。ICESat-2 任务生产的 ATLAS 产品是指该卫星上的先进地形激光高度计系统（ATLAS），该系统能够对地球表面进行高精度测量。ATLAS 产品包括参数描述和这些参数的推导，都是在 ICESat-2 科学研究者领导的处理系统（SIPS）上实现的待定 bd 处理的算法理论基础文件。 ICESat-2 ATLAS 产品的主要特点是能够生成 0 级到 4 级标准数据产品，以及相关产品质量评估和元数据信息等。这些产品都是通过 ATLAS 科学算法软件（ASAS）和调度和数据管理系统（SDMS）生成的。ASAS 负责生成 0 级到 4 级标准数据产品，而 SDMS 则负责管理这些产品的质量和元数据信息。 ICESat-2 ATLAS 产品的理论基础文件包括了海冰产品的算法理论基础文档（ATBD），该文档描述了海冰处理算法和理论基础。该文档还包括了参数描述和这些参数的推导，以及相关的ancillary 地理物理参数在海冰产品推导中的应用。 ICESat-2 ATLAS 产品的主要应用包括： * 海冰研究：ICESat-2 ATLAS 产品能够对海冰进行高精度测量，从而帮助科学家研究海冰的变化和影响。 * 地形测量：ICESat-2 ATLAS 产品能够对地球表面进行高精度测量，从而帮助科学家研究地球表面的变化和演化。 * 气候研究：ICESat-2 ATLAS 产品能够对气候变化的影响进行研究，从而帮助科学家更好地理解气候变化的机理。 ICESat-2 ATLAS 产品是一种高精度的测量产品，能够对地球表面和海冰进行高精度测量，从而帮助科学家研究地球表面的变化和演化。知识点： 1. ICESat-2 ATLAS 产品是指 ICESat-2 卫星上的先进地形激光高度计系统（ATLAS），该系统能够对地球表面进行高精度测量。 2. ICESat-2 ATLAS 产品包括参数描述和这些参数的推导，都是在 ICESat-2 科学研究者领导的处理系统（SIPS）上实现的待定 bd 处理的算法理论基础文件。 3. ATLAS 科学算法软件（ASAS）负责生成 0 级到 4 级标准数据产品，而调度和数据管理系统（SDMS）负责管理这些产品的质量和元数据信息。 4. ICESat-2 ATLAS 产品的理论基础文件包括了海冰产品的算法理论基础文档（ATBD），该文档描述了海冰处理算法和理论基础。 5. ICESat-2 ATLAS 产品的主要应用包括海冰研究、地形测量和气候研究。

ICESat-2 Algorithm Theoretical Basis Document for Sea Ice Products (ATL07/ATL10)

Release 004

xvi

List of Tables

Table Page

Table 1 ICESat-2 Instrument parameters and expected signal levels over sea ice (Dec 2014) .. 7!

Table 2 Input parameters (Source: ATL03) .............................................................................. 28!

Table 3 Input parameters (Source: ATL09) .............................................................................. 30!

Table 4 Control parameters – coarse surface finding ................................................................ 32!

Table 5 Control parameters – fine surface finding .................................................................... 38!

Table 6 Uncertainties in retrieved surface height (100 photons) .............................................. 45!

Table 7 Control parameters – surface classification (Winter, Spring transition, Summer, Fall

transition) .......................................................................................................................... 53!

Table 8 Output to ATL07 (See Appendix A for details of full product dictionary) ................. 57!

Table 9 Control parameters – freeboard estimation .................................................................. 61!

Table 10 Uncertainties due to surface tilts ................................................................................ 65!

Table 11 Control parameters – for filtering/filling of along-track reference surface estimates 65!

Table 12 Output to ATL10 (See Appendix A for full product dictionary) ............................... 68!

Table 13 Control parameters – Gridded products ..................................................................... 71!

Table 14 Output to ATL20 – Gridded monthly composite of strong beam freeboards (See

Appendix A for full product dictionaries) ......................................................................... 72!

Table 15 Output to ATL21 – Gridded monthly composite of strong beam SSH anomalies (See

Appendix A for full product dictionary) ........................................................................... 73!

Table 16 Coverage of ICESat-2 Products in the Northern Hemisphere ................................... 77!

Table 17 Coverage of ICESat-2 Products in the Southern Hemisphere ................................... 77!

Table 18 Parameters from ICESat-2 ......................................................................................... 79!

Table 19 Parameters from Ancillary Sources ........................................................................... 79!

Table 20 Data filters (ATL07, ATL10). NB: the reference surface heights filter (row 3) is still

in testing for R005 data production. ................................................................................. 81!

Table 21 Current Test Data Sets ............................................................................................... 82!

Table 22 Grid Dimensions ...................................................................................................... 100!

ICESat-2 Algorithm Theoretical Basis Document for Sea Ice Products (ATL07/ATL10)

Release 004

1 INTRODUCTION

This document describes the theoretical basis of the sea ice processing algorithms and the

parameters that are derived from the ATLAS data acquired by the ICESat-2 mission. ATLAS

(Advanced Topographic Laser Altimeter System) is the photon counting lidar onboard the

ICESat-2 Observatory.

This is a living document intended to provide 1) an up-to-date description of the data products

for end users, and 2) comprehensive instructions to the developers of the underlying

algorithms. The algorithm development is an on-going process, so there can be times when

sections of the ATBD reflect the expected state of the data products and not its current state.

We have attempted to clarify these differences in more recent versions of the ATBD.

The ATLAS-based sea ice products include:

1. Along-track sea ice and sea surface heights (Product: ATL07/L3A).

2. Along-track sea ice freeboard (ATL10/L3A).

3. Gridded monthly sea ice freeboard (ATL20/L3B).

4. Gridded monthly sea surface height (ATL21/L3B, expected spring/summer 2021).

Section 2 provides an overview of the retrieval approaches and a short discussion of the

phenomenology of photon clouds from sea ice using data acquired by the Multiple Altimeter

Beam Experimental Lidar (MABEL), a technology demonstrator for the ATLAS instrument.

Section 3 describes the geophysical parameters of interest that reside in each product.

Section 4 specifies the sea ice algorithms used in the derivation of surface heights and surface

types (ATL07).

Section 5 specifies the sea ice algorithms used in the derivation of sea ice freeboard (ATL10).

Section 6 specifies the sea ice algorithms used in the derivation of the gridded sea ice products

(ATL20/21).

Section 7 addresses specific implementation details/considerations and data requirements that

are not addressed in the above sections. Topics include: software architecture for multi-beam

system; bounds of geographic and seasonal coverage by the different products in the Arctic

and Southern Oceans; and, parameters required by the algorithms that are produced by the

ICESat-2 project or from an external source.

Section 8 describes the browse products for ATL07, ATL10, ATL20, ATL21.

Section 9 describes the data filtering/quality assessments.

Section 10 describes the data used to design and test the pre-launch algorithms.

Section 11 discusses the constraints, limitations, and assumptions associated with algorithm

performance.

ICESat-2 Algorithm Theoretical Basis Document for Sea Ice Products (ATL07/ATL10)

Release 004

2 BACKGROUND

This section describes the sea ice parameters derived from ICESat2/ATLAS altimetry and

provides an overview of the approaches used to retrieve these variables. Also, a short

discussion of the phenomenology of photon clouds over sea ice is provided.

When conditions allow, the ICESat-2 sea ice products provide estimates of the following

parameters of the ice-covered seas of the northern and southern hemispheres:

1. Surface height and type.

2. Total freeboard.

These parameters are provided in along-track and gridded formats.

Even though a sea ice thickness product will not be available as a routine product from the

ICESat-2 mission, one of the ICESat-2 science requirements is to produce sea ice parameters

that will facilitate the conversion of freeboard to thickness by future investigations. The next

section discusses derivation of freeboard and its conversion to sea ice thickness.

2.1 Overview: sea ice freeboard and thickness

We first describe the geometric relationships between the different parameters used in the

freeboard and thickness calculations, and then briefly the approach to estimate ice thickness

assuming isostatic equilibrium. From altimetric heights, there are three steps in the estimation

of sea ice thickness:

1. Discrimination of ice and open water returns;

2. Retrieval of freeboard; and,

3. Conversion of freeboard to ice thickness.

Total freeboard, as defined here, is the height of the air-snow interface above the local sea

surface (Figure 1). For the Arctic Ocean, the total freeboard is typically assumed to consist of a

snow layer superimposed on the freeboard of floating sea ice, i.e., a two-layered system. This

total freeboard height, h

, above the sea surface can be written as the sum of two terms:

(1)

where and are the thicknesses of the snow and ice layers above the sea surface; h

commonly referred to as the sea ice freeboard. For Antarctic sea ice, the situation is more

complex because of layering and snow-ice formation (so called “flooded ice” from freezing of

water-soaked snow due to infiltration of rain, meltwater, or seawater above the snow-ice

interface) during the winter. The efficacy of using a simple two-layered freeboard model of the

Arctic (as shown in figure) as a model for the Antarctic ice cover remains to be demonstrated.

From altimetry, the total freeboard (h

) can be estimated by differencing the ice surface height,

, and from the local sea surface height, h

ssh

(2)

= h

+ h

= h

− h

ssh

ICESat-2 Algorithm Theoretical Basis Document for Sea Ice Products (ATL07/ATL10)

Release 004

Both h

and h

ssh

are typically measured relative to the level of a particular reference ellipsoid

(e.g., WGS84). The ATLAS instrument measures both variables. The space-time varying sea

surface height is the sum of contributions from a number of physical processes. It can be

written as the sum of the following terms:

(3)

In this equation, h

is associated with geoid undulations, h

represents the sea surface response

to atmospheric pressure loading, h

is due to tides, h

is the dynamic ocean topography (DOT)

associated with geostrophic surface currents, and higher order terms. All these terms vary in

time and space and possess their own characteristic length scales. The reader is referred to

Kwok et al. [2006] for a brief discussion of the various sea surface models and the expected

uncertainties of each of these terms.

Since centimeter-level knowledge of the sea surface height (h

ssh

) in Equation 3 is lacking, the

first step in the retrieval of freeboard is the identification of available sea surface samples in

the altimetry data that are usable as ocean surface references. This involves separating the

altimetric returns into those from ice and from open water (leads). After identification of the

returns into the two surface types, h

can be then be calculated from the observed heights.

Since the sea surface height varies along an altimeter track, an important consideration is the

spatial length scale over which one could assume the sea surface height to be nearly constant:

this is dependent on the acceptable freeboard uncertainty, the expected spatial variability of sea

surface height (i.e., topography), and the expected number of tiepoints (sea surface reference)

within an altimeter segment. The spatial variability of h

deserves special attention as it is

more energetic than h

, h

, and h

at all length scales. That is, the small-scale variability of the

geoid could introduce significant errors in the freeboard retrieval process if not accounted for.

Once h

and h

ssh

are determined, ice thickness (h

) can be estimated with the assumption that

the floating sea ice is in isostatic balance:

ssh

(x,t) = h

(x) + h

(x,t) + h

(x,t) + O

Figure 1 Geometric relationships

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